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  • ASTROPHYSICS  (16,581)
  • 1
    Publication Date: 2011-08-24
    Description: Astrometry with the Hubble space telescope and Hipparcos satellite advanced the optical astrometric accuracies to the milliarcsec level. The global astrometric interferometer for astrophysics (GAIA) satellite and other proposed optical interferometry space missions would advance the optical astrometric accuracy to better than 10 microarcsec. A catalog figure of merit is defined which allows a quantitative comparison to be carried out for astrometric catalogs. Using two specific astrophysical problems, the level of the expected scientific contribution from a GAIA-type mission is assessed. The two problems are: the age of the globular clusters in relation to the age of the Universe, and the initial mass function compared with fluctuations in the star formation rate with time.
    Keywords: ASTROPHYSICS
    Type: ESA, Future Possibilities for Astrometry in Space; p 149-151
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  • 2
    Publication Date: 2011-08-24
    Description: We compute optical and infrared light curves of the pulsating class of delayed detonation models for Type Ia supernovae (SN Ia's) using an elaborate treatment of the Local Thermodynamic Equilbrium (LTE) radiation transport, equation of state and ionization balance, expansion opacity including the cooling by CO, Co(+), and SiO, and a Monte Carlo gamma-ray deposition scheme. The models have an amount of Ni-56 in the range from approximately or equal to 0.1 solar mass up to 0.7 solar mass depending on the density at which the transition from a deflagration to a detonation occurs. Models with a large nickel production give light curves comparable to those of typical Type Ia supernovae. Subluminous supernovae can be explained by models with a low nickel production. Multiband light curves are presented in comparison with the normally bright event SN 1992bc and the subluminous events Sn 1991bg and SN 1992bo to establish the principle that the delayed detonation paradigm in Chandrasekhar mass models may give a common explosion mechanism accounting for both normal and subluminous SN Ia's. Secondary IR-maxima are formed in the models of normal SN Ia's as a photospheric effect if the photospheric radius continues to increase well after maximum light. Secondary maxima appear later and stronger in models with moderate expansion velocities and with radioactive material closer to the surface. Model light curves for subluminous SN Ia's tend to show only one 'late' IR-maximum. In some delayed detonation models shell-like envelopes form, which consist of unburned carbon and oxygen. The formation of molecules in these envelopes is addressed. If the model retains a C/O-envelope and is subluminous, strong vibration bands of CO may appear, typically several weeks past maximum light. CO should be very weak or absent in normal Sn Ia's.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 444; 2; p. 831-847
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  • 3
    Publication Date: 2011-08-24
    Description: We consider the galactic evolutionary history of He-3 in models which deplete deuterium by as much as a factor of 2 to approximately 15 from its primordial value to its present-day observed value in the interstellar medium (ISM). We show that when He-3 production in low-mass stars (1-3 solar mass) is included over the history of the galaxy, He-3 is greatly overproduced and exceeds the inferred solar values and the abundances determined in galactic H II regions. Furthermore, the ISM abundances show a disturbing dispersion which is difficult to understand from the point of view of standard chemical evolution models. In principle, resolution of the problem may lie in either (1) the calculated He-3 production in low-mass stars; (2) the observations of the He-3 abundance; or (3) an observational bias twoard regions of depleted He-3. Since He-3 observations in planetary nebula support the calculated He-3 production in low-mass stars, option (1) is unlikely. We will argue for option (3) since the He-3 interstellar observations are indeed made in regions dominated by massive stars in which He-3 is destroyed. In conclusion, we note that the problem with He-3 seems to be galactic and not cosmological.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 444; 2; p. 680-685
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  • 4
    Publication Date: 2011-08-24
    Description: Many features of Herbig-Haro objects can be reproduced using a kinematical bow shock model. We use the model to generate position-velocity (PV) diagrams of flux in H-alpha (O I) lambda lambda 6300+63, (S II) lambda lambda 6716+31, (O III) lambda lambda 4959+5007, and (C I) lambda lambda 9823+50, line ratios of (O I)/H-alpha, (O I)/(S II), (S II)/H-alpha, H-alpha(S II), and (O III)/H-alpha, electron density N(sub e), and electron temperature T(sub e). We show how position-velocity diagrams of N(sub e) and flux vary with shock velocity. By matching the diagrams from single lines, the line ratios, and N(sub e) with observations, we determined a narrow range of shock parameters for HH 1F, 2(A' + H), and 43 (B + C). We model the N(sub e) features of HH 2(A' + H) as a superposition of two bowshocks. We also show that the effects of slight misalignments of the two diagrams to be divided can produce artifacts in the line ratios and N(sub e) which obliterate the physical features. We show that N(sub e) in HH 1 can only be explained using the kinematical model by taking these misalignments into account.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 109; 2; p. 752-761
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  • 5
    Publication Date: 2011-08-24
    Description: We calculated bolometric temperature (T(sub bol)) and luminosity (L(sub bol)) for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the Ophiuchus 'off-core' region. We have constructed the bolometric luminosity-temperature (BLT) diagram, the log-log plot of L(sub bol) versus T(sub bol), for the three samples. T(sub bol) is defined as the temperature of a blackbody having the same frequency as the observed continuum spectrum. It measures the redness (or coldness) of an astronomical source. The BLT diagram is analogous to the H-R diagram and allows for a direct and quantitative comparison of YSOs at a wide variety of evolutionary states, ranging from the most deeply embedded stars to T Tauri stars nearly on the main sequence. We found (1) T(sub bol) increases monotonically from embedded sources (approximately 60-500 K) to classical T Tauri stars (approximately 1000-3000 K) to weak-line T Tauri stars (approximately 2000-5000 K); (2) T(sub bol) correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars. There is no significant correlation for the classical T Tauri stars. These results can be understood in terms of dissipation of circumstellar dust envelope and disk during the early stages of stellar evolution. Sources in the three regions have different distributions in the BLT diagram. The Ophiuchus core has the highest fraction of cold sources among the three regions. These cold sources are also more luminous than the YSOs in the other regions. The Ophiuchus off-core sample is dominated by the more evolved pre-main-sequence stars. The Taurus sources have distributions intermediate in L(sub bol), T(sub bol), and age between the Ophiuchus core and off-core distributions. These may suggest differences in the star formation history, and possibly in the stellar masses and mass accretion rates in these star-forming regions.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal, Part 1 (ISSN 0004-637X); 445; 1; p. 377-392
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  • 6
    Publication Date: 2011-08-24
    Description: Asymptotic giant branch (AGB) stars are known to be sites of dust formation and undergo significant mass loss. The outflow is believed to be driven by radiation pressure on grains and momentum coupling between the grains and gas. While the physics of shell dynamics and grain formation are closely coupled, most previous models of circumstellar shells have treated the problem separately. Studies of shell dynamics typically assume the existence of grains needed to drive the outflow, while most grain formation models assume a constant veolcity wind in which grains form. Furthermore, models of grain formation have relied primarily on classical nucleation theory instead of using a more realistic approach based on chemical kinetics. To model grain formation in carbon-rich AGB stars, we have coupled the kinetic equations governing small cluster growth to moment equations which determine the growth of large particles. Phenomenological models assuming stationary outflow are presented to demonstrate the differences between the classical nucleation approach and the kinetic equation method. It is found that classical nucleation theory predicts nucleation at a lower supersaturation ratio than is predicted by the kinetic equations, resulting in significant differences in grain properties. Coagulation of clusters larger than monomers is unimportant for grain formation in high mass-loss models but becomes more important to grain growth in low mass-loss situations. The properties of the dust grains are altered considerably if differential drift velocities are ignored in modeling grain formation. The effect of stellar temperature, stellar luminosity, and different outflow velocities are investigated. The models indicate that changing the stellar temperature while keeping the stellar luminosity constant has little effect on the physical parameters of the dust shell formed. Increasing the stellar luminosity while keeping the stellar temperature constant results in large differences in grain properties. For small outflow velocities, grains form at lower supersaturation ratios and close to the stellar photosphere, resulting in larger but fewer grains. The reverse is true when grains form under high outflow velocities, i.e., they form at higher supersaturation ratios, farther from the star, and are much smaller but at larger quantities.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 444; 1; p. 251-269
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  • 7
    Publication Date: 2011-08-24
    Description: The globular clusters that we observe in galaxies may be only a fraction of the initial population. Among the evolutionary influences on the population is the destruction of globular clusters by tidal forces as the cluster moves through the field of influence of a disk, a bulge, and/or a putative nuclear component (black hole). We have conducted a series of N-body simulations of globular clusters on bound and marginally bound orbits through poetentials that include black hole and speroidal components. The degree of concentration of the spheroidal component can have a considerable impact on the extent to which a globular cluster is disrupted. If half the mass of a 10(exp 10) solar mass spheroid is concentrated within 800 pc, then only black holes with masses greater than 10(exp 9) solar mass can have a significant tidal influence over that already exerted by the bulge. However, if the matter in the spheroidal component is not so strongly concentrated toward the center of the galaxy, a more modest central black hole (down to 10(exp 8) solar mass) could have a dominant influence on the globular cluster distribution, particularly if many of the clusters were initially on highly radial orbits. Our simulations show that the stars that are stripped from a globular cluster follow orbits with roughly the same eccentricity as the initial cluster orbit, spreading out along the orbit like a 'string of pearls.' Since only clusters on close to radial orbits will suffer substantial disruption, the population of stripped stars will be on orbits of high eccentricity.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 444; 1; p. 193-199
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  • 8
    Publication Date: 2011-08-24
    Description: We investigate observable effects of anisotropic turbulence on the velocity profiles and eclipse behavior of emission lines from accretion disks. Turbulence expands the local line broadening profile, enhancing the surface brightness of saturated emission lines. Anisotropic turbulence produces anisotropic emission in such lines. The effects become observable when the turbulence exceeds the thermal velocity. Each term in the velocity-velocity correlation matrix produces a distinctive azimuthal pattern of enhanced emission-line surface brightness on the face of the accretion disk. These patterns express themselves as changes in the observable shapes of the disk's emission lines. The best place to look for turbulence effects is in saturated emission lines of heavy elements such as Ca, Mg, and Fe, which have a smaller thermal velocity at a given sound speed and at moderate inclination (60-70 degrees), since the Keplerian shear broadening dominates at higher inclinations.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 297; 1; p. 273-284
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  • 9
    Publication Date: 2011-08-24
    Description: The satellite observations at comet Halley have shown strong heating of solar wind alpha particles over an extended region dominated by high-intensity, low-frequency turbulence. These waves are excited by the water group pickup ions and can energize the solar wind plasma by different heating processes. The alpha particle heating by the Landau damping of kinetic Alfven waves and the transit time damping of low-frequency hydromagnetic waves in this region of high plasma beta are studied in this paper. The Alfven wave heating was shown to be the dominant mechanism for the observed proton heating, but it is found to be insufficient to account for the observed alpha particle heating. The transit time damping due to the interaction of the ions with the electric fields associated with the magnetic field compressions of magnetohydrodynamic waves is found to heat the alpha particles preferentially over the protons. Comparison of the calculated heating times for the transit time damping with the observations from comet Halley shows good agreement. These processes contribute to the thermalization of the solar wind by the conversion of its directed energy into the thermal energy in the transition region at comet-solar wind interaction.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; A5; p. 7891-7897
