ALBERT

Description: Millisecond pulsars are galactic objects that exhibit a very stable spinning period. Several tens of these celestial clocks have now been discovered, which opens the possibility that an average time scale may be deduced through a long-term stability algorithm. Such an ensemble average makes it possible to reduce the level of the instabilities originating from the pulsars or from other sources of noise, which are unknown but independent. The basis for such an algorithm is presented and applied to real pulsar data. It is shown that pulsar time could shortly become more stable than the present atomic time, for averaging times of a few years. Pulsar time can also be used as a flywheel to maintain the accuracy of atomic time in case of temporary failure of the primary standards, or to transfer the improved accuracy of future standards back to the present.

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.

Description: Large-amplitude ultralow-frequency wave structure observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi-perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring-type distributions of the heavy (water group) pickup cometary ions.

Description: Solid state SiS2 is proposed as the material responsible for the recently discovered 21 micrometer emission feature that is observed in the carbon-rich circumstellar shells of certain protoplanetary nebulae. Sulfurized SiC, or SiS2 mantles on grains of either SiC or a:C-H are discussed as possible forms for which no spectroscopic laboratory observations yet exist. The identification with a relatively minor species and required special abundance ratios are consistent with the low incidence rate that the 21 micrometer feature presents in the population of carbon rich objects. It is also consistent with the lack of a good correlation between the 21 micrometer feature and the other solid-state spectroscopic features that have been observed in protoplanetaries that would be expected if the feature arose from molecules composed of H, C, N, and O. SiS2 condensate is consistent with the circumstellar shell temperature range, T(sub CS) approximately equal to or less than 150 K, at which the feature appears, and the available mass of SiS2, M(sub SiS2) approx. = 5 x 10(exp -6) solar mass, that is possible in the circumstellar shell.

Description: In the present work, the high Reynolds number flow past an inclined plate with a splitter plate placed in its wake is considered numerically. A numerical conformal mapping technique is employed to transform the two-plate system into the same number of cylinders: the flow field is assumed to be two-dimensional. The vortex shedding from the inclined plate is modelled using the discrete vortex method. It is shown that the splitter plate has a profound effect on the development of the flow over a range of values of a suitably defined offset parameter and for a range of positions of the leading edge of the splitter plate. The acoustic field is also calculated and the spectrum reflects the flow results.

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.

Description: NASA's Cosmic Background Explorer (COBE) carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background (CMB) radiation on angular scales greater than 7 deg and to conduct a search for a diffuse cosmic infrared background (CIB) radiation with 0.7 deg angular resolution. Data from the Far-Infrared Absolute Spectrophotometer (FIRAS) show that the spectrum of the CMB is that of a blackbody of temperature T = 2.73 +/- 0.06 K, with no deviation from a blackbody spectrum greater than 0.25% of the peak brightness. The first year of data from the Differential Microwave Radiometers (DMR) show statistically significant CMB anisotropy. The anisotropy is consistent with a scale invariant primordial density fluctuation spectrum. Infrared sky brightness measurements from the Diffuse Infrared Background Experiment (DIRBE) provide new conservative upper limits to the CIB. Extensive modeling of solar system and galactic infrared foregrounds is required for further improvement in the CIB limits.

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.

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.

Description: In 1970 the International Astronomical Union (IAU) defined any object's north pole to be that axis of rotation which lies north of the solar system's invariable plane. A competing definition in widespread use at some institutions followed the 'right hand rule' whereby the 'north' axis of rotation was generally said to be that of the rotational angular momentum. In the case of the latter definition, the planet Neptune and its satellite Triton would have their 'north' poles in opposite hemispheres because Triton's angular momentum vector is in the hemisphere opposite from that of Neptune's rotation angular momentum. The IAU resolutions have been somewhat controversial in some quarters ever since their adoption. A Working Group has periodically updated the recommended values of planet and satellite poles and rotation rates in accordance with the IAU definition of north and the IAU definition of prime meridian. Neither system is completely satisfactory in the perception of all scientists, and some confusion has been generated by publishing data in the two different systems. In this paper we review the IAU definitions of north and of the location of prime meridian and we present the algorithm which has been employed in determining the rotational parameters of the natural satellites. The IAU definition of the prime meridian contains some ambiguities which in practice have been 'specified' by the numerical values published by the IAU working group but which have not yet been explicitly documented. The purpose of this paper is to explicitly document the algorithm employed by the IAU working group in specifying satellite poles and rotation rates.

Description: We have considered equilibrium configurations of a white dwarf and a red dwarf companion in AM Herculis type binary system where the red dwarf is assumed to have an intrinsic centered dipole field, and the white dwarf an offset dipole or a centered dipole plus a quadrupole field, and the interaction is purely magnetic. The restoring torque, due to the system being perturbed from its stable equilibrium configuration, is compared with the accretion torque. The results are used to show that the observed alignment of the main cyclotron emission region relative to the red star can be explained quite simply by these models without having to invoke gravitational torques. We also provide a possible explanation of the observation that the more strongly accreting pole has the weaker field strength in the few systems where fields have been determined at both poles.

Description: A detailed spectral analysis of five X-ray observations of Mrk 841 with the EXOSAT, Ginga, and ROSAT satellites is reported. Variability is apparent in both the soft (0.1-1.0 keV) and medium (1-20 keV) energy bands. Above, 1 keV, the spectra are adequately modeled by a power law with a strong emission line of equivalent width 450 eV. The large equivalent width of the emission line indicates a strongly enhanced reflection component of the source compared with other Seyferts observed with Ginga. The implications of the results of the analysis for physical models of the emission regions in this and other X-ray bright Seyferts are briefly examined.

Description: We explore the implications of the IR-radio correlation in star-forming galaxies, using a simple physical model constrained by the constant global ratio q of IR to radio emission and by the radial falloff of this ratio in disks of galaxies. The modeling takes into account the diffusion, radiative decay, and escape of cosmic-ray electrons responsible for the synchrotron emission, and the full range of optical depths to dust-heating photons. We introduce two assumptions: that dust-heating photons and radio-emitting cosmic-ray electrons are created in constant proportion to each other as part of the star formation activity, and that gas and magnetic field are well coupled locally, expressed as B proportional to n exp beta, with beta between 1/3 and 2/3. We conclude that disk galaxies would maintain the observed constant ratio q under these assumptions if the disk scale height h(0) and the escape scale length l(esc) for cosmic-ray electrons followed a relation of the form l(esc) proportional to h(0) exp 1/2; the IR-to-radio ratio will then depend very weakly on interstellar density, and, therefore, on magnetic field strength or mean optical depth.

Description: Energy of a high harmonic standing wave tends to be distributed equally over the whole wave even in a stratified medium where the wave's peak amplitude can be much larger near the upper boundary than the lower one. This fact is generalized to the many diverse physical problems which solve second-order differential equations of Sturm-Liouville type. For any such solution y(z) whose sign fluctuates along the z-axis, quantities are found which have the same value between any two neighboring zeros of y. One of the equidistributed quantities for an oscillating fluid sphere is similar to kinetic energy but is identical only in limiting cases. The acoustic midpoint of a cavity can be a unique place where some nonlinear perturbations have extra strength. This may apply to the puzzling solar phenomenon called supergranulation.

Description: It is proposed that H2O masers in star-forming regions occur early in the expansion of thin shells swept up by high-velocity winds from young massive stars. In W49N, confinement of the shell by a density distribution with an axial cavity can explain both the velocity field and the shape of the mass distribution. A modified version of the thin-shell code of Mac Low and McCray (1988), including radiation cooling, is used to model dynamically the expanding shell.

