ALBERT

Description: Our universe is most luminous at far-infrared and submillimeter wavelengths (100 GHz - 10 THz) after the Cosmic Microwave Background (CMB) radiation. This region of the electromagnetic spectrum provides critical tracers for the study of a wide range of astrophysical and planetary phenomena. This spectral range contains information on the origin of the planets, stars, galaxies, and clusters; the geometry and matter/energy content of the Universe, atmospheric constituents and dynamics of the planets and comets and tracers for global monitoring and the ultimate health of the Earth. Sensors at far-infrared and submillimeter wavelengths provide unprecedented sensitivity for astrophysical, planetary, and earth observing instruments. Very often, for a spaced based platform where the instruments are not limited by atmospheric losses and absorption, the overall instrument sensitivity is dictated by the sensitivity of the sensors themselves. Moreover, some of the cryogenic sensors at submillimeter wavelengths provide almost quantum-limited sensitivity. This paper provides an overview of the submillimeter-wave sensors and their performance and capabilities for space applications.

Description: The large numbers of nucleus fragments observed are a spectacular illustration of the process of cascading fragmentation in progress, a concept introduced to interpret the properties of the Kreutz system of sungrazers and comet D/1993 F2. The objective is to describe the fragmentation sequence and hierarchy of comet 73P, the nature of the fragmentation process and observed events, and the expected future evolution of this comet. The orbital arc populated by the fragments refers to an interval of 3.74 days in the perihelion time. This result suggests that they all could be products (but not necessarily first-generation fragments) of two 1995 events, in early September (involving an enormous outburst) and at the beginning of November. The interval of perihelion times is equivalent to a range of about 2.5 m/s in separation velocity or 0.00012 the Sun's attraction in nongravitational deceleration. Their combined effect suggests minor orbital momentum changes acquired during fragmentation and decelerations compatible with survival over two revolutions about the Sun. Fragment B is a likely first-generation product of one of the 1995 events. From the behavior of the primary fragment C, 73P is not a dying comet, even though fragment B and others were episodically breaking up into many pieces. Each episode began with the sudden appearance of a starlike nucleus condensation and a rapidly expanding outburst, followed by a development of jets, and a gradual tailward extension of the fading condensation, until the discrete masses embedded in it could be resolved. In April-May, this debris traveled first to the southwest, but models show their eventual motion toward the projected orbit. Fainter fragments were imaged over limited time, apparently because of their erratic activity (interspersed with periods of dormancy) rather than improptu disintegration. A dust trail joining the fragments and reminiscent of comet 141P/Machholz suggests that cascading fragmentation exerts itself profoundly over an extremely broad mass range of particulate debris.

Description: The National Academy of Sciences (NAS) was chartered by Congress, under the leadership of President Abraham Lincoln, to provide scientific and technical advice to the government of the United States. Over the years, the advisory program of the institution has expanded, leading to the establishment of the National Academy of Engineering (NAE) and the Institute of Medicine, and of the National Research Council (NRC), the operational arm of the National Academies. The original charter of the Space Science Board was established in June 1958, three months before NASA opened its doors. The Space Science Board and its successor, the Space Studies Board (SSB), have provided expert external and independent scientific and programmatic advice to NASA on a continuous basis from NASA's inception until the present. The Board has also provided such advice to other agencies, including the National Oceanic and Atmospheric Administration, the National Science Foundation, the U.S. Geological Survey, and the Department of Defense, and responds to requests from Congress. Early in 2005, the leadership of NASA changed, and with it new emphases emerged. Some of the early interpretations of the 2004 Vision for Space Exploration, in which only certain aspects of space science were encouraged, disappeared and a broader mandate for science emerged. But what also emerged was fiscal reality, which precluded many of the exciting activities that were planned for NASA's science programs. In this environment of change, there has been a continuing need to evaluate NASA's plans against the strategies for science that have been laid down in the various NRC decadal surveys, and to assist NASA in determining how best to proceed given the reduced level of resources it will have. Coupled with this has been a continuing need to provide Congress with the assessments of NASA's plans that it requests.

