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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Studies of extra-solar Oort clouds and the Kuiper disk

We are conducting research designed to enhance our understanding of the evolution and detectability of comet clouds and disks. According to 'standard' theory, both the Kuiper Belt and the Oort Cloud are (at least in part) natural products of the planetary accumulation stage of solar system formation. One expects such assemblages to be a common attribute of other solar systems. Therefore, searches for comet disks and clouds orbiting other stars offer a new method for inferring the presence of planetary systems. This project consists of two efforts: (1) observational work to predict and search for the signatures of Oort Clouds and comet disks around other stars; and (2) modelling studies of the formation and evolution of the Kuiper Belt (KB) and similar assemblages that may reside around other stars, including beta Pic.

Stern, S. Alan↗
NASA NTRS2012

Gas, Stars, and Star Formation in Alfalfa Dwarf Galaxies

We examine the global properties of the stellar and Hi components of 229 low H i mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H i masses <10(sup 7.7) solar mass and Hi line widths <80 kilometers per second. Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M*) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M* approximately less than10(exp 8)M(sub 0) is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper Hi mass limit yields the selection of a sample with lower gas fractions for their M* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H i depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that Hi disks are more extended than stellar ones.

ALFALFA↗
NASA NTRS1988

X-ray sources in regions of star formation. III - Naked T Tauri stars associated with the Taurus-Auriga complex

Ground-based and IRAS optical and IR spectroscopic and photometric observations are reported for 90 stars in or near 59 Einstein Observatory X-ray error circles in the Tau-Aur region. The data are presented in extensive tables and sample spectra and characterized in detail, with particular attention to 28 newly discovered 'naked' T Tau stars, which are shown to be normal stars with no significant IR or UV excess and ages of 1-40 Myr. These stars are found to outnumber normal T Tau stars by a factor of 10 in an area near the Tau-Aur dark clouds, and it is argued that their evolution toward the ZAMS is typical for low-mass stars. The implications of this finding for the time scales of circumstellar-disk dissipation and planet formation are discussed.

Walter, Frederick M.↗
NASA NTRS1999

A Deep HRI Survey of Low-Mass PMS Stars in NGC 2264

Brian Patten is the Principal Investigator of the NASA ADP project 'A Deep HRI Survey of Low-Mass PMS Stars in NGC 2264'. This project was funded to support primarily the data reduction and analysis for new ROSAT data to be acquired in ROSAT AO8. For AO8 we were awarded two deep (100 ks) exposures with the ROSAT HRI instrument of a rotation and proper-motion selected sample of young (3 Myr - 15 Myr), low-mass, PMS stars in the populous star-forming region NGC 2264. These X-ray data were to be combined with an extensive rotation database for members of this cluster to allow us, for the first time, to probe the early evolution of magnetic dynamo activity for both fully convective stars and those stars found lower on the Hayashi tracks which have developed radiative cores. This database would have been used to study the interrelationship between coronal activity level, interior structure, and rotation rate as a function of mass and age.in the PMS and to define empirical constraints for theoretical models of angular momentum and magnetic dynamo evolution.

Patten, Brian↗
NASA NTRS1964

Stellar evolution. i. a program for the solution of the time-dependent basic equations <sternentwicklung. i. ein program zur losung der zeitabhingigen aufbaugleichungen<

In this paper a program is described for stellar evolution computations, based on the Henyey method. The program shall be used for very different phases of stellar evolution. Therefore, the main program for the solution of the time dependent basic equations is kept as general as possible, while all assumptions about special physical conditions in the star occur only in subprograms. Hence the range of applicability of the program can easily be extended. The system of difference equations and details of the integration method used in the program are given in two appendices. For a first test the program has been applied to the post-main sequence evolution of a star of 7 M; the results will appear in part II and III. Some technical details of these calculations are given and discussed here. The program turned out to be applicable to calculations of the evolution near the main sequence, of phases of core contraction and of nuclear burning in shell sources.

