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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Modeling Circumstellar Disks of B-Type Stars with Observations from the Palomar Testbed Interferometer

Geometrical (uniform disk) and numerical models were calculated for a set of B-emission (Be) stars observed with the Palomar Testbed Interferometer (PTI). Physical extents have been estimated for the disks of a total of15 stars via uniform disk models. Our numerical non-LTE models used parameters for the B0, B2, B5, and B8spectral classes and following the framework laid by previous studies, we have compared them to infrared K-band interferometric observations taken at PTI. This is the first time such an extensive set of Be stars observed with long-baseline interferometry has been analyzed with self-consistent non-LTE numerical disk models.

Grzenia, B. J.↗
NASA NTRS1994

Two-dimensional hydrodynamical simulations of wind-compressed disks around rapidly rotating B stars

We use a two-dimensional piecewise parabolic method (PPM) code to simulate numerically the hydrodynamics of a radiation-driven stellar wind from a rapidly rotating Be star. The results generally confirm predictions of the semianalytic 'wind-compressed disk' model recently proposed by Bjorkman and Cassinelli to explain the circumstellar disks inferred observationally to exist around such rapidly rotating stars. However, this numerical simulation is able to incorporate several important effects not accounted for in the simple model, including a dynamical treatment of the outward radiative driving and gas pressure, as well as a rotationally distorted, oblate stellar surface. This enables us to model quantitatively the compressed wind and shock that forms the equatorial disk. The simulation results thus do differ in several important details from the simple method, showing, for example, an inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. There is also no evidence for the predicted detachment of the disk that arises in the fixed outflow picture. The peak equatorward velocity in the dynamical models is furthermore about a factor of 2 smaller than the lytically predicted value of approximately 50% of the stellar equatorial rotation speed. As a result, the dynamical disks are somewhat weaker than predicted, with a wider opening angle, lower disk/pole density ratio, and smaller shock velocity jump. The principal cause of these latter differences appears to be an artificially strong equatorward drift of the subsonic outflow in the original analytic model. Much better agreement with the dynamical results can be obtained, however, from a slightly modified, analytic wind-compression model with a more detailed specification of the fixed wind outflow and a lower boundary set to the sonic radius along a rotationally oblate stellar surface. Hence, despite these detailed differences, the general predicted effect of disk formation by wind compression toward the equator is substantially confirmed.

Owocki, Stanley P.↗
NASA NTRS1975

Viscous effects in rapidly rotating stars with application to white-dwarf models. III - Further numerical results

Improved viscous evolutionary sequences of differentially rotating, axisymmetric, nonmagnetic, zero-temperature white-dwarf models are constructed using the relativistically corrected degenerate electron viscosity. The results support the earlier conclusion that angular momentum transport due to viscosity does not lead to overall uniform rotation in many interesting cases. Qualitatively different behaviors are obtained, depending on how the total mass M and angular momentum J compare with the M and J values for which uniformly rotating models exist. Evolutions roughly determine the region in M and J for which models with a particular initial angular momentum distribution can reach carbon-ignition densities in 10 b.y. Such models may represent Type I supernova precursors.

Durisen, R. H.↗
NASA NTRS1979

Occultation of Epsilon Geminorum by Mars. III - Temperature structure of the Martian upper atmosphere

High-quality photoelectric observations of the April 8, 1976, occultation of Epsilon Gem by Mars are analyzed which were obtained at five different observatories. Temperature profiles are derived by numerical inversion of the star's light curves and found to be in mutual agreement. This is taken as evidence for the presence of strong atmospheric waves in the Martial upper atmosphere between number densities of 10 to the 13th and 10 to the 15th per cu cm. It is estimated that the vertical wavelength is about 20 km, the horizontal wavelength is greater than 600 km, and the peak-to-peak wave amplitude is approximately 40 K. The results are shown to be qualitatively consistent with Zurek's (1976) model of thermally driven tidal modes. No compelling evidence for turbulence effects is found in the occultation data.

