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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1971

Observations of O and Of stars.

Evaluation of spectrograms with a dispersion of 16.2 A/mm covering the spectral range from 3300 to 4900 A obtained for three early O stars and three early Of stars. Detailed line profiles of selected hydrogen and helium lines were obtained, and equivalent widths of nearly all visible lines were measured. Comparison of the spectroscopic properties between Of stars and O stars suggests that Of stars have lower surface gravities than do O stars. Comparison of their photometric properties confirms this suggestion. Hence present temperature scales for O5-O8 stars should not be applied to Of stars until the effect of gravity in determining spectral type has been determined. In addition, the spectra suggest that atmospheric models of Of stars should take into account spherical and hydrodynamic effects.

Heap, S. R.↗
NASA NTRS1974

Investigation of ultraviolet fluxes of normal and peculiar stars

Data from Project Celescope, a program that photographed the ultraviolet sky, in order to study several problems in current astrophysics are analyzed. Two star clusters, the Pleiades and the Hyades, reveal differences between the two that we are unable to explain simply from their differences in chemical abundance, rotation, or reddening. Data for Orion show large scatter, which appears to be in the sense that the Orion stars are too faint for their ground-based photometry. Similarly, many supergiants in the association Sco OB1 are too faint in the ultraviolet, but the ultraviolet brightness appears to be only poorly correlated with spectral type. Ultraviolet Celescope data for several groups of peculiar stars have also been analyzed. The strong He I stars are too faint in the ultraviolet, possibly owing to enhancement of O II continuous opacity due to oxygen overabundance. The Be stars appear to have ultraviolet colors normal for their MK spectral types. The P Cygni stars are considerably fainter than main-sequence stars of comparable spectral type, probably owing, at least in part, to line blocking by resonance lines of multiply ionized light metals. The Wolf-Rayet stars have ultraviolet color temperatures of O stars.

Deutschman, W. A.↗
NASA NTRS1978

Spectral types for early-type stars observed by Skylab

MK spectral types are presented for 246 early-type stars observed with the S-019 ultraviolet stellar astronomy experiment on Skylab. K-line types are also given where applicable, and various peculiar stars are identified. The peculiar stars include five silicon stars, a shell star, a helium-rich star, a silicon-strontium star, a chromium-europium star, and two marginal metallic-line stars.

Roman, N. G.↗
NASA NTRS1982

Copernicus observations of the N v resonance doublet in 53 early-type stars

UV spectra in the wavelength interval 1170-1270 A are presented for 53 early-type stars ranging in spectral type from O6.5 V to B2.5 IV. The sample includes four Wolf-Rayet stars, seven known Oe-Be stars, and six galactic halo OB stars. A qualitative analysis of the stellar N v doublet reveals that: (1) N v is present in all stars hotter and more luminous than type B0 for the main sequence, B1 for giants, and B2 for supergiants; (2) shell components of N v and an unidentified absorption feature at 1230 A are present in about half of the stars; (3) the column density of N v is well correlated with bolometric luminosity over the spectral range O6 to B2; and (4) the ratio of emission to absorption equivalent width is a factor of 2 smaller in the main sequence stars than in supergiants, which suggests that the wind structure changes as a star evolves. For several stars, this ratio is too small to be explained by traditional wind models.

Abbott, D. C.↗
NASA NTRS1982

Subluminous Wolf-Rayet stars - Observations

Even though the loss of the outer, H-rich envelope may be a necessary condition for forming WR stars, it is clearly not a sufficient one. This is because the majority of planetary nuclei do not have a WR-type spectrum. The question why some central stars are WR stars while others are, say, O stars is addressed here. The question is approached by ascertaining how the properties of WR-type central stars differ from those of O-type stars. The study therefore begins with the classification and calibration of WR spectra. It is found that whereas the WR phenomenon may occur in stars of any mass, it is limited to stars that have been stripped (or mixed) down to their cores. The fact of prior envelope loss, however, is not sufficient to predict which stars will develop WR characteristics. Instead, a comprehensive description of envelope loss, including a detailed accounting of interior conditions and processes at the core envelope interface, will be required to explain the onset of WR characteristics.