STELLAR EVOLUTION↗
NASA NTRS2016

Galaxy Evolution Spectroscopic Explorer: Scientific Rationale

GESE is a mission concept consisting of a 1.5-m space telescope and UV multi-object slit spectrograph designed to help understand galaxy evolution in a critical era in the history of the universe, where the rate of star-formation stopped increasing and started to decline. To isolate and identify the various processes driving the evolution of these galaxies, GESE will obtain rest-frame far-UV spectra of 100,000 galaxies at redshifts, z approximately 1-2. To obtain such a large number of spectra, multiplexing over a wide field is an absolute necessity. A slit device such as a digital micro-mirror device (DMD) or a micro-shutter array (MSA) enables spectroscopy of a hundred or more sources in a single exposure while eliminating overlapping spectra of other sources and blocking unwanted background like zodiacal light. We find that a 1.5-m space telescope with a MSA slit device combined with a custom orbit enabling long, uninterrupted exposures (approximately 10 hr) are optimal for this spectroscopic survey. GESE will not be operating alone in this endeavor. Together with x-ray telescopes and optical/near-IR telescopes like Subaru/Prime Focus Spectrograph, GESE will detect "feedback" from young massive stars and massive black holes (AGN's), and other drivers of galaxy evolution.

digital micro-mirror device (DMD)↗
NASA NTRS1989

Interacting winds and the shaping of planetary nebulae

Using two-dimensional hydrodynamics, the problem of shaping of planetary nebulae by the interacting winds model is studied. The shock structure is obtained and it is shown that interacting winds are capable of producing morphologies similar to the ones observed in planetary nebulae, provided that a density contrast exists between the equatorial and polar directions. This confirms recent suggestions by Balick. Mechanisms that can produce the required density contrast are discussed and it is shown that binary central stars provide the most natural mechanism, especially via common envelope evolution. It is not yet entirely clear whether stellar rotation is sufficient to generate the required contrast in the case of single central stars.

Soker, Noam↗
NASA NTRS1971

Dynamics of plane stellar systems.

The evolution of initially balanced rotating disks of stars is investigated with a computer model for isolated disks of stars. An isolated, initially cold balanced disk is found to be violently unstable. Adding a sufficient amount of velocity dispersion will stabilize all small-scale disturbances. However, the disks are still unstable against slowly growing long wave-length modes and after about two rotations most disks tend to assume a bar-shaped structure. It is found that the final mass distribution over most of the disk can be closely approximated by an exponential variation, irrespective of the initial mass distribution. The gravitational two-stream instability is investigated by means of a modified computer model for infinite doubly periodic stellar systems.

Hohl, F.↗
NASA NTRS1972

Dynamics of plane stellar systems.

The evolution of initially balanced rotating disks of stars is investigated with a computer model for isolated disks of stars. An isolated, initially cold balanced disk is found to be violently unstable. Adding a sufficient amount of velocity dispersion will stabilize all small-scale disturbances. However, the disks are still unstable against slowly growing long wave-length modes and after about two rotations most disks tend to assume a bar-shaped structure. It is found that the final mass distribution over most of the disk can be closely approximated by an exponential variation, irrespective of the initial mass distribution. The gravitational two-stream instability is investigated by means of a modified computer model for infinite doubly periodic stellar systems.

Hohl, F.↗
NASA NTRS1981

Astronomical imaging investigations for Shuttle and Spacelab missions

General objectives of space astronomical imagery are briefly examined, taking into account the improved angular resolution achievable in space observations, the utilization of an accessibility of the far ultraviolet spectral range for an easier detection and more accurate measurement of very hot stars, the determination of the structure and evolution of galaxies, and the determination of the cosmic distance scale. Attention is given to the Ultraviolet Imaging Telescope, the Spacelab Wide Angle Telescope, and the Starlab UV/optical telescope facility. It is pointed out that Starlab is envisioned as a multi-instrument, multi-purpose astronomical facility based on a near-diffraction-limited Ritchey-Chretien telescope of about 1 meter aperture and f/15 focal ratio. These resolutions are a factor of 10 better than typically achieved with ground-based telescopes.