French, R. G.↗
NASA NTRS1982

IUE spectroscopic investigation of interacting binary systems

The IUE observations of binaries consisting of at least one early type star, to the Algol systems, and to W Serpentis systems are discussed. The OB close binaries almost invaribly show a stellar wind. The difference between an unevolved early type system and a slightly evolved system is difficult to discern observationally and probably not meaningful in terms of Roche lobes. the zeta Aur stars show complex gas motions and shocks. Many of the Algol systems show peculiarities in their ultraviolet continua and the resonance lines, particularly Si IV, are of abnormal and variable strength. Emission lines were detected in several systems during the total eclipse. The W Ser stars are characterized by numerous strong emission lines in their ultraviolet spectra. It appears that these systems may be related to beta Lyrae as they seem to be in a phase of rapid mass transfer and their spectra show similarities to that of beta Lyrae.

Mccluskey, G. E., Jr.↗
NASA NTRS1981

Thermonuclear processes on accreting neutron stars

Theoretical models for X-ray burst sources that invoke thermonuclear flashes on the surface layers of an accreting neutron star are discussed. The historical development of X-ray burst observation is summarized, and a physical picture of a neutron star undergoing accretion is drawn. Detailed numerical computations of the evolution of the surface layers of such a star are reviewed. The need for general relativistic corrections to the model is pointed out. Finally, comparisons are made with observations of X-ray bursts, the rapid burster, fast X-ray transients, X-ray pulsars, and gamma-ray burst sources.

Joss, P. C.↗
NASA NTRS1991

Cooling flows and the formation of massive halos in cD galaxies

X-ray brightness profiles of galaxy clusters frequently indicate the presence of a hot gaseous envelope, and theoretical models predict that numerous low-mass stars will be formed as these gaseous halos cool and contract toward the center of the cluster potential. In this Letter, the optical surface brightness profiles of the halos seen in six cooling flow galaxies are modeled, and it is concluded that the optical profiles and mass-to-light ratios are consistent with the cooling flow accretion hypothesis.

Prestwich, Andrea H.↗
NASA NTRS1994

Dense stellar cores in merger remnants

We use numerical models which include star formation to analyze the mass profiles of remnants formed by mergers of disk galaxies. During a merger, dissipation in gas and ensuing star formation leave behind a dense stellar core in the remnant. Rather than joining smoothly onto a de Vaucouleurs profile, the starburst population leads to a sharp break in the surface density profile at a few percent of the effective radius. While our results are preliminary, the lack of such signatures in most elliptical galaxies suggests that mergers of gas-rich disk galaxies may not have contributed greatly to the populations of present-day ellipticals.

Mihos, J. Christopher↗
NASA NTRS1998

Stellar Occultation Studies of the Solar System

Earth-based observations of stellar occultations provide extremely high spatial resolution for bodies in the outer solar system, about 10,000 times better than that of traditional imaging observations. Stellar occultation data can be used to establish the structure of atmospheres and rings of solar system bodies at high spatial resolution. Airborne occultation observations are particularly effective, since the controlled mobility of the observing platform allows the observer to fly within the optimum part of the occultation shadow for most events that are visible from Earth. Airborne observations are carried out above any clouds and are nearly free of scintillation noise from the Earth's atmosphere. KAO occultation observations resulted in the first detection of gravity waves in the Martian atmosphere, discovery of the Uranian rings, the first detection of Pluto's atmosphere, the first Earth-based investigations of Triton's atmosphere, and the discovery of narrow jets from Chiron's nucleus. The first SOFIA occultation opportunity will be an investigation of Pluto's atmospheric structure in November, 2002, and will resolve a problem that has lingered since the KAO discovery observation fourteen years earlier. We plan to continue our successful airborne occultation program with the greatly enhanced capability provided by SOFIA. We propose here to replace our KAO occultation photometer with one having twice the throughput, half the noise, a somewhat wider wavelength range, four times the field of view, and ten times the frame rate to optimize its performance and to capitalize on the larger collecting area offered by SOFIA. It will also allow for simultaneous visible and IR occultation observations, greatly enriching the results that we can obtain from occultations. We call this new imaging occultation photometer HOPI (High-speed Occultation Photometer and Imager). HOPI will provide a signal-to-noise ratio two to four times that of our present photometer for a given event, will permit useful observations of the more numerous events involving fainter stars, and will allow higher time resolution observations to be made. HOPI's characteristics also make it an ideal instrument for initial and ongoing evaluations of the SOFIA telescope's performance.