Description: A computational code for the vorticity-potential method is developed for a three-dimensional bounded vorticity field. The evaluation of the boundary data for the vector potential in the code is improved so that the numerical solution simulates that in an unbounded domain to a high order. The time evolution of two vortex rings and that of an elliptic ring are investigated with this code. The cut-and-connect phenomena of vortex rings are successfully captured. The results are compared with those of asymptotic theory and the experiment. They also highlight the need for additional theoretical and numerical investigations.

Description: Observations of the IMF carried out from October 1990 to February 1992 during the in-ecliptic phase of the Ulysses mission are presented. The observations were made between 1 and 5.4 AU during a different phase of the solar cycle from previous missions. The large-scale reorganization of the solar coronal fields following the maximum in Solar Cycle 22 generated numerous transient events, causing large-scale disturbances in interplanetary space, particularly during the first half of 1991. From about the middle of 1991, solar activity apparently decreased, resulting in fewer transient events, allowing the formation of corotating interaction regions (CIRs) recurring in successive solar rotations. Observations of the IMF reflect this temporal evolution in solar activity. However, the average orientation of the IMF, the observed sector structure, and the evolution of the CIRs show that the dynamic interaction of transient events with evolving stream-stream structures in this heliocentric distance range results in significant deviations from the expected IMF topology, even when signature of the transients is less apparent.

Description: The paper discusses the occurrence and properties of disks around young stars, emphasizing in particular how these may relate to planet formation and the evolution of the solar system. The global properties of such disks often resemble those attributed to the primitive solar nebula, suggesting that conditions appropriate for planet formation commonly accompany the birth of low-mass stars. Disk masses, between 0.001 and 1 solar mass, are generally lower than those of the stars, and may represent only a fraction (less than about 10 percent) of the total system mass. From the paucity of near-IR radiation from some disks it is inferred that the inner regions there are gaps where the opacity from small particles becomes vanishingly small. Evidence is presented to the effect that gaps in the inner disks develop preferentially in the oldest objects, suggesting that, with time, matter is lost or accumulates into large particles such as planetesimals, which cannot yet be detected.

Description: Theoretical results predicting yields of various isotopes in nova outbursts are reviewed. Emphasis is placed on the stardust and gas-phase chemical anomalies that may be contributed to the Galactic ISM by classic novae. It is concluded that most of the heavy isotope anomalies are produced by about 25 percent of novae that occur in systems containing massive O-Ne-Mg white dwarfs. The dust-production scenario for novae is summarized, and it is argued that explosions on O-Ne-Mg white dwarfs may lead to the formation of dust grains that carry the Ne-E and Mg-26 anomalies. Quantitative constraints are placed on the degree to which classical novae participate in the production of chemical anomalies, both in the primitive solar system and on a Galactic scale.

Description: We have detected forbidden O I 63 micron and forbidden Si II 35 micron emission from the oxygen-rich, M2 lab supergiant, Alpha Orionis (Betelgeuse). The forbidden O I line flux is 2.4 +/- 0.2 x 10 exp -18 W/sq cm, and the forbidden Si II line flux is 0.9 +/- 0.4 x 10 exp -18 W/sq cm. These fluxes are consistent with the thermal model of Rodgers and Glassgold (1991), which indicates that the emission arises in dense, warm gas in Alpha Ori's inner envelope and implies that nearly all of the available O and Si is in atomic form. This is the first reported detection of FIR, fine-structure emission from the inner or transition region of a circumstellar envelope, where molecules and dust are expected to form.

Description: Simple and easy to implement elementary function approximations are introduced to the spectral window functions needed in calculations of model predictions of the cosmic microwave backgrond (CMB) anisotropy. These approximations allow the investigator to obtain model delta T/T predictions in terms of single integrals over the power spectrum of cosmological perturbations and to avoid the necessity of performing the additional integrations. The high accuracy of these approximations is demonstrated here for the CDM theory-based calculations of the expected delta T/T signal in several experiments searching for the CMB anisotropy.

Description: New observations were obtained for the OH/IR star IRC +10420, including optical/infrared imaging, spectroscopy, polarimetry, and photometry. The observations, which indicate a very rich and complex circumstellar environment for the star. A model for IRC +10420 star is proposed, in which a true core-burning supergiant is surrounded by a rotating equatorial disk of gas and dust that is viewed from about midway above the plate.

Description: Results of a search for extended CO(7-6) line emission in the Orion A molecular cloud using the University of Texas submillimeter laser heterodyne receiver are presented. The large-scale distribution of quiescent CO(7-6) emission is mapped along a cut across the Trapezium cluster extending more than 13 arcmin (nearly 2 pc) in R.A. The receiver, observing method, and calibration techniques are described, and the CO(7-6) spectra and maps are presented. The physical parameters and energetics of the molecular gas are discussed. The findings are compared with observations of other high-luminosity star-forming regions.

Description: The Astro-1 Space Shuttle payload's Broad Band X-ray telescope has been used to obtain high-quality, moderate-resolution spectroscopy of Cygnus X-2 which allow the resolving of the physical width of the 6.7 keV Fe K-alpha feature with a factor-of-4 energy resolution improvement over past experiments. Three possible sites are noted for the Fe K-alpha emission: the accretion disk, its corona, and the source itself. It is judged that reflection from the accretion disk can generate a line of the observed energy, width, and equivalent width, provided that the disk surface is highly ionized.

Description: Spectral images in the 3.3 and 11.3 micron IR emission band around the star SVS 3 in NGC 1333 indicate that the two bands have their origin in different locations, rendering them useless in estimates of the sizes of the molecules emitting the bands. This is in keeping with the emission bands' generation by a mixture of variously sized polycyclic aromatic hydrocarbons (PAHs) whose smallest species have been dehydrogenated in the vicinity of SVS 3. Hot bands and overtone/combination bands may account for the 3.4-micron band, rather than molecular side-groups of the PAHs.

Description: Hydrodynamic pulsation models are matched with observations of globular cluster RRc stars to derive masses, luminosities and temperatures as functions of two observables: pulsation period, P1, and Fourier phase parameter, phi sub 31. We find that mean RRc masses and luminosities increase, and mean temperatures fall, with decreasing cluster metallicity. The Oosterhoff dichotomy is explained as mainly a temperature effect, while hints of a new dichotomy (in RRc mass and luminosity) are found among certain Oo II clusters. A provisional RR Lyrae distance scale emerges for the RRc stars in the form of a relation between luminosity and the two observables, P1 and phi sub 31. Finally, the scheme we propose passes three independent tests: (1) it reproduces the observed hierarchy of relative luminosity among a large sample of RRc stars in Omega Centauri; (2) it yields mean RRc masses which are fully consistent with the RRd masses derived for the clusters M68 and M15; and (3) it gives an LMC distance modulus in agreement with that obtained by other methods.

Description: The slope of a gamma-ray burst spectrum determines how the burst flux decreases as the cosmological redshift z increases. This provides a test for the presence of a cosmological redshift in a sample of gamma-ray bursts. Many bursts have spectra characterized by a frequency-dependent power-law index, so a burst subset can be created by requiring that each subset member have a power-law index of the given value at a low frequency and an index of a different given value at a high frequency. If all bursts in this subset are at z much less than 0.1, then the flux at low frequency will change proportionally with the flux at high frequency. If, however, z greater than 0.1 for most bursts, then the linear dependence of the high-frequency flux on the low-frequency flux disappears at the flux characterizing the spatial cutoff in the log N-log C(max)/C(min) diagram. This signature of the cosmological redshift generally persists when gamma-ray bursts are given a broad distribution of intrinsic fluxes and spectral shapes. The characteristics that a burst distribution must have to mask the cosmological redshift are discussed. The limitations of this test are observational, arising primarily from errors in measuring the photon flux and in determining the spectral index of the burst spectrum.