Carruthers, G. R.↗
NASA NTRS1985

The young SMC cluster NGC 330

A color-magnitude diagram has been obtained for the young SMC cluster NGC 330. The diagram shows a well-defined main sequence, a group of blue supergiants, a group of red supergiants between B-V = 1.2 m and 1.6 m about one magnitude fainter, and an empty Hertzsprung gap. The surrounding field is a composite of a very gold population resembling galactic globular clusters and a very young population. DDO and infrared photometry strongly suggest that the cluster is metal-poor, but a definitive measure could not be made because of calibration difficulties. The cluster's age is estimated at 12 million years, with the surrounding field about 50 percent older. The cluster will prove very useful in testing stellar evolution models for young, metal-poor stars if the cluster's metallicity can be established via high-resolution spectroscopy.

Carney, B. W.↗
NASA NTRS1986

The lower atmosphere of solar flares; Proceedings of the Solar Maximum Mission Symposium, Sunspot, NM, Aug. 20-24, 1985

The topics discussed by the present conference encompass the chromospheric flare phenomenon, white light flares, UV emission and the flare transition region, the flare corona and high energy emissions, stellar flares, and flare energy release and transport. Attention is given to radiative shocks and condensation in flares, impulsive brightening of H-alpha flare points, the structure and response of the chromosphere to radiation backwarming during solar flares, the interpretation of continuum emissions in white light flares, and the radiation properties of solar plasmas. Also discussed are EUV images of a solar flare and C III intensity, an active region survey in H-alpha and X-rays, dynamic thermal plasma conditions in large flares, the evolution of the flare mechanism in dwarf stars, the evidence concerning electron beams in solar flares, the energetics of the nonlinear tearing mode, macroscopic electric fields during two-ribbon flares, and the low temperature signatures of energetic particles.

Neidig, Donald F.↗
NASA NTRS1987

Computer aided indexing at NASA

The application of computer technology to the construction of the NASA Thesaurus and in NASA Lexical Dictionary development is discussed in a brief overview. Consideration is given to the printed and online versions of the Thesaurus, retrospective indexing, the NASA RECON frequency command, demand indexing, lists of terms by category, and the STAR and IAA annual subject indexes. The evolution of computer methods in the Lexical Dictionary program is traced, from DOD and DOE subject switching to LCSH machine-aided indexing and current techniques for handling natural language (e.g., the elimination of verbs to facilitate breakdown of sentences into words and phrases).

Buchan, Ronald L.↗
NASA NTRS1987

Theoretical horizontal-branch evolution

The general features of the theoretical evolution of canonical horizontal-branch (HB) stars are briefly reviewed with specific emphasis on the track morphology in the HR diagram and the determination of the globular cluster helium abundance. The observational evidence for the occurrence of semiconvection is discussed together with some remaining theoretical uncertainty.

Sweigart, Allen V.↗
NASA NTRS1989

AGB evolution in the Large Magellanic Cloud

Observations and theoretical studies of the evolution of massive asymptotic giant branch (AGB) stars in the Magellanic Clouds are reviewed. Recent results support the hypothesis that AGB is truncated, probably through mass loss, before these stars can attain the theoretically predicted maximum luminosity. In particular, IRAS observations of the LMC provide little evidence for the presence of highly obscured 'cocoon' stars.

Reid, Neill↗
NASA NTRS1990

Luminous asymptotic giant branch stars in the Large Magellanic Cloud

A search has been conducted for optically obscured asymptotic giant branch (AGB) stars in the LMC. The results obtained are noted to rule out the presence of sufficient high-luminosity 'cocoon' stars to explain the observed deficit of several hundred luminous AGB stars between the predictions of standard AGB evolution models and the observed luminosity function. Bolometric magnitudes as low as -5 are inferred for these sources; it is suggested that this phase can be triggered at low luminosities, truncating AGB evolution and leading to the observed scarcity of AGBs with M(bol) greater than -6.0 mag.

Reid, Neill↗
NASA NTRS1992

The radial distribution of oxygen in disk galaxies

The shape of the oxygen abundance profile is examined for seven nearby spiral galaxies in which at least 15 H II regions have been observed. The radial abundance profile for three of these galaxies has a distinct change in slope. The others show no deviation from a single scale-length exponential. This behavior is explained within the framework of the star-forming viscous disk model of galaxy evolution. A prediction is made that radial abundance profiles of noninteracting unbarred spiral galaxies change slope at the radius where the rotation curve changes from linearly rising to flat. For barred and interacting galaxies the situation is complicated by additional mixing mechanisms of the interstellar medium.

Zaritsky, Dennis↗