Elliot, James L.↗
NASA NTRS1998

Stellar Occultation Studies of the Solar System

Earth-based observations of stellar occultations provide extremely high spatial resolution for bodies in the outer solar system, about 10,000 times better than that of traditional imaging observations. Stellar occultation data can be used to establish the structure of atmospheres and rings of solar system bodies at high spatial resolution. Airborne occultation observations are particularly effective, since the controlled mobility of the observing platform allows the observer to fly within the optimum part of the occultation shadow for most events that are visible from Earth. Airborne observations are carried out above any clouds and are nearly free of scintillation noise from the Earth's atmosphere. KAO occultation observations resulted in the first detection of gravity waves in the Martian atmosphere, discovery of the Uranian rings, the first detection of Pluto's atmosphere, the first Earth-based investigations of Triton's atmosphere, and the discovery of narrow jets from Chiron's nucleus. The first SOFIA occultation opportunity will be an investigation of Pluto's atmospheric structure in November, 2002, and will resolve a problem that has lingered since the KAO discovery observation fourteen years earlier. We plan to continue our successful airborne occultation program with the greatly enhanced capability provided by SOFIA. We propose here to replace our KAO occultation photometer with one having twice the throughput, half the noise, a somewhat wider wavelength range, four times the field of view, and ten times the frame rate to optimize its performance and to capitalize on the larger collecting area offered by SOFIA. It will also allow for simultaneous visible and IR occultation observations, greatly enriching the results that we can obtain from occultations. We call this new imaging occultation photometer HOPI (High-speed Occultation Photometer and Imager). HOPI will provide a signal-to-noise ratio two to four times that of our present photometer for a given event, will permit useful observations of the more numerous events involving fainter stars, and will allow higher time resolution observations to be made. HOPI's characteristics also make it an ideal instrument for initial and ongoing evaluations of the SOFIA telescope's performance.

Elliot, James L.↗
NASA NTRS2018

Surface Deposition of Molecular Contaminants in the Mars 2020 Rover Wake

The Mars 2020 rover has very strict contamination control requirements in order to prepare samples for a potential Mars Sample Return. It is known that the rover will out gas a low-level of molecular contaminant on the surface of Mars, and numerical simulations using STAR-CCM+ are performed in order to predict the magnitude of the contaminant deposition to the Martian surface in the vicinity of the rover. This is done in order to determine whether or not the M2020 rover could self-contaminate a potential sampling site if it were to remain in the same location for a long period of time. Simulations are performed under a variety of surface conditions, and it is determined that the predicted level of molecular contaminant deposition to the surface near the rover will not be a significant factor in the overall sample contamination budget.

Katz, Ira↗
NASA NTRSDate not supplied

A Summary of Recent SBIR and STTR Projects in Support of Cryogenic Fluid Management Modeling

Over the last five years, NASA has invested in development of sub-models to improve the current capabilities of Computational Fluid Dynamics (CFD) codes utilized for cryogenic fluid management (CFM) predictions. The goal is for the submodels to be modular so that they can easily be implemented into NASA’s CFD codes such as ANSYS Fluent, Siemens STAR-CCM+, and Streamline Numerics Loci-Stream. These technologies are being developed through Small Business Innovative Research (SBIR) and Small Business Technology Transfer (STTR) contracts under the CFM subtopic solicitation and funded by the CFM Portfolio Project. All contracts presented include cryogenic experiments to enable model validation. A sub-grid boiling model has been developed by Combustion Research and Flow Technology Inc. and Massachusetts Institute of Technology for cryogenic line chilldown predictions and has completed Phase III. A spray chill-and-fill model for cryogenic tankage is being developed by Combustion Research and Flow Technology Inc. and University of Connecticut and is currently in Phase II. Last, film condensation modeling to support cryogenic liquefaction was progressed through a Phase I contract with Combustion Research and Flow Technology Inc. and University of Connecticut. A summary of accomplishments for each contract is presented.