Description: The effects of external heating on the stability of hot accretion disks are studied in some detail. It is known that geometrically thin, optically thick, nonirradiated accretion disks have two distinct branches in the surface density mass-accretion rate plane: the upper branch is radiation pressure-dominated and is unstable against thermal and secular perturbations, while the lower one is gas-pressure-dominated and is stable. We show quite generally that, even when disks are strongly irradiated, the upper branch remains unstable and the lower branch remains stable; the lower branch, however, can become radiation pressure dominated, if the irradiating flux, F(irr), is kept constant. A stable, radiation pressure dominated state thus appears. If F(irr) changes in proportion to the mass-accretion rate through the disk, the instabilities associated with radiation pressure dominated disks cannot be removed. Some observational implications are discussed in the context of long-term variations of low-mass X-ray binaries.

Description: Inflation is found to naturally provide mechanisms for the direct generation of cosmic turbulence on the same scales as those on which density perturbations are formed. Since the observational constraints on turbulence could translate into constraints on inflation itself, these results call for a reexamination of the issue of cosmic turbulence. It is found that by the end of inflation, the amplitude of the generated turbulent velocity has been suppressed by a factor approximately greater than 10 exp 100. Inflation therefore assures the absence of turbulence on scales of galaxies and clusters of galaxies. In addition, it is shown that the density fluctuations generated by inflation can excite longitudinal turbulence after they reenter the Hubble radius at later cosmic epochs.

Description: Important problems are discussed concerning the effects of shock geometry on the injection of thermal particles and the process of standard Fermi particle acceleration in general. In pursuit of a self-consistent model that simultaneously describes shock structure and particle acceleration in shocks of arbitrary obliquity, a Monte Carlo simulation developed for parallel shocks is generalized to address oblique geometries. Attention is given to initial results concerning the ways in which injection and acceleration efficiency varies with obliquity and Mach number.

Description: The paper examines the (C-12)/(C-13) isotope ratio in the solar neighborhood on the basis of observations of the (C-12)(O-18) and (C-13)(O-18) J = 1-0 transitions in four interstellar clouds located within 500 pc. The (C-12)/(C-13) ratio in these sources ranges from 57 to 74, and its weighted average is 62 +/- 4, with thermal noise and line formation uncertainties contributing about equally to the probable error. These values indicate moderate chemical evolution in the local star neighborhood since the formation of the solar system, in good agreement with model prediction of the current carbon ratio in the local interstellar medium.

Description: In the present grid-independent approximate Riemann solver for 2D and 3D flows that are governed by the Euler or Navier-Stokes equations, fluxes on grid faces are obtained by wave decomposition; the assumption of information-propagation in the velocity-difference directions leads to a more accurate resolution of shear and shock waves, when these are are oblique to the grid. The model, which yields significantly greater accuracy in both supersonic and subsonic first-order spatially accurate computations, describes the difference in states at each grid interface by the action of five waves.

Description: The action of weak, streamwise vortices on a plane, incompressible, steady mixing layer is examined in the large Reynolds number limit. The outer, inviscid region is bounded by a vortex sheet to which the viscous region is confined. It is shown that the local linear analysis becomes invalid at streamwise distances O(epsilon sup -1), where (epsilon much less than 1) is the crossflow amplitude, and a new nonlinear analysis is constructed for this region. Numerical solutions of the nonlinear problem show that the vortex sheet undergoes an O(1) change in position and that the solution is ultimately terminated by a breakdown in the numerical procedure. The corresponding viscous layer shows downstream thickening, but appears to remain well behaved up to the terminal location.

Description: This paper investigates the linear stability of confined mixing layers with special emphasis on the effects of heat release and compressibility. The results show that reflection of supersonic disturbances by the walls makes the confined supersonic mixing layer more unstable than the unconfined free shear layer. Decreasing the distance between the walls makes the flow more unstable. However, subsonic disturbances are relatively unaffected by the walls. Heat release and Mach number hardly change the growth rates of supersonic disturbances. The most unstable supersonic disturbances are two-dimensional in rectangular channel flows, but three-dimensional in partially confined flows. Finally, the reactants are not strongly mixed by supersonic instabilities, which mainly disturb one side of the layer.

Description: It has been suggested that gamma-ray burst light curves may consist of many superposed flares with a duration shorter than 30/microsec. If true, the implications for the interpretation of burst data are enormous. With the launch of the Compton Gamma-Ray Observatory, four predictions of Mitrofanov's (1989) suggestion can be tested. Our results which contradict this suggestion are (1) the photon arrival times are not correlated between independent detectors, (2) the spectral hardness and intensity does not depend on the detector area, (3) the bursts seen by detectors which measure photon positions do not see microsecond flares, and (4) burst positions deduced from detectors with different projected areas are close to the positions deduced from time-of-flight differences between separated spacecraft. We conclude, therefore, that gamma-ray bursts are not composed of microsecond flares.

Description: We report on a series of ultraviolet spectroscopic observations made with the IUE satellite, which sampled critical outburst stages of the soft X-ray transient Nova Muscae 1991. Continuum light curves, and a multi-epoch line analysis are presented. In addition, a single epoch optical-UV spectra obtained with the HST/FOS is discussed. The optical-UV continuum fits approximately the 'canonical' fv varies as V exp 1/3 power low expected for thermal emission from steady state accretion disk. The emission line spectrum contains prominent high ionization lines of C IV and N V. Weaker lines are seen at intermittent epochs, including some of lower ionization. The derivation of galactic reddening which has significant implications for distance estimates is discussed. Comparisons with previous SXT sources are considered.

Description: Recent observations of the Galactic center region indicate the presence of a narrow gamma-ray line feature at 170 keV, and theoretical speculations suggest it may result from Compton backscattering of the 511 keV annihilation radiation. The high-resolution gamma-ray spectrometer on HEAO 3 observed the Galactic center in the fall of 1979 and in the spring of 1980. In view of the recent developments, the HEAO data were re-examined to search for this new feature and to look for possible correlations with the 511 keV line emisison. No evidence for such Compton backscattered radiation was found and the derived upper limits for emission in a line feature near 170 keV were well below previously reported fluxes, indicating possible time variability.

Description: We investigate the origin of diffuse low-energy Galactic gamma-ray continuum down to about 30 keV. We calculate gamma-ray emission via bremsstrahlung and inverse Compton scattering by propagating an unbroken electron power law injection spectrum and employing a Galactic emmissivity model derived from COSB observations. To maintain the low energy electron population capable of producing the observed continuum via bremsstrahlung, a total power input of 4 x 10 exp 41 erg/s is required. This exceeds the total power supplied to the nuclear cosmic rays by about an order of magnitude.

Description: The study reports observations of positron annihilation radiation from the inner region of the Galaxy which show that there are two components of the radiation: a steady, diffuse Galactic component and a variable component from discrete, presumably compact sources. The existence of the variable component is supported by the ensemble of all narrow FOV 511 keV line observations, including recent detections with OSSE. The fit of this ensemble to a time-independent source distribution can be excluded at the approximately 3-sigma level. The same ensemble, combined with the broad FOV SMM observations of Galactic 511 keV line emission, sets constraints on the Galactic distribution of the diffuse component.

Description: We calculate the spectral hardness ratios for several intense gamma-ray bursts (GRBs) which have sufficient statistics in four energy channels. We study the evolution of these hardness ratios during the events using color-color diagrams (CCDs) and we attempt a preliminary classification of GRBs based on their CCD evolution.