Heap, S. R.↗
NASA NTRS1983

Einstein Observations of X-ray emission from A stars

Results are reported from the combined CfA Stellar Survey of selected bright A stars and an Einstein Guest Observer program for Ap and Am stars. In an initial report of results from the CfA Stellar Surveys by Vaiana et al. (1981) it was noted that the spread in observed X-ray luminosities among the few A stars observed was quite large. The reasons for this large spread was studied by Pallavicini et al. (1981). It was found that the X-ray emission from normal stars is related very strongly to bolometric luminosity for early-type stars and to rotation rate for late-type stars. However, an exception to this rule has been the apparently anomalous behavior of A star X-ray emission, for which the large spread in luminosity showed no apparent correlation with either bolometric luminosity or stellar rotation rate. In the present study, it is shown that the level of emission from normal A stars agrees with the correlation observed for O and B stars.

Golub, L.↗
NASA NTRS1983

Observational knowledge about the physical properties of O stars

Information about the effective temperatures, radii, and masses of O-type stars is presented. It is argued that rapid variations in the amount of light from O stars and the spectral distribution are a result chiefly of changes which occur in the envelope of the star. The stability of the photospheric layers of O stars against convection is reviewed and it is noted that late O stars and early B stars have a convection zone in the deeper parts of the photosphere. This convection zone is due to the second ionization of helium. Evidence is reviewed that most of the line-profile changes seen for O stars are generated by changes in the physical state of the mantle of the star, that is of the outer atmosphere where the deposition of non-radiative energy and momentum controls the physical state of the atmosphere. The physical state of the mantle may change in response to changes in the upper envelope of a star with a different time constant than the photosphere does.

Underhill, A. B.↗
NASA NTRS1986

CSTAR star catalogue development

The Continuous Stellar Tracking Attitude Reference (CSTAR) system is an in-house project for the Space Station to provide high accuracy, drift free attitude and angular rate information for the GN&C system. Constraints exist on the star catalogue incorporated in the system. These constraints include the following: mass memory allocated for catalogue storage, star tracker imaging sensitivity, the minimum resolvable separation angle between stars, the width of the field of view of the star tracker, and the desired number of stars to be tracked in a field of view. The Smithsonian Astrophysical Observatory (SAO) catalogue is the basis reference for this study. As it stands, the SAO does not meet the requirements of any of the above constraints. Star selection algorithms have been devised for catalogue optimization. Star distribution statistics have been obtained to aid in the development of these rules. VAX based software has been developed to implement the star selection algorithms. The software is modular and provides a design tool to tailor the catalogue to available star tracker technology. The SAO catalogue has been optimized for the requirements of the present CSTAR system.

Uhde-Lacovara, J. A.↗
NASA NTRS1986

Success potential of automated star pattern recognition

A quasi-analytical model is presented for calculating the success probability of automated star pattern recognition systems for attitude control of spacecraft. The star data is gathered by an imaging star tracker (STR) with a circular FOV capable of detecting 20 stars. The success potential is evaluated in terms of the equivalent diameters of the FOV and the target star area ('uniqueness area'). Recognition is carried out as a function of the position and brightness of selected stars in an area around each guide star. The success of the system is dependent on the resultant pointing error, and is calculated by generating a probability distribution of reaching a threshold probability of an unacceptable pointing error. The method yields data which are equivalent to data available with Monte Carlo simulatins. When applied to the recognition system intended for use on the Space IR Telescope Facility it is shown that acceptable pointing, to a level of nearly 100 percent certainty, can be obtained using a single star tracker and about 4000 guide stars.