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  • 10
    Publication Date: 2011-08-24
    Description: We use a maximum-entropy method to show that the transfer function (TF) of the broad-line region (BLR) in the Seyfert-1 galaxy NGC 3516 is time-varying. The TF decreased by nearly a factor of two over a time scale less than half a year during a monitoring campaign in 1990. We conclude that the observed time dependency is likely to be either due to variations of the spectral index of the ionizing continuum spectrum; due to a change in the number or covering fraction of broad-line clouds; due to nonlinear line response; or due to nonstationary anisotropy of the continuum source on time scales of several weeks to months. An extended continuum source could also explain the observed time-dependency. The symmetry of the time lag for variations in the red and blue wings of H-alpha indicates that the BLR kinematics is not dominated by organized radial inflow or outflow. If we assume circular orbits, the observed time lag 14 days at velocity v = 4000 km/s suggests a mass of the central object of approximately 2 x 10(exp 7) solar mass. The H-alpha TF peaks away from zero delay, indicating that the H-alpa BLR is either non-spherical or inwardly emitting.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 289; 1; p. 76-82
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  • 11
    Publication Date: 2011-08-24
    Description: Magellan altimetry data reveal that the center of figure (CF) of Venus is displaced approximately 280 m from its center of mass (CM) toward 4.4 deg S, 135.8 deg E, a location in Aphrodite Terra. This offset is smaller than those of other terrestrial planets but larger than the estimated error, which is no more than a few tens of meters. We examine the possibility that the CF-CM offset is related to specific geologic provinces on Venus by deriving three simple models for the offset: a thick-crust model, a hotspot model, and a thick-lithosphere model. The offset caused by a region of thick crust depends upon the region's extent, the crust-mantle density contrast, and the thickness of excess crust. A hotspot-related offset depends on the extent of the thermally anomalous region and the magnitude of the thermal anomaly. Offset due to a region of thick lithosphere depends upon the extent of the region, the average temperature contrast across the lithosphere, and the amount of excess lithosphere. We apply the three models to Venus plateau-shaped highlands, volcanic rises, and lowlands, respectively, in an attempt to match the observed CF-CM offset location and magnitude. The influence of most volcanic rises and of Ishtar Terra on the CF-CM offset must be quite small if we are to explain the direction of the observed offset. The lack of influence of volcanic rises can be explained if the related thermal anomalies are limited to a few hundred degrees or less and are plume-shaped (i.e., characterized by a flattened sublithospheric `head' with a narrow cylindrical feeder `tail'). The unimportance of Ishtar Terra is most easily explained if it lies atop a significant mantle downwelling.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 111; 2; p. 417-432
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  • 12
    Publication Date: 2011-08-24
    Description: There are now a large number of small bodies in the outer solar system that are known to be covered with dark material. Attempts to identify that material have been thwarted by the absence of discrete absorption features in the reflection spectra of these planetesimals. An absorption at 2.2 micrometers that appeared to be present in several objects has not been confirmed by new observations. Three absorptions in the spectrum of the unusually red planetesimal 5145 Pholus are well-established, but their identity remains a mystery.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 2; p. (2)41-(2)49
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  • 13
    Publication Date: 2011-08-24
    Description: We explore the factors that determine the bow shock standoff distance. These factors include the parameters of the solar wind, as well as the size and shape of the obstacle. In this report we develop a semiempirical Mach number relation for the bow shock standoff distance in order to take into account the shock's behavior at low Mach numbers. This is done by determining which properties of the shock are most important in controlling the standoff distance and using this knowledge to modify the current Mach number relation. While the present relation has proven useful at higher Mach numbers, it has lacked effectiveness at the low Mach number limit. We also analyze the bow shock dependence upon the size and shape of the obstacle, noting that it is most appropriate to compare the standoff distance of the bow shock to the radius of curvature of the obstacle, as opposed to the distance from the focus of the object to the nose. Last, we focus our attention on the use of bow shock models in determining the standoff distance. We note that the physical behavior of the shock must correctly be taken into account, specifically the behavior as a function of solar wind dynamic pressure; otherwise, erroneous results can be obtained for the bow shock standoff distance.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A9; p. 17,681-17,689
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  • 14
    Publication Date: 2011-08-24
    Description: The adequacy of the power spectrum to characterize the variations of a parameter depends on whether or not the parameter has a Gaussian distribution. We here perform very simple tests of Gaussianity on the distribution. We here perform very simple tests of Gaussianity on the distributions of the magnitudes of the interplanetary magnetic field, and on the distributions of the components; that is, we find the first four cumulants of the distributions (mean, variance, skewness, and kurtosis) and their solar cycle variations. We find, consistent with other recent analyses, that the traditional distributions of the 1-hour averaged magnitude are not distributed normally or lognomally as has often been assumed and the 1-hour averaged z component is found to have a nonzero kurtosis. Thus the power spectrum is insufficient to completely characterize these variations and polyspectra are needed. We have isolated variations in the 1/f frequency region of the spectrum and show that the distributions of the magnitudes have nonzero skewness and kurtosis, the magnitudes are not distributed lognormally, and the distributions of the components have nonzero kurtosis. Thus higher-order spectra are again needed for a full characterization.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A9; p. 17,645-17,651
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  • 15
    Publication Date: 2011-08-24
    Description: Acceleration of interstellar pickup H(+) and He(+) as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at approximately 4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H(+) and He(+)) and the low thermal speed solar wind ions (H(+) and He(++) are derived using velocity distribution functions of protons, pickup He(+) and alpha particles from less than 1 to 60 keV/e and of ions (principally protons) above approximately 60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H(+) and He(+) are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He(+), and intermediate for alpha particles, (4) both H(+) and He(+) have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F(sub o)w(sup -4) for 1 is less than w is less than approximately 5, where w is the ion speed divided by the solar wind speed. Above w approximately 5-10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A9; p. 17,637-17,643
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  • 16
    Publication Date: 2011-08-24
    Description: A recent ultraviolet snaphsot imaging survey of the nuclei of nearby galaxies detected a compact nuclear ultraviolet source in only five of the 26 LINERs (low-ionization nuclear emission-line regions) included in the observed sample. Motivated by this observational result, we examine the possibility that all LINERs are powered by photoionization from a nuclear source, which is, however, active only for 20% of the time. We show that decay times of low-ionization species can be of the order of one to a few centuries, and we demonstrate through time-dependent photoionization calculations that if the nuclear ionizing source is active for only a fraction of the time, this would not be readily noticeable in the emission-line spectrum. We suggest that the activity cycle is related to episodic accretion events which are associated with the tidal disruption of stars by a central black hole. The time interval between tidal disruptions is of the same order as the emission-line decay time, with the accretion episode following each disruption lasting a few decades. These estimates appear to support the duty cycle hypothesis. Some observational consequences of the proposed scenario are also discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 445; 1; p. L1-L5
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  • 17
    Publication Date: 2011-08-24
    Description: A unified model is formulated to interpret quantitatively the observed characteristics of the nuclear train, the two dust trails, and the tail region of P/Shoemaker-Levy 9 in terms of a collisionally modified rotation velocity distribution of the comet's debris. Since there is no evidence for the comet's out-gassing, the model does not assume activity. The discruption of the parent comet was due primarily to tidal stesses during its extremely close approach to Juptier in July 1992. The original nucleus is found to have been most probably approximately 5 km in radius or, equivalently, approximately 10(exp 17) g in mass. The dynamical separation of the debris occurred apparently approximately 2.2 hr after the perijove passage, even though the actual fragmentation of the original mass is likely to have begun before closest approach. Physical breakp was accompanied by ubiquitous low-velocity collisions among the particulates, resulting in a rearrangement of the initial rotaional velocities into a rapidly 'thermalized' distribution, characterized by a long tail of relatively high velocities for the debris that populates the dust trails far from the nuclear train. Compelling evidence is presented for secondary fragmentation events, indicative of the comet's continuing disintegration. Secondary fragmentation may be caused by rotational bursting of the massive fragments that had been cracked but unbroken during the tidal disruption. Impact conditions and possible interactions of the comet's debris with the jovian system during the encounter in July 1994 are described.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 289; 2; p. 607-636
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  • 18
    Publication Date: 2011-08-24
    Description: Beta Lyr is an enigmatic object. Despite several decades of concentrated observational and theoretical effort, our understanding of the system is still rather poor. We discuss two existing structural models of the system, the massive torus model of Wilson (1981), and the low mass accretion disk model of Hubeny & Plavec (1991). In particular, we answer recent criticism of the low mass disk model expressed by the proponents of the massive disk hypothesis. We show that although both theories have a large degree of internal consistency, there are several serious physical objections against the massive disk model. In particular, such a model requires unrealistically low viscosity (large Reynolds number). Moreover, such massive disk is likely to be dynamically unstable. Finally, we propose several observational and theoretical approaches that could lead to construction of a more physically realistic model of the Beta Lyr system.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 289; 2; p. 411-418
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  • 19
    Publication Date: 2011-08-24
    Description: We report on observations of the Earth's bow shock at unprecedentedly large downtail distances, some as remote as 360 R(sub E). Suprisingly, we find that even at these large distances, the bow shock signature remains clear. The cases we report are among the weakest shocks ever clearly identified. These shocks reveal patterns of field changes remarkably similar to those observed for stronger shocks. Indeed, several of the shocks could serve as textbook examples because they occur in unusually quiet and steady solar wind conditions. The quasi- perpendicular shocks are, in some cases preceded by whistler wave trains. Several of the shocks which have normal vectors in the transitional region between quasiperpendicular and quasiparallel are associated with large amplitude wave disturbances in the downstream plasma. Although low Mach number, these shocks are the source of large amplitude turbulence. We show a range of fits to the locations of the shock crossings, and argue that, for steady solar wind conditions, there is no sign of tail flapping. The displacement of the shock from the tail axis appears to be governed by the interplanetary magnetic field orientation as previously reported for Venus.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 4; p. (4)197-(4)204
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  • 20
    Publication Date: 2011-08-24