Description: An overview of the early results of EGRET, one of four instruments carried on the Compton Gamma Ray Observatory, is presented with regard to observations of pulsars, bursts, galactic diffuse radiation, and active galaxies. The June 11, 1992 solar flare was observed in gamma rays with energies from the threshold of detection to energies over 1 GeV. Detailed time histories and energy spectra were obtained for the Crab and Vela pulsars, with one of the Vela pulses being quite sharp, approximately 2 percent of the total period. Several gamma-ray bursts were recorded. The one on May 3, 1991 was seen in EGRET's spark chamber system, which made it possible to determine a position. The quasars 3C 279, which has a redshift of 0.538, was seen in high energy gamma rays in both June and October 1992.

Description: We present a detailed spectral and temporal analysis of a long ROSAT PSPC pointing centered on the bright Seyfert 1 galaxy NGC 5548. The spectrum is particularly complex, with a two-component model favored, confirming the existence of the soft excess in this source. In addition, an absorption feature arising from highly ionized oxygen in the line of sight has been detected, consistent with an origin in the 'warm absorber' which has been found to be a common feature in Seyfert galaxies observed by Ginga. The energy and depth of the feature should allow us to constrain the density and geometry of the absorbing gas, which may lie close to the central continuum source. NGC 5548 was in a low X-ray state during the observation, brightening by a factor of about 2 on a time-scale of about 2 d. Time-resolved spectral analysis shows a change in the ratio of the hard and soft fluxes, consistent with reprocessing models.

Description: The Gamma Ray Imaging Spectrometer (GRIS) was flown on balloons over Alice Springs, Australia on 1992 April 26 and May 7. A full Galactic center transit (about 12 hr) was achieved on both flights with the instrument working normally. The electron/positron annihilation line was detected on both flights. The line fluxes and line widths were found to be (7.7 +/- 1.2) x 10 exp -4 and (8.9 +/- 1.1) x 10 exp -4 photons/sq cm per sec and 1.3 +/- 0.7 and 3.6 +/- 1.0 keV, respectively. These results are compared to each other and earlier (1988) GRIS results to produce suggestive evidence for source variability. Near-contemporaneous OSSE/CGRO Galactic center observations indicate that the GRIS results cannot be due solely to a single point source like IE 1740.7-2942 within a few degrees of the Galactic center. The GRIS 1992 results represent the first time that successive high-resolution balloon measurements have been achieved on a time scale of days.

Description: Results of an 18 cm VLBI survey of 31 luminous, radio-compact IR galaxies are presented. Over half the sample galaxies show high-brightness temperature emission, with T sub b greater than 10 exp 5 K and structure on scales of 5-150 mas. The limits for nondetected sources are similar, consistent with a picture in which most of these galaxies have compact cores at a level of a few percent of the total radio flux density. Characteristics of the extended radio structure, infrared properties, and optical excitation are not good indicators of the detectability of VLBI-scale emission. Structural information and energetic considerations rule out a single supernova interpretation of the compact emission in these galaxies, although we cannot exclude the possibility of several simultaneous extraordinarily luminous radio supernovae within the central few hundred cubic parsec. Our results instead favor the presence of an AGN obscured by starburst-related dust.

Description: The detection by the Rosat satellite of GQ Muscae as a very soft blackbodylike source is reported. If the observed X-ray flux is being radiated at the Eddington luminosity from a one solar mass white dwarf, its effective temperature must be about 350,000 K. It is concluded that the white dwarf is burning hydrogen-rich material near its surface. GQ Mus is, however, the only one of 26 recent novae detected in the all-sky Rosat study; this suggests that either most novae eject all their accreted material during outburst or GQ Mus is now burning recently accreted material. GQ Mus appears identical to the supersoft X-ray sources CAL83, CAL87, and RX J0527.8-6954, lending support to the suggestion that these sources are white dwarfs accreting and burning material from a companion.

Description: It is proposed that the Local Bubble was caused by the SN that produced the Geminga pulsar. The pulsar's 300,000-year age is used in conjunction with the motion of a recently discovered optical counterpart to verify that the SN was well-positioned for production of the Local Bubble if the explosion occurred within 60 pc of the solar system.

Description: We have measured the millimeter polarization of IRAS 16293-2422, a low-mass protostar candidate embedded in the east streamer of the Rho Ophiuchi dark cloud. The source has a linear polarization at 1.1 mm of (2.8 +/- 0.5) percent at a position angle of 144 deg +/- 5 deg with a 19 arcsec beam. If this polarization is due to thermal emission from magnetically aligned nonspherical grains in a dust disk around the central (proto)star, then the magnetic field threads across the disk at a position angle of 54 deg +/- 5 deg projected on the sky. This direction is the same as that of the minor axis of the elongated circumstellar gas disk (of scale 1600 AU) and of the direction of one of the quadratic outflows. There is no positive evidence for a twisted magnetic field in the dust disk, which constrains the hydromagnetic models for molecular outflows, at least for this source.

Description: We have obtained a simple analytical approximation to the relationship between a rest-frame equivalent-width distribution for Ly-alpha forest absorption lines, N(W), and an H I column density distribution of the observed cloud number, N(N). Assuming a simple power-law form for N(N) proportional to N exp (1-beta), it is shown that beta = 1.4 turns out to agree fairly well with the observed form of N(W) in a broad range of column densities. We present a theoretical analysis of how the 'proximity effect' influences a W-limited sample of Ly-alpha forest lines. It is shown that this influence is considerably smaller than has been found before for a N-limited sample, for which an approximate value of beta was assumed rather than derived as has been done, for a W-limited sample, in the present paper. As a result, available observational data appear to be still consistent with the conjecture that the observed population of QSOs is the major source of the UV background at redshifts z about 2-4.

Description: Isoline maps of the Style II nongravitational parameters A1 and A2 of periodic comets are presented for a baseline model as functions of the parameters of the inertially fixed spin vector and the source's location-dependent thrust angle. The parameters A1 and A2 for a collection of sources are equal to the sum of their values for the individual contributors, thus depending on the source distribution. The model is consistent with even the most extreme among detected perturbations, including instances of rapid temporal changes in A2 and the discontinuous orbital anomalies, which are interpreted as products of a sudden redistribution of the transferred momentum due to episodic activation of short-lived sources. It is argued that the lifetimes of active regions vary considerably from case to case, and forced precession of the nucleus strongly influences the comet's activity pattern.

Description: We report the confirmation of X-ray substructure in two rich clusters of galaxies, A1656 (Coma) and A2256. Using data from the Einstein Observatory imaging proportional counter, we present a technique of fitting the cluster profile and searching for contributions to the X-ray emission from small regions within the cluster. Using this method we find substructure in both A1656 and A2256 while simultaneously fitting the cluster profile. We obtain the ellipticity and position angle of both clusters as a function of radius and find that both have ellipticity and position angles which vary with radius. We interpret the existence of substructure along with the variable ellipticity and position angle as evidence for a recent merger in both of these clusters.

Description: A study of medium-to-bright early-type galaxies in six bandpasses from 3500 A to 2.2 microns is presented in order to quantify their colors and color gradients and relate these to metallicity and properties of the underlying stellar population. The Stromgren filter system chosen makes it possible to introduce a new calibration to the Mg(2) system from the present narrow-band v - y indices. A comparison is presented of narrow-band colors centered on particular spectral features vs a color dominated by the mean temperature of the giant branch (i.e., J - K) to test the effects of light vs heavy element abundances on knowledge of the total system metallicity, Z, and the effects of reddening. A good correlation is found between v - y and Mg(2); it provides a connection between one light element metallicity indicator (v - y centers on the CN blend) and another, Mg. The color-magnitude relations for all five optical and near-IR colors are shown. The strongest correlation exists for the metallicity colors, v - y and J - K.

