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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Phase space simulation of collisionless stellar systems on the massively parallel processor

A numerical technique for solving the collisionless Boltzmann equation describing the time evolution of a self gravitating fluid in phase space was implemented on the Massively Parallel Processor (MPP). The code performs calculations for a two dimensional phase space grid (with one space and one velocity dimension). Some results from calculations are presented. The execution speed of the code is comparable to the speed of a single processor of a Cray-XMP. Advantages and disadvantages of the MPP architecture for this type of problem are discussed. The nearest neighbor connectivity of the MPP array does not pose a significant obstacle. Future MPP-like machines should have much more local memory and easier access to staging memory and disks in order to be effective for this type of problem.

White, Richard L.↗
NASA NTRS1986

On star formation in stellar systems. I - Photoionization effects in protoglobular clusters

The progressive ionization and subsequent dynamical evolution of nonhomogeneously distributed low-metal-abundance diffuse gas after star formation in globular clusters are investigated analytically, taking the gravitational acceleration due to the stars into account. The basic equations are derived; the underlying assumptions, input parameters, and solution methods are explained; and numerical results for three standard cases (ionization during star formation, ionization during expansion, and evolution resulting in a stable H II region at its equilibrium Stromgren radius) are presented in graphs and characterized in detail. The time scale of residual-gas loss in typical clusters is found to be about the same as the lifetime of a massive star on the main sequence.

Tenorio-Tagle, G.↗
NASA NTRS1989

On star formation in stellar systems. II - Photoionization in protodwarf galaxies

Numerical hydrodynamical calculations are used to study the effects of the onset of star formation on the residual gas in a primordial low-mass Local-Group dwarf spheroidal galaxy in the size range 0.3-1.0 kpc. It is demonstrated that photoionization in the presence of a moderate gas-density gradient can be responsible for gas ejection on a time-scale of a few times 10 to the 7th yr. The results indicate that, given a normal initial mass function, many protodwarf galaxies may have been dispersed by the onset of star formation.

Noriega-Crespo, A.↗
NASA NTRS1990

Dynamical instabilities in axisymmetric stellar systems. I - Oblate E6 models

The stability of a set of models based on isothermal oblate E6 elliptical galaxies is studied using N-body techniques. The only stable models are those that are near the isotropic model and have nearly equal number of stars in retrograde and prograde orbits. Fast rotators are unstable to modes that appear to be analogous to the classical streaming instability seen in many disk systems. Systems with a large velocity dispersion in the direction of the cylindrical radius are unstable to modes that appear to be similar to the radial orbit instability observed in some spherical systems. However, evidence is presented that these two instabilities may be related, and an instability criterion that applies to both is constructed.

Levison, Harold F.↗
NASA NTRS1992

Equilibrium stellar systems with spindle singularities

Equilibrium sequences of axisymmetric Newtonian clusters that tend toward singular states are constructed. The distribution functions are chosen to be of the form f = f(E, Jz). The numerical method then determines the density and gravitational potential self-consistently to satisfy Poisson's equation. For the prolate models, spindle singularities arise from the depletion of angular momentum near the symmetry axis. While the resulting density enhancement is confined to the region near the axis, the influence of the spindle extends much further out through its tidal gravitational field. Centrally condensed prolate clusters may contain strong-field regions even though the spindle mass is small and the mean cluster eccentricity is not extreme. While the calculations performed here are entirely Newtonian, the issue of singularities is an important topic in general relativity. Equilibrium solutions for relativistic star clusters can provide a testing ground for exploring this issue. The methods used in this paper for building nonspherical clusters can be extended to relativistic systems.

Shapiro, Stuart L.↗
NASA NTRS1998

HD 98800: A Unique Stellar System of Post-T Tauri Stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Soderblom, David R.↗
NASA NTRS1999

Infalling Planetesimals in Pre-Main Sequence Stellar Systems

These are exciting times in the study of planetary system formation with a steadily expanding inventory of exo-planet detections, and imaging of dust disks around nearby young and main sequence stars. While these discoveries imply that our Solar System is far from unique, linking the data for the protoplanetary and debris disks to mature planetary systems requires a demonstration that disk evolution proceeds via planetesimal production and growth to the formation of planets. Theoretical studies of planet formation indicate that planetesimals grow, via runaway accretion, to lunar-sized (approx. = 2000 km) embryos in 10(exp 5) years. Recent gas giant planet formation studies have suggested that most of the action in planet formation occurs over 1-16 Myr, with formation of planets similar to Jupiter in t less than 10 Myr, within the time interval that infrared (IR) and optical emission line studies have demonstrated that circumstellar material remains detectable around both solar mass and intermediate mass stars. Direct imaging of exo-planetesimals is not feasible with current and foreseeable technology, since such bodies have substantially less surface area than micron-sized grains distributed in a disk, and thus are inefficient IR emitters. However, such bodies may be indirectly detectable.