    Description: The region upstream of a planetary bow shock, known as the foreshock, contains a variety of phenomena. Electrons and ions are reflected and energized at the shock. As these stream back upstream, they generate both VLF and ULF waves. Studies of the terrestrial foreshock have provided most of our understanding of these phenomena. However, comparisons with other planetary foreshocks are beneficial, even though the instrumentation used to provide the data may be less sophisticated than that flown on Earth orbiting spacecraft. In particular, maps of the VLF emissions upstream of the Venus bow shock, using data acquired by the Pioneer Venus Orbiter are particularly illuminating. These maps show that the tangent field line is clearly marked by the presence of plasma oscillations. Of additional interest is evidence that the emissions only extend some 15 Venus radii away from the shock, indicating that the emissions are controlled by the shock scale size. Lower frequency ion acoustic waves are observed deep in the ion foreshock. Only close to the shock do both the ion acoustic waves and ULF waves occur simultaneously. The ULF waves mark the ion foreshock boundary where ion beams should be present. The ion acoustic waves tend to be observed further downstream, where diffuse ion distributions are expected to occur. A similar mapping of the terrestrial foreshock, using data from the ISEE-3 spacecraft shows similar results for the electron foreshock. An extensions of this study to include ULF and ion acoustic waves would be helpful.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 4; p. (4)125-(4)136
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  • 21
    Publication Date: 2011-08-24
    Description: The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of comet Halley. A narrow transition layer with enhanced plasma density is formed at this shock-like boundary as a flux of cometary ions flowing into it from within the cavity is removed by electron-ion recombination. We examine the structure of this layer using both a two-dimensional magnetohydrodynamical model and a one- dimensional hybrid code.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 4; p. (4)19-(4)24
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  • 22
    Publication Date: 2011-08-24
    Description: We compare the burst distribution of the Burst and Transient Source Experiment (BATSE)-2B catalog to a cosmological distribution. The observed distribution agrees well with a cosmological one, however, it is insensitive to cosmological parameters such as omega and lambda. The bursts are not necessarily standard candles, and their luminosity can vary by up to a factor of 10. The maximal redshift, z(sub max), of bursts longer than 2 s is 2.1(sup +1)(sub -0.7) (assuming no evolution). The present data is insufficient to determine maximal redshift, z(sub max), of bursts shorter than 2 s.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 444; 1; p. L25-L28
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  • 23
    Publication Date: 2011-08-24
    Description: Images of Io taken by the Hubble Space Telescope (HST) Planetary Camera (PC) using filters at 7120 and 3577 A in April and May 1992 shortly after Io emerged from Jupiter's shadow have been analyzed for the effect of posteclipse brightening first report by Binder and Cruikshank (1964). The classic effect displays a 10% increase in brightening lasting for 10 to 15 min. This was not seen in any of the eclipses that were studied. A comparison of the 3577-A images with UV maps produced from images taken with Voyager 1 (McEwen 1988), showed little change in 13 years. The striking, detailed similarity of the two demonstrate that the Hubble Space Telescope Planetary Camera has the capability to monitor temporal changes on Io's surface.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 111; 1; p. 73-78
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  • 24
    Publication Date: 2011-08-24
    Description: Femtolensoing is a gravitational lensing effect in which the magnification is a function not only of the position and sizes of the source and lens, but also of the wavelength of light. Femtolensing is the only known effect of 10(exp -13) - 10(exp -16) solar mass) dark-matter objects and may possibly be detectable in cosmological gamma-ray burst spectra. We present a new and efficient algorithm for femtolensing calculation in general potentials. The physical optics results presented here differ at low frequencies from the semiclassical approximation, in which the flux is attributed to a finite number of mutually coherent images. At higher frequencies, our results agree well with the semicalssical predictions. Applying our method to a point-mass lens with external shear, we find complex events that have structure at both large and small spectral resolution. In this way, we show that femtolensing may be observable for lenses up to 10(exp -11) solar mass, much larger than previously believed. Additionally, we discuss the possibility of a search femtolensing of white dwarfs in the Large Magellanic Cloud at optical wavelengths.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal, Part 1 (ISSN 0004-637X); 442; 1; p. 67-75
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  • 25
    Publication Date: 2011-08-24
    Description: The main sources for the ions in the interplanetary medium are the solar wind and the interstellar gas intruding into the heliosphere. Other sources, i.e. comets and planets can, however, contribute significantly locally. We present approximate expressions for deriving fluxes of pick-up ions originating from local neutral gas sources. Since the Io-torus is thought to be a relatively strong emitter of fast atoms, particular O and S, we used the O(+) data obtained by SWICS-Ulysses as a function of distance from Jupiter, both in and out-off the ecliptic plane in order to recognize a possible contribution of the Ionian source to the measured O(+). We find that - except for a very limited zone near the Jovian magnetosphere - the contribution of the Io-torus to the O(+) fluxes is minor, thus confirming our previous results on the pick-up H(+), He(+), N(+), O(+), and Ne(+) fluxes of interstellar origin.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 289; 3; p. 933-936
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  • 26
    Publication Date: 2011-08-24
    Description: It has been suggested by Verigin et al. (1993) that the response of the Martian magnetotail to changes in the solar wind ram pressure indicates the presence of an intrinsic dipole magnetic field. 3-D hybrid particle simulations of Mars were performed including the magnetotail regions. The simulations are in agreement with published Phobos 2 data from the 3 elliptical orbits and reproduce the magnetopause diameter dependence on the solar wind ram pressure reported by Verigin et al. (1993). However, the simulations were performed with no intrinsic field present, indicating that the dependence of the magnetotail width on ram pressure, is not a discriminator for the presence of the intrinsic field.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 22; 10; p. 1181-1184
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  • 27
    Publication Date: 2011-08-24
    Description: New model atmosphere calculations for very cool white dwarfs with mixed H/He and pure He compositions are presented. The hydrogen-rich models incorporate improved cross section calculations of the collision-induced absorption by molecular hydrogen due to collisions with H2, H, and He. The effects associated with variations in the effective temperature (T(sub eff) greater than or equal to 4000 and less than or equal to 10,000 K), the surface gravity (log g greater than or equal to 7.5 and less than or equal to 9.5), and the chemical composition (N(He)/N(H) greater than or equal to O and less than or equal to 100) are investigated. Results from earlier calculations are confirmed qualitatively, but a more detailed comparison reveals large quantitative deviations. Cool white dwarfs with mixed H/He chemical compositions are shown to be easily recognizable from their predicted strong infrared flux deficiency. Pure helium model calculations are described as well. These include a modified version of the recently developed equation of state of D. Saumon and G. Chabrier. Nonideal effects brought about by various equations of state are explored in detail. For the purpose of this analysis, a model of pressure ionization based on an accurate description of the interactions in a mostly atomic helium fluid is developed. The effects of pressure ionization are shown to be the most important issue in the model calculations. A critical discussion of previous generations of pure helium model calculations is presented. Finally, broadband color indices are provided for the complete model grid.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 2; p. 764-779
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  • 28
    Publication Date: 2011-08-24
    Description: Laboratory studies of infrared emission from gas-phase naphthalene in the 3.3 micrometer region following ultraviolet laser excitation are used to interpret the unidentified infrared bands observed in many astronomical objects. A time-resolved Fourier transform infrared emission technique acquires the time and spectrally resolved data. Two excitation wavelengths are employed: 193 nm and 248 nm. The infrared emission features are strongly dependent on the initial excitation energy. Wavelength-resolved spectra recorded 6.8 microseconds after the laser pulse show a 45/cm redshift from the gas-phase absorption spectra for 193 nm excitation and 25/cm for 248 nm excitation. We hypothesize that a series of sequence bands originating from the highly vibrationally excited ensemble of molecules is responsible for the observed shift. As collisional and radiative deactivation removes energy from the highly vibrationally excited molecules, the maximum in the emission profile gradually approaches the customary absorption maximum. This indicates that the amount of redshift is strongly dependent on the amount of internal vibrational energy in the molecule at the time of the vibrational transition.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 2; p. 675-681
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  • 29
    Publication Date: 2011-08-24
    Description: We present spectroscopic observations of the nucleus of the Seyfert/low-ionization nuclear emission-line region galaxy NGC 1097 spanning the period 1991-1994. The goal was to monitor anticipated variations of the broad, double-peaked Balmer lines which appeared abruptly in 1991. We find that the broad Balmer lines have varied significantly over the monitoring period, both in their integrated fluxes and in their profile shapes. The integrated H-alpha flux has decreased by a factor of 2, the (H-alpha)/(H-beta) ratio has increased, and the originally asymmetric H-alpha profile has become symmetric. The decline of the H-alpha flux and the change in the (H-alpha)/(H-beta) ratio can be interpreted as consequences of either increased obscuration along the line of sight, or a decline in the ionizing continuum, but neither of these scenarios can account for the change in profile shapes. A model attributing the line emission to a precessing elliptical ring around a 10(exp 6) solar mass nuclear black hole can reproduce the observed profile variations. In this scenario, the line-emitting ring is the result of the tidal disruption of a star by the black hole. Alternative scenarios associating the broad-line emission with a collimated bipolar outflow also remain viable, but binary black holes and inhomogeneous accretion disks are disfavored by the observed pattern of variability.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 2; p. 617-624
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  • 30
    Publication Date: 2011-08-24
    Description: We analyze previous results on the stability of uniformly and differentialy rotating, self-gravitating, gaseous and stellar, axisymmetric systems to derive a new stability criterion for the appearance of torodial, m = 2 intermediate or I-modes and bar modes. In the process, we demonstrate that the bar modes in stellar systems and the m = 2 I-modes in gaseous systems have many common physical characteristics and only one substantial difference: because of the anisotropy of the stress tensor, dynamical instability sets in at lower rotation in stellar systems. This difference is reflected also in the new stability criterion. The new stability parameter alpha equals (T(sub J))/(absolute value of W) is formulated first for uniformly rotating systems and is based on the angular momentum content rather than on the energy content of a system. (T(sub J) is defined as ((L)(Omega(sub J)))/2; L is the total angular momentum; Omega(sub J) is the Jeans frequency introduced by self-gravity; and W is the total gravitational potential energy.) For stability of stellar systems alpha less than or equal to 0.254-0.258 while alpha less than or equal to 0.341-0.354 for stability of gaseous systems. For uniform rotation, one can write alpha = ((ft)/2)(exp 1/2), where t is defined as T/(absolute value of W), T is the total kinetic energy due to rotation, and f is a function characteristic of the topology/connectedness and the geometric shape of a system. Equivalently, alpha equals t/(chi), where chi is defined as Omega/Omega(sub J) and Omega is the rotation frequency. Using these forms, alpha can be extended to and calculated for a variety of differentially rotating, gaseous and stellar, axisymmetric disk and spheroidal models whose equilibrium structures and stability characteristics are known. In this paper, we also estimate alpha for gaseous torodial models and for stellar disk systems embedded in an inert or responsive 'halo.' We find that the new stability criterion holds equally well for all these previously published axisymmetric models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 2; p. 551-562
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  • 31
    Publication Date: 2011-08-24
    Description: Numerical simulations have been used to study high velocity two-body impacts. In this paper, a two-dimensional Largrangian finite difference hydro-code and a three-dimensional smooth particle hydro-code (SPH) are described and initial results reported. These codes can be, and have been, used to make specific predictions about particular objects in our solar system. But more significantly, they allow us to explore a broad range of collisional events. Certain parameters (size, time) can be studied only over a very restricted range within the laboratory; other parameters (initial spin, low gravity, exotic structure or composition) are difficult to study at all experimentally. The outcomes of numerical simulations lead to a more general and accurate understanding of impacts in their many forms.