Description: Spectral local isotropy tests are applied to direct numerical simulation data, mainly at the centerline of a fully developed turbulent channel flow. Despite the small Reynolds number of the simulation, the high-wavenumber behavior of velocity and vorticity spectra is consistent with local isotropy. This consistency is verified by the relationship between streamwise wavenumber spectra and spanwise wavenumber spectra. The high-wavenumber behavior of the pressure spectrum is also consistent with local isotropy and compares favorably with the calculation of Batchelor (1951), which assumes isotropy and joint normality of the velocity field at two points in space. The latter assumption is validated by the shape but not the magnitude of the quadruple correlation of the streamwise velocity fluctuation at small separations. There is only partial support for local spectral isotropy away from the centerline as the magnitude of the mean strain rate increases.

Description: A survey is presented of the current understanding of dark matter invoked by astrophysical theory and cosmology. Einstein's equivalence principle asserts that local measurements cannot distinguish a system at rest in a gravitational field from one that is in uniform acceleration in empty space. Recent test-methods for the equivalence principle are presently discussed as bases for testing of dark matter scenarios involving the long-range forces between either baryonic or nonbaryonic dark matter and ordinary matter.

Description: A previously derived analytical model for the small-scale structure of turbulence is reformulated in such a way that the energy spectrum may be computed. The model is an ensemble of two-dimensional (2D) vortices with internal spiral structure, each stretched by an axially symmetric strain flow. Stretching and differential rotation produce an energy cascade to smaller scales in which the stretching represents the effect of instabilities and the spiral structure is the source of dissipation at the end of the cascade. The energy spectrum of the resulting flow may be expressed as a time integration involving only the enstrophy spectrum of the time evolving 2D cross section flow, which may be obtained numerically. Examples are given in which a k exp -5/3 spectrum is obtained by this method. The k exp -5/3 inertial range spectrum is shown to be related to the existence of a self-similar enstrophy preserving range in the 2D enstrophy spectrum. The results are found to be insensitive to time dependence of the strain rate, including even intermittent on-or-off strains.

Description: Recent developments concerning the accretion-outflow connection and the role of magnetic fields are examined. It is argued that the weakly ionized wind most likely represents an MHD outflow driven centrifugally from the disk surfaces or from the boundary between the disk and the star. Specific wind models for each of these alternatives are presented, and it is contended that both provide a natural explanation of the observed correlation between accretion and outflow. The kinematic, thermal, and chemical wind properties predicted by these models are described and their observational implications are considered. It is suggested that the wind characteristics may be reflected in the observed forbidden line and IR continuum emission of T Tauri stars and in the measured abundances of various molecular species.

Description: A k-epsilon model is proposed for wall bonded turbulent flows. In this model, the eddy viscosity is characterized by a turbulent velocity scale and a turbulent time scale. The time scale is bounded from below by the Kolmogorov time scale. The dissipation equation is reformulated using this time scale and no singularity exists at the wall. The damping function used in the eddy viscosity is chosen to be a function of R(sub y) = (k(sup 1/2)y)/v instead of y(+). Hence, the model could be used for flows with separation. The model constants used are the same as in the high Reynolds number standard k-epsilon model. Thus, the proposed model will be also suitable for flows far from the wall. Turbulent channel flows at different Reynolds numbers and turbulent boundary layer flows with and without pressure gradient are calculated. Results show that the model predictions are in good agreement with direct numerical simulation and experimental data.

Description: We predict the spectrum and flux of the high-energy diffuse cosmic gamma-ray background radiation produced by blazars. Our calculations are based on the Compton Observatory EGRET observations of high-energy gamma-rays from these objects, the Whipple observation of TeV gamma-rays from Mrk 421, and radio population studies of flat-spectrum radio sources. Our statistical analysis gives a result consistent with a linear correlation. We then use the radio luminosity function and redshift evolution to calculate the gamma-ray background. We find that blazars cannot account for the flux level or steep spectrum of the background observed by SAS 2 in the sub-GeV range, but could likely provide the dominant background component in the multi-GeV energy range and above. We predict a differential photon spectral index for this component of about 2 up to about 10 GeV. Above that energy, the spectrum should gradually steepen owing to pair production by interactions of the gamma-rays with intergalactic infrared photons. Using the calculations of this effect by Stecker et al. (1992), we estimate a spectral index of about 3.5 for energies above a TeV.

Description: Several models of the R Aqr northeast jet have been proposed which attempt to explain the presence of shock excitation, resulting in a confusing picture of this nearest astrophysical jet. This paper compares observations of the jet in the R Aqr system with the HST's Faint Object Camera (FOC) and radio continuum images acquired with the VLA. The forbidden-O III jet structure is derived from restored FOC imagery, which has been convolved with an elliptical Gaussian of the same size as the restoring beam of the 6-cm VLA maps, allowing image comparison at the same spatial resolution. It is found that, at increasing distances from the central star, the forbidden O III emission knots that comprise the optical jet occur at systematically larger position angles when compared with corresponding features in the radio images. The angular separation of forbidden O III and radio continuum emission can be understood in terms of a shock formed when ejecta in the stream interacts with previously existing circumstellar material and subsequently cools by nebular line emission.

Description: H2 is the most abundant molecule in the universe. We demonstrate that this molecule may be an important component of interstellar and possibly intergalactic ices, both because it can be formed in situ, within the ices, and because gas-phase H2 can freeze out onto dust grains in some astrophysical environments. The condensation-sublimation and infrared spectral properties of ices containing H2 are presented. We show that solid H2 in H2O-rich ices can be detected by an infrared absorption band at 4137/cm (2.417 microns). The surface binding energy of H2 to H2O ice was measured to be Delta-H(s)/k = 555 +/- 35 K. Surface binding energies can be used to calculate the residence times of H2 on grain surfaces as a function of temperature. Some of the implications of these results are considered.

Description: Results are presented of an experimental and numerical study of a turbulent boundary layer with pressure gradients conducted using the recent 'fringe method' with its numerical advantages and good inflow quality. After an inflow transient good agreement is observed; the differences, of up to 13 percent, are discussed. Moderate deviations from the law of the wall are found in the velocity profiles of the simulation. They are fully correlated with the pressure gradient, are in fair quantitative agreement with the experimental results of Nagano et al. (1992), and are roughly the opposite of uncorrected mixing-length-model predictions. Large deviations from the wall scaling are observed for other quantities, notably for the turbulence dissipation rate. The a(1) structure parameter drops mildly in the upper layer with adverse pressure gradient.

Description: Numerical solution of the MHD wave equations for stability of the cometary sheath determined by the balance between the inward Lorentz body force and the outward ion-neutral drag force is obtained by using a two-point boundary value method. The eigenvalues and the eigenfunctions are obtained numerically by treating the cometary inner sheath as a layer of finite thickness, bounded by the contact surface, i.e., the diamagnetic cavity boundary. The magnetic field structure discovered in the ionosphere of Comets Halley and Giacobini-Zinner is found to be unstable. The effects of finite plasma pressure, dissociative recombination, and mass loading due to photoionization are found to be stabilizing but are unable to quench the instability completely. It is also found that the higher the neutral production rate the lesser is the growth rate for the instability.

Description: The paper formulates a tensorially consistent near-wall second-order closure model. Redistributive terms in the Reynolds stress equations are modeled by an elliptic relaxation equation in order to represent strongly nonhomogeneous effects produced by the presence of walls; this replaces the quasi-homogeneous algebraic models that are usually employed, and avoids the need for ad hoc damping functions. The model is solved for channel flow and boundary layers with zero and adverse pressure gradients. Good predictions of Reynolds stress components, mean flow, skin friction, and displacement thickness are obtained in various comparisons to experimental and direct numerical simulation data. The model is also applied to a boundary layer flowing along a wall with a 90-deg, constant-radius, convex bend.