Grady, Carol A.↗
NASA NTRS2004

The Structure of Young Stellar Systems: Establishing the Initial Conditions for Planet Formation

FYO1-FY03 Origins support to the PI had been as part of an effort involving Profs. Blake, van Dishoeck and L.G. Mundy of the University of Maryland. As outlined below, the merging of BIMA+OVRO into the Combined Array for Research in Millimeter Astronomy (CARMA) and our mutual involvement in the Spitzer Legacy Science program now provides a more natural means of continuing the Caltech/Maryland collaboration. In addition, the spectroscopy/radiative transfer programs led by Profs. Blake, Hogerheijde, and van Dishoeck form a more cohesive basis for a single proposal. Students supported by our previous Origins grant have been involved in making some of the first sub-arcsecond resolution images of the morphology and chemistry of individual YSOs at OVRO and BIMA, in the analysis of IR spectra taken by ISO, and in continuing exploratory IR diffraction-limited imaging and spectroscopy at the Keck and VLT observatories. Notable scientific accomplishments in the past grant period include: 1) detecting high density and temperature hot cores in the dynamically accreting envelopes of low mass protostars that contain significant quantities of complex, prebioitc molecules, 2) imaging chemical and isotopic zonation in the outer regions of T Tauri star accretion disks, particularly fractional ionization and D/H studies, 3) acquiring and modeling the first extensive CO v = 1 approaches 0 spectroscopy survey of the terrestrial planet-forming region of circumstellar accretion disks, and 4) optimizing detailed radiative transfer modeling of the molecular and dust emission from YSO envelopes and from comets, including a new parallelized implementation.

Blake, Geoffrey A.↗
NASA NTRS2001

The AstroBiology Explorer (ABE) MIDEX Mission Concept

The Astrobiology Explorer (ABE) is a MIDEX mission concept under study at NASA's Ames Research Center in collaboration with Ball Aerospace & Technologies, Corp. ABE will conduct IR spectroscopic observations to address important problems in astrobiology, astrochemistry, and astrophysics. The core observational program would make fundamental scientific progress in understanding the distribution, identity, and evolution of ices and organic matter in dense molecular clouds, young forming stellar systems, stellar outflows, the general diffuse ISM, HII regions, Solar System bodies, and external galaxies. The ABE instrument concept includes a 0.6 m aperture Cassegrain telescope and two moderate resolution (R = 2000-3000) spectrographs covering the 2.5-16 micron spectral region. Large format (1024x 1024 pixel or larger) IR detector arrays and bandpass filters will allow each spectrograph to cover an entire octave of spectral range or more per exposure without any moving parts. The telescope will be cooled below 50K by a cryogenic dewar shielded by a sunshade. The detectors will be cooled to approximately 8K. The optimum orbital configuration for achieving the scientific objectives of the ABE mission is a low background, 1 AU Earth driftaway orbit requiring a Delta II launch vehicle. This configuration provides a low thermal background and allows adequate communications bandwidth and good access to the entire sky over the approximate 1-2 year mission lifetime.

Ennico, Kimberly↗
NASA NTRS2002

The AstroBiology Explorer (ABE) MIDEX Mission Concept: Identifying Organic Molecules in Space

The Astrobiology Explorer (ABE) is a MIDEX mission concept, currently under Concept Phase A study at NASA's Ames Research Center in collaboration with Ball Aerospace & Technologies, Corp., and managed by NASA's Jet Propulsion Laboratory. ABE will conduct infrared spectroscopic observations to address important problems in astrobiology, astrochemistry, and astrophysics. The core observational program would make fundamental scientific progress in understanding the distribution, identity, and evolution of ices and organic matter in dense molecular clouds, young forming stellar systems, stellar outflows, the general diffuse ISM, HII regions, Solar System bodies, and external galaxies. The ABE instrument concept includes a 0.6 m aperture Ritchey-Chretien telescope and three moderate resolution (R = 2000-3000) spectrometers together covering the 2.5-20 micron spectral region. Large format (1024 x 1024 pixel) IR detector arrays will allow each spectrometer to cover an entire octave of spectral range per exposure without any moving parts. The telescope will be cooled below 50 K by a cryogenic dewar shielded by a sunshade. The detectors will be cooled to approx. 7.5 K by a solid hydrogen cryostat. The optimum orbital configuration for achieving the scientific objectives of the ABE mission is a low background, 1 AU Earth driftaway orbit requiring a Delta II launch vehicle. This configuration provides a low thermal background and allows adequate communications bandwidth and good access to the entire sky over the approx. 1.5 year mission lifetime.

Ennico, Kimberly↗
NASA NTRS1971

A stellar tracking reference system

A stellar attitude reference system concept for satellites was studied which promises to permit continuous precision pointing of payloads with accuracies of 0.001 degree without the use of gyroscopes. It is accomplished with the use of a single, clustered star tracker assembly mounted on a non-orthogonal, two gimbal mechanism, driven so as to unwind satellite orbital and orbit precession rates. A set of eight stars was found which assures the presence of an adequate inertial reference on a continuous basis in an arbitrary orbit. Acquisition and operational considerations were investigated and inherent reference redundancy/reliability was established. Preliminary designs for the gimbal mechanism, its servo drive, and the star tracker cluster with its associated signal processing were developed for a baseline sun-synchronous, noon-midnight orbit. The functions required of the onboard computer were determined and the equations to be solved were found. In addition detailed error analyses were carried out, based on structural, thermal and other operational considerations.

Klestadt, B.↗
NASA NTRSDate not supplied

Hardware Demonstration and Improvements of the Stellar Positioning System

As the number of Lunar and Martian surface-exploration missions increases, precise surface navigation is becoming critical. Of most interest is navigation techniques that can generate an absolute state without reliance on Earth-based tracking. One such navigation technique is the Stellar Positioning System. Based on the practice of celestial navigation, this approach combines measurements of the body, star field orientation, and time, to calculate an absolute position on the surface of any planetary body with a known gravity field and known orientation in celestial space. A hardware prototype consisting of an inertial measurement unit, star tracker, and accurate time keeping was developed to demonstrate this concept. The stellar positioning system model was refined to fit this hardware, and was demonstrated by conducting live-sky tests in multiple locations around Marshall Space Flight Center in Huntsville, AL. This effort discusses the preliminary testing results, improvements of the stellar positioning system, feasibility for surface exploration missions, and planned further refinements that will improve the performance.

Joel Amert↗