    Keywords: ASTROPHYSICS
    Type: Planetary and Space Science (ISSN 0032-0633); 42; 12; p. 1053-1066
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  • 32
    Publication Date: 2011-08-24
    Description: We present laboratory measurements of the radiation yields of complex organic solids produced from N2/CH4 gas mixtures containing 10 or 0.1% CH4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton tholin. High-energy magnetospheric electrons responsible for tholin production represents a class distinct from the plasma electrons considered in models of Titan's aiglow. Electrons with E greater than 20 keV provide an energy flux approximately 1 x 10(exp -2) erg/cm/sec, implying from our measured tholin yields a mass flux of 0.5 to 4.0 x 10(exp -14) g/sq cm/sec of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m). This figure is in agreement with required mass fluxes computed from recent radiative transfer and sedimentation models. If, however, theses results, derived from experiments at approximately 2 mb, are applied to lower pressure levels toward peak auroral electron energy deposition and scaled with pressure as the gas-phase organic yields, the derived tholin mass flux is at least an order of magnitude less. We attrribute this difference to the fact that tholin synthesis occurs well below the level of maximum electron energy depositon and to possible contributions to tholis from UV-derived C2-hydrocarbons. We conclude that Tita tholin, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze-a result consistent with the fact that the optical properties of Titan tholin, among all proposed material, are best at reproducing Titan's geometric albedo spectrum from near UV to mid-IR in light-scattering models.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 112; 2; p. 376-381
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  • 33
    Publication Date: 2011-08-24
    Description: We use a two-dimensional piecewise parabolic method (PPM) code to simulate numerically the hydrodynamics of a radiation-driven stellar wind from a rapidly rotating Be star. The results generally confirm predictions of the semianalytic 'wind-compressed disk' model recently proposed by Bjorkman and Cassinelli to explain the circumstellar disks inferred observationally to exist around such rapidly rotating stars. However, this numerical simulation is able to incorporate several important effects not accounted for in the simple model, including a dynamical treatment of the outward radiative driving and gas pressure, as well as a rotationally distorted, oblate stellar surface. This enables us to model quantitatively the compressed wind and shock that forms the equatorial disk. The simulation results thus do differ in several important details from the simple method, showing, for example, an inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. There is also no evidence for the predicted detachment of the disk that arises in the fixed outflow picture. The peak equatorward velocity in the dynamical models is furthermore about a factor of 2 smaller than the lytically predicted value of approximately 50% of the stellar equatorial rotation speed. As a result, the dynamical disks are somewhat weaker than predicted, with a wider opening angle, lower disk/pole density ratio, and smaller shock velocity jump. The principal cause of these latter differences appears to be an artificially strong equatorward drift of the subsonic outflow in the original analytic model. Much better agreement with the dynamical results can be obtained, however, from a slightly modified, analytic wind-compression model with a more detailed specification of the fixed wind outflow and a lower boundary set to the sonic radius along a rotationally oblate stellar surface. Hence, despite these detailed differences, the general predicted effect of disk formation by wind compression toward the equator is substantially confirmed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 424; 2; p. 887-904
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  • 34
    Publication Date: 2011-08-24
    Description: We construct equilibrium sequences of rotating neutron stars in general relativity. We compare results for 14 nuclear matter equations of state. We determine a number of important physical parameters for such stars, including the maximum mass and maximum spin rate. The stability of the configurations to quasi-radial perturbations is assessed. We employ a numerical scheme particularly well suited to handle rapid rotation and large departures from spherical symmetry. We provide an extensive tabulation of models for future reference. Two classes of evolutionary sequences of fixed baryon rest mass and entropy are explored: normal sequences, which behave very much like Newtonian sequences, and supramassive sequences, which exist for neutron stars solely because of general relativistic effects. Adiabatic dissipation of energy and angular momentum causes a star to evolve in quasi-stationary fashion along an evolutionary sequence. Supramassive sequences have masses exceeding the maximum mass of a nonrotating neutron star. A supramassive star evolves toward eventual catastrophic collapse to a black hole. Prior to collapse, the star actually spins up as it loses angular momentum, an effect that may provide an observable precursor to gravitational collapse to a black hole.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 424; 2; p. 823-845
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  • 35
    Publication Date: 2011-08-24
    Description: We have analyzed 56 Einstein Observatory Imaging Proportional Counter (IPC) observations within +/- 3 deg of the Galactic plane in order to determine the low-latitude soft X-ray background flux in the 0.56-1.73 keV band. Any detected X-ray point source which fell within our regions of study was removed from the image, enabling us to present maps of the background flux as a function of Galactic latitude along 18 meridians. These maps reveal considerable structure to the background in the Galactic plane on an angular scale of approximately 1 deg. Our results are compared with those of an earlier study of the 1 keV X-ray background along l = 25 deg by Kahn & Caillault. The double-peaked structure they found is not discernible in our results, possibly because of the presence of solar backscattered flux in their data. A model which takes into account contributions to the background by extragalactic and stellar sources, the distribution of both atomic and molecular absorbing material with the Galaxy, the energy dependence of the cross section for absorption of X-rays, and the energy dependence of the detector has been constructed and fitted to these new data to derive constraints on the scale height, temperature, and volume emissivity of the unaccounted-for X-ray-emitting material. The results of this model along l = 25 deg are roughly similar to those of the model of Kahn & Caillault along the same meridian.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 424; 2; p. 671-687
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  • 36
    Publication Date: 2011-08-24
    Description: Microlensing by compact objects with masses between approximately 0.001 solar masses and approximately 300 solar masses will amplify the continuum emission of a quasar, without significantly changing its line emission. Thus, compact objects with masses associated with stars, subdwarfs, and stellar remnants will reduce the apparent equivalent widths of quasar emission lines. It is possible to detect this population of lenses by searching for an increase in the number of small equivalent width quasars with redshift. This increase was looked for, but not found, in quasar samples taken from the Einstein Medium Sensitivity Survey and the Steidel & Sargent absorption-line studies. Thus, Omega(sub c), the cosmological density of compact objects relative to the critical density, is less than or approximately equal to 0.1 in the mass range 0.01 solar masses-20 solar masses (for Omega less than 0.6). For any value of Omega, Omega(sub c) less than or approximately equal to 0.2 in the larger mass range 0.001 solar masses-60 solar masses, and Omega(sub c) less than 1 for 0.001 solar masses-300 solar masses. Subdwarfs, stellar objects, or their remnants (e.g., MACHOS) cannot close the universe.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 424; 2; p. 550-568
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  • 37
    Publication Date: 2011-08-24
    Description: The Ulysses spacecraft has gathered data from within flows from the Sun's southern polar coronal hole, the first in situ measurement of this region. We present a brief analysis of the heliospheric magnetic field data from this region, using a fractal method. As is the case near the ecliptic, estimated spectral exponents are near 5/3 on spacecraft scales of seconds to minutes. On longer time scales, however, there appears to be a significantly different population in polar flows, which is similar to that found by the Helios spacecraft in fast solar wind flows at 0.3 AU.
    Keywords: ASTROPHYSICS
    Type: Annales Geophysicae (ISSN 0992-7689); 13; 1; p. 105-107
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  • 38
    Publication Date: 2011-08-24
    Description: This paper reports on a Workshop on Supernova Remnants and the Physics of Strong Shock Waves hosted by North Carolina State University at Raleigh, North Carolina, September 16-18, 1993. The workshop brought together observers, shock theorists, cosmic-ray specialists, and simulators to address the role supernova remnants can play in furthering our understanding of the complex plasma physics associated with collisionless shocks and particle acceleration. Over fifty scientists presented papers on various aspects of supernova remnants. In lieu of a proceedings volume, we present here a synopsis of the workshop, in the form of brief summaries of each workshop session.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 106; 701; p. 780-797
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  • 39
    Publication Date: 2011-08-24
    Description: We present the first two-dimensional X-ray temperature map of the rich cluster of galaxies Abell 754. We also present an X-ray surface brightness map with improved spatial resolution and sensitivity compared with previous maps. Both the temperature map and the surface brightness map show that A754 is in the throes of a violent merger; it is probably far from hydrostatic equilibrium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 443; 1; p. L9-L12
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  • 40
    Publication Date: 2011-08-24
    Description: We have investigated the properties of the OMC-2 and OMC-3 cores in the Orion giant molecular cloud using high spatial spectral resolution observations of several transitions of the (13)CO, C(18)O, C(S-32) and C(S-34) molecules taken with the SEST telescope. The OMC-2 core consists of one clump (22 solar mass) with a radius of 0.11 pc surrounded by a cluster of 11 discrete infrared sources. The H2 column density and volume density in the center of this clump are 2 x 10(exp 22)/sq cm and 9 x 10(exp 5)/cu cm respectively. From a comparison between physical parameters derived from C(18)O and C(S-32) observations we conclude that the molecular envelope around the core has been completely removed by these sources and that only the very dense gas is left. OMC-3 shows a more complex elongated structure in C(18)O and CS than OMC-2. The C(S-32) and C(S-34) maps show that the denser region can be separated into at least sub-cores of roughly equal sizes (radius approximately equals 0.13 pc), with n(H2) = 6 x 10(exp 5)/cu cm, and a mass of 10 solar mass (from C(S-32)). The very different masses obtained for the central core from C(18)O and C(S-32) (55 and 12 solar mass respectively) indicate that a massive envelope is still present around the very dense sub-cores. We report the first detection of several molecular outflows in OMC-3. The presence of an IRAS source and the first detection of these outflows confirm that star formation is going on in OMC-3. Based on the different physical properties of these regions compared with OMC-1, OMC-2 appears to be in an intermediate evolutionary stage between OMC-1 and OMC-3.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 294; 3; p. 835-854
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  • 41
    Publication Date: 2011-08-24
    Description: We show that the mid- to far-infrared fluxes of 'flat spectrum' T Tauri stars can be explained by radiative equilibrium emission from infalling dusty envelopes. Infall eliminates the need for accretion disks with non-standard temperature distributions. The simplicity and power of this explanantion indicates that models employing 'active' disks, in which the temperature distribution is a parameterized power law, should be invoked with caution. Infall also naturally explains the scattered light nebulae detected around many flat spectrum sources. To match the observed spectra, material must fall onto a disk rather than the central star, as expected for collapse of a rotating molecular cloud. It may be necessary to invoke cavities in the envelopes to explain the strength of optical and near-infrared emission; these cavities could be produced by the powerful bipolar outflows commonly observed from young stars. If viewed along the cavity, a source may be lightly extincted at visual wavelengths, while still accreting substantial amounts of material from the envelope. Infall may also be needed to explain the infrared-bright companions of many optical T Tauri stars. This picture suggests that many of the flat spectrum sources are 'protostars'-young stellar objects surrounded by dust infalling envelopes of substantial mass.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 434; 1; p. 330-340
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  • 42
    Publication Date: 2011-08-24
    Description: We formulate and test a series of algorithms for echo mapping the emission-line regions near active galactic nuclei from measurements of correlated variability in their line and continuum light curves. The linear regularization method (LRM) employs a direct inversion of evenly spaced light-curve data, with a regularization parameter that can be used to control the trade-off between noise and resolution. Matrix formulas express the formal solution as well as its variance and covariance in terms of uncertainties in the measurements. Unlike the maximum-entropy method (MEM), LRM applies to kernels with both positive and negative values, but the results are somewhat limited by ringing effects. A positivity constraint proves effective in controlling the ringing. MEM combines regularization and positivity in a natural way, but similar results are also found using positivity constraints with nonentropic regularization functions. Direct inversions of unevenly sampled light curves require interpolating the noisy data. In this case better results are found by solving for both the continuum light curve and kernel function in a simultaneous fit to the data. Our conclusion is that while echo mapping currently gives ambiguous results, the algorithms are not the limiting factor. Progress depends on efforts to increase the accuracy and completeness of sampling of the observed light curves.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 106; 704; p. 1091-1103