Description: A computer code for solving the Reynolds-averaged full Navier-Stokes equations has been developed and applied using H- and C-type grids. The Baldwin-Lomax eddy-viscosity model is used for turbulence closure. The integration in time is based on an explicit four-stage Runge-Kutta scheme. Local time stepping, variable coefficient implicit residual smoothing, and a full multigrid method have been implemented to accelerate steady-state calculations. A grid independence analysis is presented for a transonic rotor blade. Comparisons with experimental data show that the code is an accurate viscous solver and can give very good blade-to-blade predictions for engineering applications.

Description: Low-resolution UV observations of the bright, low-redshift BL Lac object PKS 2155-304 have been obtained by the HST's high-resolution spectrograph. Attention is here given to results on the number of intergalactic H I Ly-alpha components in the line-of-sight toward this object. The highest-redshift of the observed Ly-alpha systems furnishes a lower limit on the distance of the BL Lac object; this distance is consistent with the approximate redshift deduced from CCD imagery of the probable host galaxy for PKS 2155-304.

Description: Space and astrophysical plasmas often comprise a number of massive ion components in addition to a tenuous, negatively charged dust component and an electron component. Stationary electrostatic double layers in a dusty plasma are investigated in a model treating the ion components as Boltzmann-distributed (inertialess) fluids. On comparison with the inertialess theory, one finds considerably reduced double layer existence parameter regimes. Significantly, highly nonlinear double layers are ruled out when ion inertia is incorporated. However, in the restricted parameter regimes in which the inertial theory predicts double layers for small ion/dust mass ratios (about 10 exp -15-10 exp -8) there is good qualitative agreement with inertialess theory. The reasons for these, and other discrepancies and similarities, are discussed.

Description: A new type of stellar envelope structure has been computationally discovered at very high stellar masses. The outer part of the envelope resembles a nearly detached, diffusely filled shell overlying an ultrahot surface of small radius. This structural anomaly is caused by a large iron bump occurring in the new opacities of Iglesias et al. (1992). The new stellar models with normal metallicity encounter a strong ionization-induced dynamical instability in the outer envelope as they rapidly transit the H-R diagram after the end of central hydrogen burning. Preliminary evolutionary and hydrodynamical calculations successfully mimic the most basic observed properties of Eta Carinae and other very luminous blue variables. The Humphreys-Davidson sloped line in the H-R diagram, however, seems to be unrelated to these variables, and is instead the observed terminus of the main-sequence phase of evolution if convective core overshooting is insignificant.

Description: We observed the 2-0 (C-12)O absorption bandhead from integrated starlight from the nuclear bulge of the starburst galaxy M82. The velocity dispersion of the stellar component in the central arcsecond is significantly larger than that of the ionized gas in the same region, implying that the stars and gas represent different kinematic systems. The M/L(K) in this nuclear region is essentially identical to that in our own nuclear bulge, but is much higher than in the starburst disk of M82 at larger radii. This implies that the near-infrared continuum from the central r is less than 7.5 pc of M82 is dominated by a nuclear bulge population that predates the enveloping starburst. This work is a first step in separating these two distinct populations in the central region of this well-observed starburst galaxy.

Description: High-resolution spectra of the chromospheric Na I D lines, Ca I 4227, Mg I 4571, 5167, and 5172, the Ca II H and K resonance and IR 8542.144 and 8662.170 lines, and H-alpha and H-beta, all observed simultaneously at the AAT, are presented. These data are presented as the observational basis for the self-consistent, semiempirical modeling of the outer photospheres and chromospheres of the target stars. Stellar activity is found in the low-chromosphere lines as core-filling and, in some stars, as line-broadening. Integrated fluxes are derived from the difference spectra, formed by subtracting quiet from active spectra, in the Ca II H and K resonance and IR 8542 and 8662 lines and in H-alpha, which are presented as chromospheric activity indicators. All the activity indices exhibit strong correlations, with the exception of the equivalent widths of H-alpha and H-beta, which are found to be relatively poor activity discriminators.

Description: A numerical study was conducted to investigate how bleed through a two-dimensional slot affects shock-wave induced, boundary-layer separation on a flat plate. This study is based on the ensemble-averaged, compressible, Navier-Stokes equations closed by the Baldwin-Lomax, algebraic turbulence model. The algorithm used to obtain solutions was the implicit, partially split, two-factored scheme of Steger. This study examined the effects of the following parameters in controlling shock-wave induced flow separation: location of slot in relation to where the incident shock wave impinged on the boundary layer, size of slot in relation to the boundary-layer thickness, number of slots, spacings between slots, and strength of the incident shock wave. This study also showed the nature of the very complex flowfield about the slot or slots and how the plenum affects the bleed process. The results of this study are relevant to problems where bleed is used to control shock-wave induced, boundary-layer separation (e.g., inside jet engine inlets and wind tunnels).

Description: Ground-based infrared observations of SN 1987A on day 612 after the explosion have yielded resolved line profiles of Co II, Ni I, Fe II at 10.52, 11.31, and 17.94 micron, respectively. The spectra were taken at a resolving power of about 1000 with an array grating spectrometer on the 4 m telescope of Cerro Tololo Inter-American Observatory. Based on the observed line intensities we have estimated the minimum mass of each ion: M(Co II) = (6.0 +/- 1.8) x 10 exp -5 solar mass; M(Ni I) = (1.1 +/- 0.1) x 10 exp -3 solar mass; and M(Fe II) = (8.0 +/- 1.5) x 10 exp -3 solar mass. From these we infer total masses for cobalt, nickel, and iron in the ejecta. The nickel and iron line profiles are markedly asymmetric. We interpret these as arising from two components, one centered on the stellar rest velocity with an approximately 3250 km/s full width, and the second at about +1200 km/s with an approximately 1100 km/s full width. The asymmetry may represent a large-scale fracturing of the ejecta by Rayleigh-Taylor instabilities.

Description: The interacting spiral galaxy pair UGC 12914/5 was mapped with the 'VLA' for a 1.49 GHz atlas of the IRAS Bright Galaxy Sample. Half of the flux from UGC 12914/5 arises from the gap between UGC 12914 and UGC 12915, across which the radio contours are drawn like filaments of taffy. The bright radio continuum connecting UGC 12914 with UGC 12915 is a synchrotron source which therefore probably contains both cosmic rays and magnetic fields. It is suggested that the filaments were stripped from UGC 12914 and UGC 12915 as their stellar disks interpenetrated during a recent encounter; this implies that the radio bridge is a relic whose properties contain information about direct collisions of galaxies with gaseous disks. New maps have been made of the radio continuum and H I line emission from UGC 12914/5 in order to verify the stripping hypothesis.

Description: Within 1 AU from Jupiter, the dust detector aboard the Ulysses spacecraft during the flyby on February 8, 1992 recorded periodic bursts of submicron dust particles with durations ranging from several hours to two days and occurring at about monthly intervals. These particles arrived at Ulysses in collimate streams radiating from close to the line-of-sight direction to Jupiter, suggesting a Jovian origin for the periodic bursts. Ulysses also detected a flux of micron-sized dust particles moving in high-velocity retrograde orbits. These grains are identified here as being of interstellar origin.