Description: Radial velocity observations of the F8 V star nu Andromedae taken at Lick and at Whipple Observatories have revealed evidence of three periodicities in the line-of-sight velocity of the star. These periodicities have been interpreted as evidence for at least three low-mass companions (LMCs) revolving around nu Andromedae. The mass and orbital parameters inferred for these companions raise questions about the dynamical stability of the system. We report here results from our independent analysis of the published radial velocity data, as well as new unpublished data taken at Lick Observatory. Our results confirm the finding of three periods in the data. Our best fits to the data, on the assumption that these periods arise from the gravitational perturbations of companions in Keplerian orbits, are also generally in agreement but with some differences from the earlier findings. We find that the available data do not constrain well the orbital eccentricity of the middle companion in a three-companion model of the data. We also find that in order for our best-fit model to the Lick data to be dynamically stable over the lifetime of the star (approximately 2 billion years), the system must have a mean inclination to the plane of the sky greater than 13 deg. The corresponding minimum inclination for the best fit to the Whipple data set is 19 deg. These values imply that the maximum mass for the outer companion can be no greater than about 20 Jupiter masses. Our analysis of the stability of the putative systems also places constraints on the relative inclinations of the orbital planes of the companions. We comment on global versus local (i.e., method of steepest descent) means of finding best-fit orbits from radial velocity data sets.

Description: Three-dimensional numerical simulations of compressible turbulent thermally driven convection, in both slab and spheroidal geometries, are reviewed and analyzed in terms of velocity spectra and mixing-length theory. The same ideal gas model is used in both geometries, and resulting flows are compared. The piecewise-parabolic method (PPM), with either thermal conductivity or photospheric boundary conditions, is used to solve the fluid equations of motion. Fluid motions in both geometries exhibit a Kolmogorov-like k(sup -5/3) range in their velocity spectra. The longest wavelength modes are energetically dominant in both geometries, typically leading to one convection cell dominating the flow. In spheroidal geometry, a dipolar flow dominates the largest scale convective motions. Downflows are intensely turbulent and up drafts are relatively laminar in both geometries. In slab geometry, correlations between temperature and velocity fluctuations, which lead to the enthalpy flux, are fairly independent of depth. In spheroidal geometry this same correlation increases linearly with radius over the inner 70 percent by radius, in which the local pressure scale heights are a sizable fraction of the radius. The effects from the impenetrable boundary conditions in the slab geometry models are confused with the effects from non-local convection. In spheroidal geometry nonlocal effects, due to coherent plumes, are seen as far as several pressure scale heights from the lower boundary and are clearly distinguishable from boundary effects.

Description: Asteroid astrometry, like any other scientific measurement process, is subject to both random and systematic errors, not all of which are under the observer's control. To design an astrometric observing program or to improve an existing one requires knowledge of the various sources of error, how different errors affect one's results, and how various errors may be minimized by careful observation or data reduction techniques.

Description: We show that the circular character of continuum structures observed in the coma of comet Hale-Bopp around the perihelion passage is most likely due to a dust jet from a large extended active region on the surface. Coma morphology due to a wide jet is different from that due to a narrow jet. The latter shows foreshortening effects due to observing geometry, wider jet produces more circular features. This circularization effect provides a self-consistent explanation for the evolution of near-perihelion coma morphology. No changes in the direction of the rotational angular momentum vector are required during this period in contrast to the models of Schleicher et al. This circularization effect also enables us to produce near-circular coma features in the S-E quadrant during 1997 late February and therefore questions the basic premise on which Sekanina bases his morphological arguments for a gravitationally bound satellite nucleus.