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  • 43
    Publication Date: 2011-08-24
    Description: We present a time-dependent spot modeling analysis of 15 consecutive years of V-band photometry of the long-period (P(sub orb) = 28.6 days) RS CVn binary HR 7275. This baseline in time is one of the longest, uninterrupted intervals a spotted star has been observed. The spot modeling analysis yields a total of 20 different spots throughout the time span of our observations. The distribution of the observed spot migration rates is consistent with solar-type differential rotation and suggests a lower limit of the differential-rotation coefficient of 0.022 +/-0.004. The observed, maximum lifetime of a single spot (or spot group) is 4.5 years, the minimum lifetime is approximately one year, but an average spot lives for 2.2 years. If we assume that the mechanical shear by differential rotation sets the upper limit to the spot lifetime, the observed maximum lifetime in turn sets an upper limit to the differential-rotation coefficient, namely 0.04 +/- 0.01. This would be differential rotation just 5 to 8 times less than the solar value and one of the strongest among active binaries. We found no conclusive evidence for the existence of a periodic phenomenon that could be attributed to a stellar magnetic cycle.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 282; 2; p. 535-546
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  • 44
    Publication Date: 2011-08-24
    Description: New optical and near-infrared (NIR) K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by circumnuclear zone of enhanced star formation, the morphological features in the 2.2 micrometer image correspond to a newly discovered kiloparsec-size stellar bar and a pair of leading arms situated inside an ovally shaped region. Analysis of the K image confirms its symmetry: only a very small percentage of the flux, some 5%, is emitted in antisymmetric structures. This indicates that the overall morphology observed in the NIR is dominated by a global density wave. Making a first-order correction for the presence of localized dust extinction in the K light using the I-K image, we find that the observed leading arm morphology is not caused or enhanced but in fact slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Our dynamical conclusions are supported with an evolutionary stellar population model reproducing the observed optical and NIR colors in a number of star-forming zones. We argue that the observed morphology is compatible with the presence of a pair of inner Lindblad resonances in the region and show this explicitly in an accompanying paper by detailed numerical modeling. The phenomena observed in NGC 4321 may provide insight into physical process leading to central activity in galaxies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 443; 2; p. L73-L76
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  • 45
    Publication Date: 2011-08-24
    Description: We report on the evolution in the X-ray spectrum of the transient X-ray pulsar EXO 2030+375 during part of an outburst in 1985 May-August. The overall continuum spectral shape is similar to that of other accreting pulsars and can be represented by a power-law spectrum modified at low energies by significant absorption and at high-energies either by an exponential cut-off or by the effects of cyclotron scattering. As the luminosity decreased by a factor of approximately 100, the X-ray spectrum became harder with the photon index decreasing from 1.83 +/- 0.01 to 1.29 +/- 0.01. In addition, the high-energy cutoff decreased from 20 to 10 keV during the same interval. If the cutoff is interpreted in terms of cyclotron resonance scattering, then this implies a magnetic field strength that decreased from 2.6 x 10(exp 12) G to 1.3 x 10(exp 12) G. This variation implies that the cutoff energy does not provide a reliable measure of the surface magnetic field strength in this system.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 13; 12; p. (12)-355-(12)360
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  • 46
    Publication Date: 2011-08-24
    Description: In the past three years, a new era of study of globular clusters has begun with multiwavelength observations from the current generation of astronomical telescopes in space. We review the recent results obtained from our studies of compact binaries and x-ray sources in globulars with ROSAT and Hubble Space Telescope (HST) as well as our balloon-borne hard x-ray telescope EXITE (Energetic X-ray Imaging Telescope Experiment) and ground-based observations (CTIO). With ROSAT, we have obtained the most sensitive high resolution soft x-ray images of clusters which show multiple low luminosity sources in cluster cores that are likely indicative of the long-sought population of cataclysmic variables (CVs). We have obtained deep H-alpha images of two clusters with HST and found CV candiates for 3 of the ROSAT sources in the core of NGC 6397. New CTIO imaging and spectroscopy of two 'dim source' fields in omega-Cen are also described. With EXITE we carried out the first hard x-ray imaging observations of the cluster 47 Tuc; such studies can ultimately limit the populations of millisecond pulsars and pulsar emission mechanisms. A long ROSAT exposure on 47 Tuc also shows probable cluster diffuse emission, possibly due to hot gas from ablating millisecond pulsars. Multiwavelength studies of globular clusters may provide new constraints on problems as diverse as the origin of CVs and low mass X-ray binaries (LMXBs) and the origin of hot gas in globulars.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 13; 12; p. (12)597-(12)609
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  • 47
    Publication Date: 2011-08-24
    Description: We present six ROSAT Position-Sensitive Proportional Counter (PSPC) observations of Seyfert 1 galaxies chosen to have low Galactic line-of-sight absorption (N(sub H) approx. = 10(exp 20)/sq cm). As expected, it is found that all of these sources possess significantly steeper spectra below approximately 1 keV, than that observed at higher X-ray energies. In addition we find evidence for soft X-ray spectral features, which are best parameterized as line emission at approximately 0.63 keV in NGC7469 and approximately 0.75 keV in ESO198-G24. We examine these results in the light of the accuracy of the PSPC spectral calibration. We suggest that Seyfert galaxies are a class of object whose spectra are similar to the diffuse X-ray background in the soft X-ray band. We suggest these data provide the first direct evidence that the sources which dominate the background differ as a function of energy.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 13; 12; p. (12)387-(12)390
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  • 48
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: A more appropriate title for this talk would have been 'Measurements of Large Scale Structure from X-ray Background Fluctuations'. While it has long been recognized that the X-ray Background (XRB) is primarily of a cosmological origin (with z less than a few), it has recently become apparent that surface brightness fluctuations in the surveys of the XRB can be used to trace the distribution of matter in much the same way as complete catalogs of individual objects. The distance which is probed is related to the angular resolution of the detector; for the HEAO-1 A2 experiment, which provides the best all-sky data base for the XRB in the 2-20 keV band, the effective depth is a few 100 Mpc.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 13; 12; p. (12)231-(12)240
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  • 49
    Publication Date: 2011-08-24
    Description: We have obtained a 45 sec resolution, Nyquist-sampled map in CO J = 1-0 covering approximately a 3 deg x 3 deg region of the giant molecular cloud Monoceros R2. The map consists of 167,000 spectra observed with the 15 element focal-plane array system on the FCRAO 14 m telescope. The data reveal that the large-scale structure of Mon R2 is dominated by a is approximately 30 pc diameter largely hemispherical shell containing approximately 4 x 10(exp 4) solar mass of molecular material and expanding at approximately 3-4 km s(exp -1) with symmetric axis roughly along the line of sight. The dynamical timescale of the shell is estimated to be approximately 4 x 10(exp 6) yr, which is consistent with the age of main-sequence stars powering the clusters of reflection nebulea in this region. There is no evidence for a redshifted shell on the far side of the interior 'bubble,' which is largely devoid of molecular material. Distortions of the shell are obvious, suggesting inhomogeneity of the cloud and possible presence of a magnetic field prior to its formation. Dense clumps in Mon R2, including the main core and the GGD 12-15 core, appear to be condensations located on the large shell. The reflection nebulea with their illuminating stars as well as embedded IRAS sources suggest that triggered star formation has taken place over a large part of the Mon R2 shell.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal, Part 1 (ISSN 0004-637X); 430; 1; p. 252-255
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  • 50
    Publication Date: 2011-08-24
    Description: The rudimentary measurements of the instruments on Pioneer Venus over a 14-year span have provided a strong framework for the interpretation of the observations with a more sophisticated instrument package during the Galileo Venus flyby in February 1990. In some cases the Galileo results provided independent confirmation of earlier inferences. In other cases completely new data were obtained. Nevertheless, because of limitations of the instruments and the trajectory and thermal restraints some outstanding questions were not addressed. Much has been learned but there is still much more to do.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 15; 4; p. (4)5-(4)16
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  • 51
    Publication Date: 2011-08-24
    Description: Stellar models with masses ranging from 05.5 to 1.3 solar mass were constructed for comparison with young cluster observations of Li and of rotation velocities. The amount of Li depletion in cool stars is sensitive to the amount of overshoot at the base of the surface convection zone, and the exact metallicity of the models. Even when this is taken into account, the Li observations are a severe constraint for the models and rule out standard models and pure diffusion models. Stellar models which include diffusion and rotational mixing in the radiative regions of stars are able to simultaneously match the Li abundances observed in the Pleiades, the UMa Group, The Hyades, Praesepe, NGC 752, and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to simultaneously explain the presence of the rapidly rotating late G and K stars in the Pleiades and the absence of rapidly rotating late F and early G stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 2; p. 876-885
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  • 52
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: We model the evolution of the He I lines, especially He I 1.083 micrometers and He I 2.058 micrometers, from the envelope of SN 1987A during the first 3 yr after the explosion. The He I metastable 2(sup 3)S and 2(sup 1)S states are populated by fast electrons resulting from energy deposition by gamma rays and depopulated by electron collisions and two-photon decay. The 2.058 micrometer line is excited mainly by nonthermal electrons at all times. The 1.083 micrometer line is excited mainly by thermal electron impact for t less than or approximately = 450 days and by recombination afterward. We can fit the observed evolution of He I 2.058 micrometers with a model in which approximately 3 solar mass of nearly pure helium and approximately 11 solar mass of hydrogen mixed with primordial helium occupy the volume emitting most of the light (v less than or approximately = 2500 km/s). For t less than or approximately = 450 days, trace carbon in the inner part of the helium suppresses the He I 2.058 micrometer line by photoabsorbing He I lambda 584 and depopulating the 2(sup 1) P(sup 0) state. The observed 1.08 micrometer line is a blend of He I 1.083 micrometers and (S I) 1.082 micrometers. We estimate that He I 1.083 micrometers contributes 40% of the observed flux for t less than or approximately = 550 days and approximately 60% thereafter.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 2; p. 821-829
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  • 53
    Publication Date: 2011-08-24
    Description: Arp (1994) has presented redshift data for the Local Group of galaxies and for the next major group, whose largest galaxies are M31 and M81, respectively. He observed that the relative redshifts of all 22 of their companions were positive and claimed that the likelihood that this would occur is 1 in 4 x 10(exp 6). We show using the classical combinatoric paradigm of ordered samples (without replacement) that the correct probability for the dominant member of each cluster to possess the lowest observed redshift is approximately 8%.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 2; p. 505-506
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  • 54
    Publication Date: 2011-08-24
    Description: Measurements of greater than or = 1 MeV electrons in Neptune's magnetosphere from L approximately = 4 to 14 are well represented by solutions of a model radial diffusion equation that includes losses due to absorption by the satellite Proteus. The model provides estimates of the radial diffusion coefficient, which is 5 x 10(exp -8) (L/5)(exp 8)/s, and the outer boundary energy spectrum, which is an exponential with an e-folding energy of 0.1 MeV. The diffusion coefficient is consistent with theoretical estimates based on the assumption that diffusion is driven by atmospheric neutral winds.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 21; 15; p. 1579-1582
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  • 55
    Publication Date: 2011-08-24