Description: The microphysics of coagulation between two, colliding, smooth, spherical grains in the elastic limit is investigated, and the criteria for sticking as a function of particle sizes, collision velocities, elastic properties, and binding energy are calculated. Critical relative velocities for coagulation were evaluated as a function of grain sizes for solicate, icy, and carbonaceous grains. It is concluded that efficient coagulation requires coverage of grain cores by an icy grain mantle. In this case, coagulation leads to only a doubling of the mass of a large grain within a dense core lifetime. It is concluded that coagulation can have a dramatic effect on the visible and, particularly, the UV portion of the extinction curve in dense clouds and on their IR spectrum.

Description: Spectral measurements of the emission from the disk of the Galaxy from keV to GeV energies are combined. It is found that this emission has a similar spatial distribution at all energies: a longitude extent of about +/-40 deg and a latitude extent of about less than 5 deg. The emission over most of the energy range is dominated by cosmic-ray electrons and protons interacting with interstellar matter. The ridge spectrum is estimated to be near 100 keV using observations by HEAO A-4 from 1977 to 1979. To determine the emission from point sources in the region the diffuse component is substracted from Galactic center observations made by wide-field gamma-ray instruments over the past 20 years. The Galactic ridge accounts for between one-third and one-half of the observed flux for most of the measurements. Substracting the diffuse emission and GRS 1758-258 emission from the HEXAGONE observations on May 22 gives a spectrum that is dominated by lines from positron annihilation.

Description: The near-IR light curve of Periodic Comet Halley 1986 III is analyzed and compared with C2 production, water production, and the visual light curve. This is the most complete IR light curve compiled to date for any comet. The scattering phase function at small sun-comet-earth angles is shown to affect the slope of near-IR light curve significantly. P/Halley's dust production, as inferred from the IR light curve showed an increased production rate near perihelion which appears to be correlated with the onset of significant jet activity. The near-IR light curve, visual light curve, C2, and water production rates displayed different heliocentric variations, suggesting that one parameter cannot be accurately estimated from another. This is particularly true of the early preperihelion visual light curve. A peak of 0.3-0.5 magnitude in the visual magnitude, representing the integrated brightness of the comet's visible coma, lagged the other parameters by about a day. The near-IR color, J-H, was less red during periods of strong dust activity.

Description: We have observed the region in Orion containing Herbig-Haro 1 as well as a number of young, active stars. This observation shows a similar X-ray morphology to that in the first X-ray imaging observation about 10 years ago. The ROSAT High Resolution Instrument with its approximately arcsecond spatial resolution allows us in most cases to make definite optical identifications of the 0.1-2.4 keV X-ray sources. New identifications with emission-line stars are made, and prior identifications using lower resolution observations are confirmed or corrected. The X-ray emission previously detected from the vicinity of HH-1 is not associated with HH-1 but with a known T Tauri star. The observed relationships among X-ray, optical line, and infrared excess emission do not simply fall into the suggested classifications for T Tauri or T Tauri-like stars. This could be an indication of another X-ray emission region such as accretion disks that add to the emission from the stellar atmospheres.

Description: The numerical gasdynamics code ZEUS-2D is used to directly model the dynamical overstabilities in blast waves. The linear analysis is confirmed by perturbing a blast wave with a low-amplitude eigenfunction of the overstability. The amplitude of the perturbations is increased in order to determine the nonlinear behavior of the overstabilities. The overstability is found to saturate due to weak transverse shocks in the shell. Transverse velocities in the dense shell reach the postshock sound speed, and high-density regions with sizes of the order of the shell thickness form. Transverse oscillations continue even after saturation. This confirms and explains the damping of the overstability experimentally discovered by Grun et al. (1991).

Description: Ten strip maps of 1665- and 1667-MHz OH emission, traversing the outer boundaries of five molecular clouds, were made. The OH emission is found to be significantly extended relative to CO, from which it inferred that OH is to be found in abundance in the shell of partly atomic, partly molecular gas surrounding the dense molecular clouds. The fractional OH abundance is calculated using existing H I and CO observations, and detailed source models which include a complete chemistry network and a radiative transfer code. It is concluded that the extended OH is formed, not by the exothermic reaction of O with H3(+), but by the endothermic reaction, H(+) + O yields H + O(+).

Description: Turbulent flow in a 2D channel with repeated rectangular rib roughness was numerically simulated using a low Reynolds number form of the k-epsilon turbulence model. Friction factors and average Stanton numbers were calculated for various pitch to rib height ratios and bulk Reynolds numbers. Comparisons with experiment were generally adequate, with the predictions of friction superior to those for heat transfer. The effect of variable properties for channel flow was investigated, and the results showed a greater effect for friction than for heat transfer. Comparison with experiment yielded no clear conclusions. The turbulence model was also validated for a related problem, that of flow downstream of an abrupt pipe expansion.

Description: The purpose of this note is to construct a local solution that eliminates a residual velocity discontinuity in the inviscid portion of a solution obtained in a recent paper by Goldstein, Leib and Cowley (1992). This result is of importance because it shows that the solution obtained in that paper is entirely non-singular outside the viscous wall boundary layer and that any singularity in the problem will have to arise in the usual way through a breakdown in the viscous boundary layer.

Description: We use a set of equations, sometimes referred to as the 'delta-equations', to approximate the two-dimensional inviscid motion of an initially circular vortex sheet released from rest in a cross-flow. We present numerical solutions of these equations for the case with delta-square = 0 (for which the equations are exact) and for delta-square greater than 0. For small values of the smoothing parameter delta, a spectral filter must be used to eliminate spurious instabilities due to round-off error. Two singularities appear simultaneously in the vortex sheet when delta-square = 0 at a critical time t(c). After t(c), the solutions do not converge as the computational mesh is refined. With delta-square greater than 0, converged solutions were found for all values of delta-square when t is less than t(c), and for all but the two smallest values of delta-square used when t is greater than t(c). Our results show that, when delta-square is greater than 0, the vortex sheet deforms into two doubly branched spirals some time after t(c). The limiting solution as delta approaching 0 clearly exists and equals the delta = 0 solution when t is less than t(c).

Description: The evolution of three-dimensional temporally evolving plane mixing layers through as many as three pairings has been simulated numerically. All simulations were begun from a few low-wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity. Three-dimensional perturbations were used with amplitudes ranging from infinitesimal to large enough to trigger a rapid transition to turbulence. Pairing is found to inhibit the growth of infinitesimal three-dimensional disturbances, and to trigger the transition to turbulence in highly three-dimensional flows. The mechanisms responsible for the growth of three-dimensionality and onset of transition to turbulence are described. The transition to turbulence is accompanied by the formation of thin sheets of spanwise vorticity, which undergo secondary rollups. The post-transitional simulated flow fields exhibit many properties characteristic of turbulent flows.

Description: A study has been made of energetic particle data, obtained from IMP 8, in conjunction with solar wind field and plasma data at the times of reported magnetic clouds. It is shown that magnetic clouds can cause a depression of the cosmic ray flux but high fields are required. A depression of 3 percent in a neutron monitor requires a field of about 25 nT. Such high fields are found only in a subset of coronal ejecta. The principal cause for Forbush decreases associated with energetic shocks is probably turbulence in the postshock region, although some shocks will be followed by an ejecta with a high field. Each event is different. The lower-energy particles can help in identifying the dominant processes in individual events.

Description: Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Description: The young planetary nebula Hubble 12 is observed using near-IR and UV spectrophotometry. The brightness of the O I lines, which is greater than in any other planetary nebula yet measured, indicates that fluorescent excitation by stellar continuum is the principal mechanism generating these lines. Extinction, electron density, and electron temperature are determined using infrared measurements combined with UV data and published optical observations. The range in extinction, density, and temperature implies that, within the ionized region, pockets of emission with distinctly different conditions exist. Logarithmic abundances for helium, oxygen, and sulfur are presented.