cryogenics↗
NASA NTRS1999

Infrared Space Observatory Observations of Far-Infrared Rotational Emission Lines of Water Vapor Toward the Supergiant Star VY Canis Majoris

We report the detection of numerous far-infrared emission lines of water vapor toward the supergiant star VY Canis Majoris. A 29.5-45 micron grating scan of VY CMa, obtained using the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory at a spectral resolving power lambda/delat.lambda of approximately 2000, reveals at least 41 spectral features due to water vapor that together radiate a total luminosity of approximately 25 solar luminosity . In addition to pure rotational transitions within the ground vibrational state, these features include rotational transitions within the (010) excited vibrational state. The spectrum also shows the (sup 2)product(sub 1/2) (J = 5/2) left arrow (sup 2)product(sub 3/2) (J = 3/2) OH feature near 34.6 micron in absorption. Additional SWS observations of VY CMa were carried out in the instrument's Fabry-Perot mode for three water transitions: the 7(sub 25)-6(sub 16) line at 29.8367 micron, the 4(sub 41)-3(sub 12) line at 31.7721 micron, and the 4(sub 32)-3(sub 03) line at 40.6909 micron. The higher spectral resolving power lambda/delta.lambda of approximately 30,000 thereby obtained permits the line profiles to be resolved spectrally for the first time and reveals the "P Cygni" profiles that are characteristic of emission from an outflowing envelope.

Neufeld, David A.↗
NASA NTRS1995

Computing supernova collapse to neutron stars and black holes

We present a new numerical code for spherical hydrodynamics in general relativity. The code can handle gravitational collapse to a neutron star or to a black hole without the appearance of singularities. Moreover, the variables and equations in the code are very similar to those appearing in traditional Lagrangian supernova codes. Any such existing code can thus be easily adapted to treat collapse where the final fate is uncertain and may be either a neutron star or a black hole. The code is based on the formulation of Hernandez & Misner, in which retarded time is used as coordinate. This prevents the computational grid from penetrating inside any black hole that may form. We present the equations and a complete finite difference scheme for the adiabatic evolution of a fluid that obeys a gamma-law equation of state. We summarize the results of several testbed calculations performed to check our code. We also give the transformation of the analytic Oppenheimer-Snyder solution for homogeneous dust collapse to our coordinate system.

Baumgarte, Thomas W.↗
NASA NTRS1993

Tidal disruptions by supermassive black holes - Hydrodynamic evolution of stars on a Schwarzschild background

We present a three-dimensional numerical study of tidal disruption of a main-sequence star by a supermassive black hole. The simulations include general relativistic effects which are important in this regime. We analyze stars in a marginally bound orbit around the black hole with pericentric separation of a few Schwarzschild radii. We show that during a close passage, as a result of relativistic effects analogous to the perihelion shift, the trajectories of the debris of the star fan out into a crescent-like shape centered on the black hole. We also discuss the increase of the central density of the star as it approaches pericentric distance, the fraction of the debris accreted by the hole, its accretion rate, the distribution of debris orbits bound to the hole, and the velocity of unbound ejected material. We compare these results with the disruption of the star by a Newtonian point mass.

Laguna, Pablo↗
NASA NTRS1966

A study of pulsation in RR Lyrae models.

Nonlinear calculations of intermediate amplitude to analyze stability of pulsating stars of varying composition, mass, luminosity, etc, using numerical integration of motion equations

STELLAR MASS↗