    Description: We have used the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) to search for chromospheric emission in the ultraviolet C II 1335 A lines of eight A-type stars. The B-V colors of these stars range from 0.12 to 0.22. We have detected emission in the spectrum of the A7 V star Altair, but in no other star. We present tentative evidence that the emission from Altair is blueshifted by approximately 60 km per sec or more with respect to the photospheric spectrum of this star. We interpret this as expansion and suggest that the base of the A star winds that we have proposed in earlier work may lie within the chromosphere. Together with International Ultraviolet Explorer (IUE) spectra, our observations indicate that in ordinary main-sequence stars the onset of convection zones capable of supporting detectable chromospheres occurs in the close vicinity of B-V = 0.22.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 428; 1; p. 319-323
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  • 56
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: We compare the recent temperature estimate for Geminga with neutron star cooling models. Because of its age (approximately 3.4 x 10(exp 5) yr), Geminga is in the photon cooling era. We show that its surface temperature (approximately 5.2 x 10(exp 5) K) can be understood by both types of neutrino cooling scenarios, i.e., slow neutrino cooling by the modified Urca process or fast neutrino cooling by the direct Urca process or by some exotic matter, and thus does not allow us to discriminate between these two competing schemes. However, for both types of scenarios, agreement with the observed temperature can only be obtained if baryon pairing is present in most, if not all, of the core of the star. Within the slow neutrino cooling scenario, early neutrino cooling is not sufficient to explain the observed low temperature, and extensive pairing in the core is necessary to reduce the specific heat and increase the cooling rate in the present photon cooling era. Within all the fast neutrino cooling scenarios, pairing is necessary throughout the whole core to control the enormous early neutrino emission which, without pairing suppression, would result in a surface temperature at the present time much lower than observed. We also comment on the recent temperature estimates for PSR 0656+14 and PSR 1055-52, which pertain to the same photon cooling era. If one assumes that all neutron stars undergo fast neutrino cooling, then these two objects also provide evidence for extensive baryon pairing in their core; but observational uncertainties also permit a more conservative interpretation, with slow neutrino emission and no pairing at all. We argue though that observational evidence for the slow neutrino cooling model (the 'standard' model) is in fact very dim and that the interpretation of the surface temperature of all neutron stars could be done with a reasonable theoretical a priori within the fast neutrino cooling scenarios only. In this case, Geminga, PSR 0656+14, and PSR 1055-52 all show evidence of baryon pairing down to their very centers.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 428; 1; p. 250-260
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  • 57
    Publication Date: 2011-08-24
    Description: We have observed the C91 alpha radio recombination line toward the Orion H II region. This narrow (approximately 3-5 km per sec full width at half maximum (FWHM)) line is spatially very extended (approximately 8 arcmin or 1 pc). These charateristics compare well with the observed characteristics of the C II fine structure line at 158 microns. Thus, the C91 alpha line originates in the predominantly neutral photodissociation regions separating the H II region from the molecular cloud. We have developed theoretical models for the C II radio recombination lines from photodissociation regions. The results show that the I(C91 alpha)/I(C158) intensity ratio is a sensitive function of the temperature and density of the emitting gas. We have also extended theoretical models for photodissociation regions to include the C II recombination lines. Comparison with these models show that, in the central portion of the Orion region, the C91 alpha line originates in dense (10(exp 6) per cu cm), warm (500-1000 K) gas. Even at large projected distances (approximately 1 pc), the inferred density is still high (10(exp 5) per cu cm) and implies extremely high thermal pressures. As in the case of the (C II) 158 microns line, the large extent of the C91 alpha line shows that (FUV) photons can penetrate to large distances from the illuminating source. The decline of the intensity of the incident radiation field with distance from Theta(sup 1) C seems to be dominated by geometrical dilution, rather than dust extinction. Finally, we have used our models to calculate the intensity of the 9850 A recombination line of C II. The physical conditions inferred from this line are in good agreement with those determined from the radio recombination and the far-infrared fine-structure lines. We show that the ratio of the 9850 A to the C91 alpha lines is a very good probe of very high density clumps.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 428; 1; p. 209-218
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  • 58
    Publication Date: 2011-08-24
    Description: We carefully examine the techniques used to infer temperatures of stellar coronal plasmas from the count rates of several broadband instruments in the X-ray and extreme-ultraviolet spectral ranges. In particular, we determine to what extent temperatures can be constrained and the corresponding uncertainties in the luminosities and emission measures lowered by fitting simultaneously count rates from the Einstein imaging proportional counter (IPC), the ROSAT Position Sensitive Proportional Counter (PSPC), the ROSAT Wide Field Camera (WFC) (both filters), and the EXOSAT Low Energy Telescope (LET) with the 3-Lex filter. We use published plasma emissivities with solar photospheric abundances. Since it has been found that single-temperature plasmas do not fit IPC data well, we assume a two-temperature plasma model. We find that, even with count rates from all of the above filters and overly optimistic error estimates, it is still not possible to determine a unique two-temperature solution. However, since the use of count rates from many filters can reduce substantially the number of possible solutions, temperature solutions determined by other means can be tested. We carry out such an analysis on a set of 18 nearby late-type stars to determine possible two-temperature solutions using multifilter photometry, and we compare these results with the temperature solutions derived by Schmitt et al. (1990) using IPC spectral data. In general, the two-temperature fits derived from the IPC spectral data are inconsistent with our results, with our data implying that, for many stars, the two temperatures derived by the IPC may be too low by about a factor of 2. The EXOSAT transmission grating Spectrometer (TGS) spectra of capella and sigma(exp 2) CrB support this conclusion. For Procyon and 70 Oph, though, the presence of a temperature component cooler than a million degress (not detected by the IPC) is deduced. While our analysis suggests the existence of more than one temperature in the coronae of late-type stars, in many instances our WFC data appear to be inconsistent with the presence of significant emission measure over a broad temperature distribution. This, together with the success of two-temperature plasmas in fitting IPC and TGS data, implies that for many stars, the coronal emission measure distribution may in fact be dominated by two distinct temperature regimes.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 438; 1; p. 350-363
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  • 59
    Publication Date: 2011-08-24
    Description: We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 438; 1; p. 364-375
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  • 60
    Publication Date: 2011-08-24
    Description: Four spectral emission features of the N(sub KK) = 4(sub 04) -3(sub 13) rotational transition of methylene (CH2) have been detected at signal levels 5-7 sigma above noise toward the hot core of the Orion-KL nebula and the molecular cloud in proximity to the continuum source W51 M. Specifically, in both sources we have resolved the F = 6-5, 5-4, and 4-3 hyperfine transitions of the J = 5-4 fine-structure levels and detected the blended hyperfine structure of the J = 4-3 fine structure levels. At the J = 3-2 fine-structure levels, we have observed new transitions of NS, a known interstellar molecule, which severely contaminates the search for CH2 hyperfine transitions. These new sensitive observations finally confirm the existence of interstellar CH2 which was tentatively reported by us some years ago.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 438; 1; p. 259-264
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  • 61
    Publication Date: 2011-08-24
    Description: We reexamine the model proposed by Gnedin & Ostriker (1992) in which Jeans mass black holes (M(sub BH) approximately = 10(exp 6) solar mass) form shortly after decoupling. There is no nonbaryonic dark matter in this model, but we examine the possibility that Omega(sub b) is considerably larger than given by normal nucleosynthesis. Here we allow for the fact that much of the high baryon-to-photon ratio material will collapse leaving the universe of remaining material with light-element abundances more in accord with the residual baryonic density (approximately = 10(exp -2)) than with Omega(sub 0) and the initial baryonic density (approximately = 10(exp -1)). We find that no reasonable model can be made with random-phase density fluctuations, if the power on scales smaller than 10(exp 6) solar mass is as large as expected. However, phase-correlated models of the type that might occur in connection with topological singularities can be made with Omega(sub b) h(exp 2) = 0.013 +/- 0.001, 0.15 approximately less than Omega(sub 0) approximately less than 0.4, which are either flat (Omega(sub lambda) = 1 - Omega(sub 0)) or open (Omega(sub lambda) = 0) and which satisfy all the observational constraints which we apply, including the large baryon-to-total mass ratio found in the X-ray clusters. The remnant baryon density is thus close to that obtained in the standard picture (Omega(sub b) h(exp 2) = 0.0125 +/- 0.0025; Walker et al. 1991). The spectral index implied for fluctuations in the baryonic isocurvature scenario, -1 less than m less than 0, is in the range expected by other arguments based on large-scale structure and microwave fluctuation constraints. The dark matter in this picture is in the form of massive black holes. Accretion onto them at early epochs releases high-energy photons which significantly heat and reionize the universe. But photodissociation does not materially change light-element abundances. A typical model gives bar-y approximately = 1 x 10(exp -5), n(sub e)/n(sub H)(z = 30) approximately = 0.1, and a diffuse gamma-ray background at 100 keV near the Cosmic Background Explorer Satellite (COBE) limit of the order of 10% of that observed which originates from high-redshift quasars. Reionization in this model occurs at redshift 600 and reaches (H II/H(sub tot) approximately = 0.1-0.2.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 438; 1; p. 40-48
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  • 62
    Publication Date: 2011-08-24
    Description: We present non-local thermodynamic equilibrium (non-LTE) synthetic spectra for the Type Ia supernovae SN 1992A and SN 1981B, near maximum light. At this epoch both supernovae were observed from the UV through the optical. This wide spectral coverage is essential for determining the density structure of a SN Ia. Our fits are in good agreement with observation and provide some insight as to the differences between these supernovae. We also discuss the application of the expanding photosphere method to SNe Ia which gives a distance that is independent of those based on the decay of Ni-56 and Cepheid variable stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L33-L36
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  • 63
    Publication Date: 2011-08-24
    Description: The Cosmic Background Explorer Satellite (COBE) detection of microwave anisotropies provides the best way of fixing the amplitude of cosmological fluctuations on the largest scales. This normalization is usually given for an n = 1 spectrum, including only the anisotropy caused by the Sachs-Wolfe effect. This is certainly not a good approximation for a model containing any reasonable amount of baryonic matter. In fact, even tilted Sachs-Wolfe spectra are not a good fit to models like cold dark matter (CDM). Here, we normalize standard CDM (sCDM) to the two-year COBE data and quote the best amplitude in terms of the conventionally used measures of power. We also give normalizations for some specific variants of this standard model, and we indicate how the normalization depends on the assumed values on n, Omega(sub B) and H(sub 0). For sCDM we find the mean value of Q = 19.9 +/- 1.5 micro-K, corresponding to sigma(sub 8) = 1.34 +/- 0.10, with the normalization at large scales being B = (8.16 +/- 1.04) x 10(exp 5)(Mpc/h)(exp 4), and other numbers given in the table. The measured rms temperature fluctuation smoothed on 10 deg is a little low relative to this normalization. This is mainly due to the low quadrupole in the data: when the quadrupole is removed, the measured value of sigma(10 deg) is quite consistent with the best-fitting the mean value of Q. The use of the mean value of Q should be preferred over sigma(10 deg), when its value can be determined for a particular theory, since it makes full use of the data.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L9-L12
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  • 64
    Publication Date: 2011-08-24
    Description: We have simulated full-sky maps of the cosmic microwave background (CMB) anisotropy expected from cold dark matter (CDM) models at 0.5 deg and 1.0 deg angular resolution. Statistical properties of the maps are presented as a function of sky coverage, angular resolution, and instrument noise, and the implications of these results for observability of the Doppler peak are discussed. The rms fluctuations in a map are not a particularly robust probe of the existence of a Doppler peak; however, a full correlation analysis can provide reasonable sensitivity. We find that sensitivity to the Doppler peak depends primarily on the fraction of sky covered, and only secondarily on the angular resolution and noise level. Color plates of the simulated maps are presented to illustrate the anisotropies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L1-L4
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  • 65
    Publication Date: 2011-08-24