Description: An analytical solution was obtained by Siegel (1991, 1992) for local boundary heat fluxes by a radiating medium at uniform temperature in a 2D rectangular region. It is shown here that, after local fluxes from the medium to the walls have been evaluated, it is very easy to compute local fluxes arriving from the adjacent and opposite walls. This extends the previous analysis and provides convenient relations to include radiation from a black boundary, each side of the rectangle being at a different uniform temperature. The final expressions are helpful in performing spectral calculations that must be made for many spectral bands.

Description: Effects of helicity in three-dimensional incompressible inhomogeneous turbulence are examined with the aid of a two-scale direct-interaction approximation (DIA). The turbulent helicity gives a measure of the reflectional asymmetry in a turbulent flow and its inhomogeneity contributes to the sustainment of large-scale vorticity field in a three-dimensional mean flow. The importance of helicity effects is discussed in the context of flows in a rotating system and swirling flows in a pipe. A three-equation model with the turbulent helicity incorporated is proposed using the theoretical results. The validity of the model is confirmed quantitatively through the application to a decaying swirling flow in a pipe.

Description: The correlations deduced by Boulanger et al. (1990) from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary hydrogenated amorphous carbon model of interstellar dust. In particular, regions of relatively strong 12-micron emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak 12-micron emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.

Description: Observations of the SMC using the Energetic Gamma Ray Experimental Telescope (EGRET) on the Compton Observatory are reported. The findings yield an upper limit for gamma-ray emission above 100 MeV of 0.5 x 10 exp -7 photon/sq cm s. The expected flux if the cosmic rays (CR) are universal rather than Galactic in origin is (2.4 +/- 0.5) x 10 exp -7 photon/sq cm s, only a third of which arises from cosmic ray electron interactions. Thus, the bulk of the CR energy density is almost certainly neither metagalactic nor universal, but Galactic in origin. The results add to the evidence that the SMC is in a nonequilibrium state and indicate that the LMC is most likely in quasi-stable equilibrium, with a CR energy density near the maximum that can be contained.

Description: We propose a new interpretation of the behavior of the notable X-ray binary source Centaurus X-3. Based on both theoretical and observational arguments (using EXOSAT data), we suggest that an X-ray excited wind emanating from the O star is present in this system. Further, we suggest that this wind is responsible for the mass transfer in the system rather than Roche-lobe overflow or a normal radiatively driven stellar wind. We show that the ionization conditions in Cen X-3 are too extreme to permit a normal radiatively driven wind to emanate from portions of the stellar surface facing toward the neutron star. In addition, the flux of X-rays from the neutron star is strong enough to drive a thermal wind from the O star with sufficient mass-flux to power the X-ray source. We find that this model can reasonably account for the long duration of the eclipse transitions and other observed features of Cen X-3. If confirmed, this will be the first example of an X-ray excited wind in a massive binary. We also discuss the relationship between the excited wind in Cen X-3 to the situation in eclipsing millisecond pulsars, where an excited wind is also believed to be present.

Description: Line profile data are used to test a simple kinematic model - spherically symmetric gravitational free fall - in which the number of free parameters is limited by requiring physical self-consistency. The predictions of this model are fitted to high-resolution spectra of the stronger rest-frame UV emission lines in 12 quasars with z of about 2. It is found that if all the lines are radiated predominantly from the illuminated faces of the emission-line clouds, the profiles of Ly-alpha, N V 1240 A, and C IV 1549 A can be simultaneously well fitted with very similar parameters for all 12 quasars. It is concluded that spherically symmetric gravitational free fall does not correctly describe the dynamics of quasar broad emission-line regions.

Description: We present a comprehensive model for the powerful H2O masers observed in starforming regions. In this model the masers occur behind dissociative shocks propagating in dense regions. This paper focuses on high-velocity dissociative shocks in which the heat of H2 reformation on dust grains maintains a large column of 300 - 400 K gas, where the chemistry drives a considerable fraction of the oxygen not in CO to form H2O. The H2O column densities, the hydrogen densities, and the warm temperatures produced by these shocks are sufficiently high to enable powerful maser action, where the maser is excited by thermal collisions with H atoms and H2 molecules. A critical ingredient in determining the shock structure is the magnetic pressure, and the fields required by our models are in agreement with recent observations. The observed brightness temperatures are the result of coherent velocity regions which have dimensions in the shock plane that are five to 50 times the postshock thickness.

Description: A Runge-Kutta formula in time is presently used to advance schemes in which central differences are used to solve the time-dependent Euler equations; a second difference is added near shocks as an artificial viscosity to reduce the given scheme to a first-order upwind one at shocks. A matrix-valued dissipation is introduced and compared with the scalar viscosity; a connection is shown between this artificial viscosity and flux limiters. The use of various flux limiters for this central difference scheme is compared.

Description: The Broad-Band X-Ray Telescope obtained moderate-resolution (about 90 eV) X-ray spectra of the O4 f star Xi Pup during the STS 35/Astro 1 mission in 1990 December. Despite the shortness of the observation (600 s), the data show a surprising amount of detail. We report the detection of an O absorption edge produced by ionized overlying wind material and K-shell line emission from Mg produced by a thermal plasma of temperature near 6 x 10 exp 6 K. The data are used to place constraints on the location, temperature, and amount of the X-ray-emitting gas, as well as the abundance and ionization of the wind material.

Description: New ultraviolet short-wavelength (SWP) IUE observations for the proto-planetary nebula Vy 2-2 were obtained in the low-dispersion mode. An exposure of 400-min duration reveals a weak stellar continuum and several definite emission lines as well as a number of absorption features. Emission features of semiforbidden C III, semiforbidden Si III, and forbidden Ne III are clearly observed, while C IV, Si II, and possibly C II appear in absorption. Emission features of He II, semiforbidden O III, and Al III are also indicated, though less certain. The UV spectrum tends to confirm that Vy 2-2 is a young, dense planetary nebula excited by a central star with T(eff) greater than 60,000 K.

Description: Rate constants for excitation of CO by collisions with H2O are needed to understand recent observations of comet spectra. These collision rates are closely related to spectral line shape parameters, especially those for Raman Q-branch spectra. Because such spectra have become quite important for thermometry applications, much effort has been invested in understanding this process. Although it is not generally possible to extract state-to-state rate constants directly from the data as there are too many unknowns, if the matrix of state-to-state rates can be expressed in terms of a rate-law model which depends only on rotational quantum numbers plus a few parameters, the parameters can be determined from the data; this has been done with some success for many systems, especially those relevant to combustion processes. Although such an analysis has not yet been done for CO-H2O, this system is expected to behave similarly to N2-H2O which has been well studies; modifications of parameters for the latter system are suggested which should provide a reasonable description of rate constants for the former.

Description: Effects of recent opacity revisions on the evolution of Population I stars are explored over the range 1.5-60 solar masses. Opacity parameters considered include the angular momentum coupling scheme for iron, the relative iron abundance, the total metal abundance, and diatomic and triatomic molecular sources. Only the total metal abundance exerts an important control over the evolutionary tracks. Blue loops on the H-R diagram during core helium burning can be very sensitive to opacity, but only insofar as the simple formation or suppression of a blue loop is concerned. The blue loops are most robust for stellar masses around 10 solar masses. We confirm, from a comparison of stellar models with observational data, that the total metal abundance is close to solar and that convective core overshooting is likely to be very slight. The new models predict the existence of an iron convection zone in the envelope and a great widening of the main-sequence band in the H-R diagram at luminosities brighter than 100,000 solar luminosities.