Description: We studied 14 chondrules separated from LEW85332, an ungrouped type-3 carbonaceous chondrite related to CR chondrites; 23 elements were determined by neutron activation and the chondrules were characterized petrographically. Oxygen isotopic compositions were determined by R. N. Clayton and T. K. Mayeda for seven chondrules. Chondrule abundance ratios tend to form one of two distinct patterns. In low FeO chondrules, refractory lithophile patterns are flat (i.e., unfractionated); siderophile abundances are high and show a small decrease with increasing volatility. Although high FeO chondrules also have flat refractory lithophile abundance patterns, siderophile abundances are highly fractionated; refractory Ir is very low and Fe is very high relative to other siderophiles. We suggest that the low FeO chondrules in LEW85332 formed early in nebular history when metal was intimately mixed with silicates in the chondrule precursors, and that the viscosity of the liquid-solid mix was too high to permit expulsion of the metal by centrifugal action; their porphyritic structures are consistent with incomplete melting, which would result in relatively high viscosities. When the high FeO chondrules formed somewhat later, much of the Fe was oxidized and the melting of precursors was more extensive, FeO and other oxidized siderophiles were retained in the silicate liquid, and metal was lost, possibly expelled from low viscosity chondrule melts. The O isotopic compositions of the chondrules form a linear array of slope 0.93 +/- 0.05 on a three-isotope diagram, parallel to the carbonaceous chondrite anhydrous minerals (CCAM) line and a CR chondrule array, but offset from the latter by -1% in (delta)O-18. Some or all of this offset may reflect incorporation of O from Antarctic water during weathering. Chondrule (Delta)O-17 values correlate positively with FeO, possibly indicating that the (Delta)O-17 of the nebular gas composition increased with time. The chemical and O isotopic data suggest a temporal sequence extending from early, low FeO to late high FeO chondrules.

Description: From 1996 June 10 to 1996 July 29, the International AGN Watch monitored the Seyfert 1 galaxy NGC 7469 using the International Ultraviolet Explorer, the Rossi X-Ray Timing Explorer, and a network of ground-based observatories. On 1996 June 18, in the midst of this intensive monitoring period, we obtained a high signal-to-noise snapshot of the UV spectrum from 1150 to 3300 A, using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope. This spectrum allows us to disentangle the UV continuum more accurately from the broad wings of the emission lines, to identify clean continuum windows free of contaminating emission and absorption, and to deblend line complexes such as Ly(alpha) + N V, C IV + He II + O III], Si III] + C III], and Mg II + Fe II. Using the FOS spectrum as a template, we have fitted and extracted line and continuum fluxes from the IUE monitoring data. The cleaner continuum extractions c o n h the discovery of time delays between the different UV continuum bands by Wanders et al. Our new measurements show delays increasing with wavelength for continuum bands centered at 1485, 1740, and 1825 A, relative to 1315 A with delays of 0.09, 0.28, and 0.36 days, respectively. Like many other Seyfert I galaxies, the UV spectrum of NGC 7469 shows intrinsic, blue-shifted absorption in Ly(alpha), N V, and C IV. Soft X-ray absorption is also visible in archival ASCA X-ray spectra. The strength of the UV absorption, however, is not compatible with a single-zone model in which the same material absorbs both the UV and X-ray light. Similar to other Seyfert galaxies, such as NGC 3516, the UV-absorbing gas in NGC 7469 has a lower ionization parameter and column density than the X-ray-absorbing material. While the UV and X-ray absorption does not arise in the same material, the frequent occurrence of both associated UV absorption and X-ray warm absorbers in the same galaxies suggests that the gas supply for each has a common origin.

Description: The last two years have seen incredible development in numerical relativity: from fractions of an orbit, evolutions of an equal-mass binary have reached multiple orbits, and convergent gravitational waveforms have been produced from several research groups and numerical codes. We are now able to move our attention from pure numerics to astrophysics, and address scenarios relevant to current and future gravitational-wave detectors.Over the last 12 months at NASA Goddard, we have extended the accuracy of our Hahn-Dol code, and used it to move toward these goals. We have achieved high-accuracy simulations of black-hole binaries of low initial eccentricity, with enough orbits of inspiral before merger to allow us to produce hybrid waveforms that reflect accurately the entire lifetime of the BH binary. We are extending this work, looking at the effects of unequal masses and spins.