    Description: We present measurements of the shape of the stellar line-of-sight velocity distribution out to two effective radii along the major axes of the four elliptical galaxies NGC 2434, 2663, 3706, and 5018. The velocity dispersion profiles are flat or decline gently with radius. We compare the data to the predictions of f = f(E, L(sub z)) axisymmetric models with and without dark matter. Strong tangential anisotropy is ruled out at large radii. We conclude from our measurements that massive dark halos must be present in three of the four galaxies, while for the fourth galaxy (NGC 2663) the case is inconclusive.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L25-L28
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  • 66
    Publication Date: 2011-08-24
    Description: High-contrast peaks in the cosmic microwave background (CMB) anisotropy can appear as unresolved sources to observers. We fit simluated CMB maps generated with a cold dark matter model to a set of unresolved features at instrumental resolution 0.5 deg-1.5 deg to derive the integral number density per steradian n (greater than absolute value of T) of features brighter than threshold temperature absolute value of T and compare the results to recent experiments. A typical medium-scale experiment observing 0.001 sr at 0.5 deg resolution would expect to observe one feature brighter than 85 micro-K after convolution with the beam profile, with less than 5% probability to observe a source brighter than 150 micro-K. Increasing the power-law index of primordial density perturbations n from 1 to 1.5 raises these temperature limits absolute value of T by a factor of 2. The MSAM features are in agreement with standard cold dark matter models and are not necessarily evidence for processes beyond the standard model.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 441; 1; p. L5-L7
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  • 67
    Publication Date: 2011-08-24
    Description: Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; A3; p. 3455-3462
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  • 68
    Publication Date: 2011-08-24
    Description: I review polarimetric observations of presumably single, hot luminous stars. The stellar types discussed are OB stars. B(e) supergiants, Luminous Blue Variables (LBV), Wolf-Rayet (W-R) stars, and type II supernovae (SN). It is shown that variable, intrinsic polarization is a common phenomenon in that part of the Hertzsprung-Russell (HR) diagram which these stars occupy. However, much observational work remains to be done before we can answer the most basic, statistical questions about the polarimetric properties of different groups of hot, luminous stars. Insight into the diagnostic power of polarization observations has been gained, but cannot be exploited without detailed models. Thus, while polarimetric observations do tell us that the mass-loss processes of all types of massive stars are time-dependent and anisotropic, the significance that this might have for the accuracy of their stellar parameters and evolutionary paths remains elusive.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 221; 1-2; p. 347-356
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  • 69
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: Data from the Wisconsin spectropolarimetric monitoring program provide information about the polarimetric variability of numerous types of stars. Examining the data that exist thus far, we find, not surprisingly, that many OB stars, especially OB supergiants and Be stars, are polarimetric variables. Be stars show the most extreme variable as a class. Results from the Be star pi Aqr are presented which illustrate the dramatic polarimetric variability that such stars can exhibit. Contemporaneous IUE data show that the development of low velocity discrete absorption components in the UV wind lines may be associated with the onset of polarimetric outbursts. These preliminary findings add further support to the picture that winds from hot stars are non-homogeneous and variable.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 221; 1-2; p. 335-345
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  • 70
    Publication Date: 2011-08-24
    Description: The original temporal analysis of a 12 night spectral timeseries of Wolf-Rayet (WR)-134 has been found to be flawed and a re-analysis shows that the line profile variations are indeed periodic. When combined with a 4 night timeseries taken 45 days earlier, a period near 2.27 d is found in periodograms of the He II lambda 5412 line centroid, rms line width, and line skew variations. When the emission line residuals are ordered as a function of phase, a sinuous feature appears to 'snake' about the line center with an amplitude of +/-500 km/s. This is approximately equal to 20 larger than the line centroid amplitude; the calculation of which is heavily weighted by static portions of the line profile. In addition to the 'snake,' emission residuals appear that move away from line center on unbound trajectories and are thought to result from the interaction of a periodic driver with the unstable flow of the radiation driven wind.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 221; 1-2; p. 155-167
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  • 71
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: The results of a recent study of coplanarity tendencies between stellar-equatorial and binary-orbital planes are applied to the low-metal F9 V star HD 114762, for which a possible substellar companion was reported by Latham et al. (1989). High-resolution spectroscopy is performed on HD 114762 in order to extract its projected rotational velocity (v sin i). This is then combined with an expected rotational velocity determined via age-scaling, providing an estimate of the star's equatorial inclination, which then combined with the results of the coplanarity study provides an approximate indication of the companion's orbital inclination. Although the formal uncertainties in this process preclude an accurate estimate of the companion's inclination, the results suggest that the inclination is low, possibly low enough to force the companion's mass above the limit for hydrogen fusion. It is thus possible the companion may be nothing more exotic than a low-mass M star, as opposed to a brown dwarf. These results add support to a similar conclusion recently obtained by Cochran et al. (1991).
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 107; 707; p. 22-26
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  • 72
    Publication Date: 2011-08-24
    Description: On December 8, 1990, the Galileo spacecraft used the Earth for a gravity assist on its way to Jupiter. Its trajectory was such that it crossed geosynchronous orbit at approximately local midnight between 1900 and 2000 UT. At the same time, spacecraft 1984-129 was also located at geosynchronous orbit near local midnight. Several flux dropout events were observed when the two spacecraft were in the near-Earth plasma sheet in the same local time sector. Flux dropout events are associated with plasma sheet thinning in the near-profile of the near-Earth plasma sheet while 1984-129 provided an azimuthal profile. With measurements from these two spacecraft we can distinguish between spatial structures and temporal change. Our observations confirm that the geosynchronous flux dropout events are consistent with plasma sheet thinning which changes the spacecraft's magnetic connection from the trapping region to the more distant plasma sheet. However, for this period, thinning occurred on two spatial and temporal scales. The geosynchronous dropouts were highly localized phenomena of 30 min duration superimposed on a more global reconfiguration of the tail lasting approximately 4 hours.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,323-21,333
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  • 73
    Publication Date: 2011-08-24
    Description: We suggest a model to explain the unexpected recurrent dust events that were observed during the Jupiter encounter by the dust detector on board the Ulysses spacecraft. This model is based dust-magnetosphere interactions. Dust particles inside the Jovian magnetosphere collect electrostatic charges and their interaction with the magnetic and electric fields can lead to energization and subsequent ejection. We discuss the dusty regions (ring/halo, `gossamer' ring) and also Io as potential sources for the Ulysses events. This model favors Io as a source. The mass and velocity range of the escaping particles are compatible with the observations, and we also suggest internal periodicities to explain the recurrent nature of the Ulysses dust events.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,245-21,251
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  • 74
    Publication Date: 2011-08-24
    Description: We report investigations of Jovian relativistic electrons in the interplanetary medium that provide new insights into both the physical processes by which the Jovian magnetosphere releases its trapped, relativistic electrons into the interplanetary medium, and the modes of their interplanetary propagation. These studies were dependent on the unique postencounter trajectory for Ulysses. The spacecraft remained close to the radial distance of Jupiter (approximately 5.2 AU) and moved southward on the duskside by only approximately 12 deg in heliographic latitude and less than 8 deg in the heliographic azimuth relative to Jupiter for the period of approximately 100 deg days of this study. During this period the nominal Parker spiral interplanetary magnetic field with its alternating polarities sector structure established direct magnetic field line connections frequently between Jupiter and the spacecraft. These unique conditions made it possible to investigate in detail, for approximately four solar rotations, both the Jovian electron burst phenomenon and the continuous, diffusive interplanetary propagation of Jovian electrons.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,129-21,144
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  • 75
    Publication Date: 2011-08-24
    Description: In the midcometosheath of comet Halley (1 x 10(exp 5) to 2 x 10(exp 5) km from the nucleus) the center-of-mass plasma frame is approximately the bulk flow velocity of the cometary ions, and the Alfven wave speed is an appreciable fraction of the flow speed. Here, the peaks of the water group ion distributions observed by the Giotto ion mass spectrometer are at velocities consistently below the expected pickup speed. It is shown that this effect is consistent with the scattering of the new pickup ions onto a bispherical shell distribution. The model does not fit the data inside approximately 1.2 x 10(exp 5) km, however, possibly as a result of the growing importance of collisions or the presence of other processes such as scattering on obliquely propagating magnetosonic waves.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,039-21,043
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  • 76
    Publication Date: 2011-08-24
    Description: We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg-Skjellerup (G-S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity-space diffusion of the implanted heavy ion population at G-S using a transport equation including souce, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injections, waves are generated at omega approximately Omega(sub i) the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 20,995-21,002
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  • 77
    Publication Date: 2011-08-24
    Description: An equation governing the combined radial diffusion and stochastic acceleration of super-Alfvenic ions by magnetohydrodynamic waves in Jupiter's outer magnetosphere is derived. The formulation is based upon a total energy invariant of the adiabatic transport which applies to an isotropic distribution undergoing rapid pitch angle scattering by waves. An analytic solution to the double diffusion equation is obtained and numerical results are presented for two models of ion injection. The first model assumes S(+) and O(+) are injected throughout a broad region of space through photoionization of Jupiter's magnetospheric neutral wind and obtain an initial energy corresponding to the local corotation energy after pickup by the planetary magnetic field. The second model assumes a monoenergetic distribution of energetic protons is implanted in the middle magnetosphere by the action of field-aligned potential drops in Jupiter's auroral ionosphere. For both light and heavy ions the injection process creates a seed population of particles which are further accelerated nonadiabatically by the MHD waves and adiabatically through radial diffusion. A comparison of the theoretical results with a recent data analysis of Voyager low-energy charged particle measurements is made with very good agreement, thus providing a rigorous quantitative account of and definitive explanation for the high-energy ion component of Jupiter's magnetosphere.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A7; p. 13,509-13,520
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  • 78
    Publication Date: 2011-08-24
    Description: We present an analysis of whistler wave magnetic and electric field amplitude ratios from which we compute wave propagation angles and energies of electrons in resonance with the waves. To do this analysis, we compute the theoretical dependence of ratios of wave components on the whistler wave propagation angle Theta for various combinations of orthogonal wave components. Ratios of wave components that would be observed by a spinning spacecraft are determined, and the effects of arbitrary inclinations of the spacecraft to the ambient magnetic field and to the whistler wave vector are studied. This analysis clearly demonstrates that B/E, the ratio of magnetic to electric field amplitudes, cannot be assumed to be the wave index of refraction, contrary to assumptions of some earlier studies. Therefore previous interpretations of whistler wave observations based on this assumption must be reinvestigated. B/E ratios derived using three orthogonal wave components can be used to unambiguously determine Theta. Using spin plane observations alone, a significant uncertainty occurs in the determination of Theta. Nevertheless, for whistler waves observed downstream of several interplanetary shocks by the Ulysses plasma wave experiment we find that Theta is highly oblique. We suggest that the analysis of wave amplitude ratios used in conjunction with traditional stability analyses provide a promising tool for determining which particle distributions and resonances are likely to be dominant contributors to wave growth.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A7; p. 13,325-13,334
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  • 79
    facet.materialart.
    In:  Other Sources
    Publication Date: 2011-08-24
    Description: Oxygen within the heliosphere, whether neutral or ionized, comes from both the external source in the local interstellar medium (LISM) and internal sources. If transient cometary sources are neglected, Jupiter is the strongest of the internal sources by virtue of its corotation and charge exchange driven neutral wind. O(+) pickup ions are born where the penetrating neutrals lose an electron as a result of photoionization, charge exchange with solar wind protons, or solar wind electron impact ionization. The region of the heliosphere from which these pickup ions arise depends on the phase of the solar cycle as well as on the velocities of the neutrals. The present model of neutral O populations and their ionization suggests that Jovian pickup ions can dominate the inner heliospheric O(+) population if the LISM neutrals are strongly filtered at the heliopause, or are excluded by strong ionizing fluxes from the Sun. Other heliospheric species such as sulfur and nitrogen may have similar origins. These planetary sources need to be taken into account in the interpretation of interplanetary gas and ion composition observations.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A7; p. 13,285-13,305
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  • 80
    Publication Date: 2011-08-24
    Description: Spacecraft observations from the Earth's magnetotail show that the flaring angle depends on the downtail distance, the upstream solar wind dynamic pressure and the B(sub z) component of the interplanetary magnetic field (1993). Measurements from the Phobos 2 spacecraft along a circular orbit at 2.8 Mars radii allow a similar study of the Mars magnetotail. Under the assumption that the magnetic pressure in the Martian tail lobes is much greater than the plasma pressure in the lobe, we use the pressure balance condition between the tail lobe magnetic pressure and the normal component of the solar wind pressure to infer the angle at which the tail magnetopause flares. As in the case of the terrestrial magnetotail, the flaring angle of Mars tail depends on the solar wind dynamic pressure, but this angle (at 2.5 R(sub M)) is about one half of the terrestrial value (at 17 R(sub E)). The median inferred flaring angle is about 13 deg.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 21; 12; P. 1121-1124
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  • 81
    Publication Date: 2011-08-24
    Description: We have constructed a model that predicts the evolution of CO2 on Mars from the end of the heavy bombardment period to the present. The model draws on published estimates of the main processes believed to affect the fate of CO2 during this period: chemical weathering, regolith uptake, polar cap formation, and atmospheric escape. Except for escape, the rate at which these processes act is controlled by surface temperatures which we calculate using a modified version of the Gierasch and Toon energy balance model. The modifications account for the change in solar luminosity with time, the greenhouse effect, and a polar and equatorial energy budget. Using published estimates for the main parameters, we find no evolutionary scenario in which CO2 is capable of producing a warm (global mean temperatures greater than 250 K) and wet (surface pressures greater than 30 mbar) early climate, and then evolves to present conditions with approximately 7 mbar in the atmosphere, less than 300 mbar in the regolith, and less than 5 mbar in the caps. Such scenarios would only exist if the early sun were brighter than standard solar models suggest, if greenhouse gases other than CO2 were present in the early atmosphere, or if the polar albedo were significantly lower than 0.75. However, these scenarios generally require the storage of large amounts of CO2 (greater than 1 bar) in the carbonate reservoir. If the warm and wet early Mars constraint is relaxed, then we find best overall agreement with present day reservoirs for initial CO2 inventories of 0.5-1.0 bar. We also find that the polar caps can have a profound effect on how the system evolves. If the initial amount of CO2 is less than some critical value, then there is not enough heating of the poles to prevent permanent caps from forming. Once formed, these caps control how the system evolves, because they set the surface pressure and, hence, the thermal environment. If the initial amount of CO2 is greater than this critical value, then caps do not form initially, but can form later on, when weathering and escape lower the surface pressure to a point at which polar heating is no longer sufficient to prevent cap formation and the collapse of the climate system. Our modeling suggests this critical initial amount of CO2 is between 1 and 2 bar, but its true value will depend on all factors affecting the polar heat budget.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 109; 1; p. 102-120
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  • 82
    Publication Date: 2011-08-24
    Description: A coupled problem of diffusion and condensation is solved for the H2SO4-H2O system in Venus' cloud layer. The position of the lower cloud boundary and profiles of the H2O and H2SO4 vapor mixing ratios and of the H2O/H2SO4 ratio of sulfuric acid aerosol and its flux are calculated as functions of the column photochemical production rate of sulfuric acid, Phi(sub H2SO4). Variations of the lower cloud boundary are considered. Our basic model, which is constrained to yield f(sub H2O) (30 km) = 30 ppm (Pollack et al. 1993), predicts the position of the lower cloud boundary at 48.4 km coinciding with the mean Pioneer Venus value, the peak H2SO4 mixing ratio of 5.4 ppm, and the H2SO4 production rate Phi(sub H2SO4) = 2.2 x 10(exp 12)/sq cm/s. The sulfur to sulfuric acid mass flux ratio in the clouds is 1:27 in this model, and the mass loading ratio may be larger than this value if sulfur particles are smaller than those of sulfuric acid. The model suggests that the extinction coefficient of sulfuric acid particles with radius 3.7 micrometers (mode 3) is equal to 0.3/km in the middle cloud layer. The downward flux of CO is equal to 1.7 x 10(exp 12)/sq cm/s in this model. Our second model, which is constrained to yield f(sub H2O) = 10 ppm at the lower cloud boundary, close to the value measured by the Magellan radiooccultations, predicts the position of this boundary to be at 46.5 km, which agrees with the Magellan data; f(sub H2O) (30 km) = 90 ppm, close to the data of Moroz et al. (1983) at this altitude; Phi(sub H2SO4) = 6.4 x 10(exp 12)/sq cm/s; and Phi(sub co) = 4.2 x 10(exp 12)/sq cm/s. The S/H2SO4 flux mass ratio is 1:18, and the extinction coefficient of the mode 3 sulfuric acid particles is equal to 0.9 km in the middle cloud layer. A strong gradient of the H2SO4 vapor mixing ratio near the bottom of the cloud layer drives a large upward flux of H2SO4, which condenses and forms the excessive downward flux of liquid sulfuric acid, which is larger by a factor of 4-7 than the flux in the middle cloud layer. This is the mechanism of formation of the lower cloud layer. Variations of the lower cloud layer are discussed. Our modeling of the OCS and CO profiles in the lower atmosphere measured by Pollack et al. (1993) provides a reasonable explanation of these data and shows that the rate coefficient of the reaction SO3 + CO yields CO2 + SO2 is equal to 10(exp -11) exp(-(13,100 +/- 1000)/T) cc/s. The main channel of the reaction between SO3 and OCS is CO2 + (SO)2, and its rate coefficient is equal to 10(exp -11) exp(-(8900 +/- 500)/T) cc/s. In the conditions of Venus' lower atmosphere, (SO)2 is removed by the reaction (SO)2 + OCS yields CO + S2 + SO2. The model predicts an OCS mixing ratio of 28 ppm near the surface.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 109; 1; p. 58-78
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  • 83
    facet.materialart.
    In:  CASI
    Publication Date: 2013-08-31
    Description: The concept of a primordial blackhole fluid with intrinsic spin density is examined along with its consequence for supercluster-sized, i.e., large scale voids, and the missing mass question.
    Keywords: ASTROPHYSICS
    Type: NASA-TM-101788 , NAS 1.15:101788
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  • 84
    Publication Date: 2013-08-31
    Description: The propagation of light rays in a clumpy universe constructed by cosmological version of the post-Newtonian approximation was investigated. It is shown that linear approximation to the propagation equations is valid in the region where zeta is approximately less than 1 even if the density contrast is much larger than unity. Based on a gerneral order-of-magnitude statistical consideration, it is argued that the linear approximation is still valid where zeta is approximately greater than 1. A general formula for the distance-redshift relation in a clumpy universe is given. An explicit expression is derived for a simplified situation in which the effect of the gravitational potential of inhomogeneities dominates. In the light of the derived relation, the validity of the Dyer-Roeder distance is discussed. Also, statistical properties of light rays are investigated for a simple model of an inhomogeneous universe. The result of this example supports the validity of the linear approximation.
    Keywords: ASTROPHYSICS
    Type: FERMILAB-PUB-89/111-A , NAS 1.26:185041 , NASA-CR-185041
    Format: application/pdf
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