Van Bezooijen, R. W. H.↗
NASA NTRS1985

The intrinsic ultraviolet continua of O stars

Low-resolution IUE spectra of O and B stars in open clusters have been obtained. The main-sequence B stars are used to derive UV extinction curves for the program clusters, and these curves are used to determine the intrinsic UV to visual colors of O stars in NGC 2244, Trumpler 14, and NGC 6231. These data are analyzed in conjunction with the dereddened colors of O stars with color excesses less than 0.15 mag to obtain a set of reliable UV colors for O3-9, class V-I stars. The results can be summarized as follows: none of the observed O stars have UV colors significantly bluer than a normal B0 V star, and the late O supergiants have significantly redder UV colors than main-sequence O stars, although this luminosity distinction appears to vanish for the early O stars. Several possible explanations for these unexpected results are considered, but none of the presently available nonstandard models are sufficiently developed to be definitively tested.

Massa, D.↗
NASA NTRS1988

Lithium in lower-main-sequence stars of the Alpha Persei cluster

Lithium abundances are presented for main-sequence stars of spectral types F, G, and K in the young open cluster Alpha Per. For 46 cluster members, a correlation between Li abundance and projected rotational velocity v sin i is found: all of the Li-poor stars are slow rotators. Two explanations are proposed to account for the correlation: (1) that the Li depletion is introduced following a rapid spin-down phase experienced by young low-mass stars, and that this episode of Li depletion may be the dominant one determining the spread of Li abundances among young low-mass main-sequence stars, and (2) that star formation has occurred over a finite period such that the older stars have undergone a spin-down and depletion of Li by a means that may or may not depend on rotation. The Li abundance in the warm and rapidly rotating stars appears to be undepleted, as is predicted by recent models of pre-main-sequence stars. The depletion observed in the cool stars exceeds the level predicted by these models.

Balachandran, Suchitra↗
NASA NTRS1989

High-luminosity carbon stars in the galactic anticenter

K-band magnitudes of 211 carbon stars in the Galactic anticenter are presented. The surface density, brightness range, and observed colors of stars in the sample are discussed. It is suggested that the K-band extinction gradient in the Galactic plane near l = 180 deg is between 0.15 and 0.3/kpc. In contrast to the total density of stars, it is found that the density of high-luminosity carbon stars does not decrease in the anticenter region out to at least 3 kpc from the sun. It is proposed that this is due to the fact that lower metallicity stars spend more time as carbon stars compared to stars of solar metallicity. Also, it is shown that the average mass-loss rates from high-luminosity carbon stars in the anticenter direction appear to be lower by a factor of 1.7 than those for carbon stars in the solar neighborhood.

Jura, M.↗
NASA NTRS1989

Highly ionized species and circumstellar shells in B8-A1 stars

The nature of the C IV and Si IV absorption seen in the spectra of apparently normal late-type B and early-type A stars located within 200 pc of the sun was investigated by examining a total of 93 archival IUE spectra of 41 stars. In three of these objects, variable or asymmetric C IV profiles indicate that the absorption features have a stellar origin. One star, HD 119921, showed shortward-shifted discrete C IV and Si IV absorption components similar to those observed in Be stars. All of the stars with detected C IV in their spectra had large values of v sin i. Four of the stars showing C IV and Si IV also showed sharp absorption features in Si II, with two of these stars displaying variable Si II features. These results strongly suggest that these stars are unidentified Be/Ae and shell stars.

Bruhweiler, Frederick C.↗
NASA NTRS1988

Spectroscopic observations of X-ray selected late type stars

A spectroscopic survey of nine X-ray selected late type stars was conducted. These stars are serendipitously discovered EINSTEIN X-ray sources, selected from two large x-ray surveys: the Columbia Astrophysical Laboratory survey (five stars) and the CFA Medium Sensitivity survey (four stars). Four of the Columbia survey stars were found to be short period binaries. The fifth was found to be an active single G dwarf. None of the Medium Sensitivity survey stars were found to be either binaries or active stars. Activity was measured by comparing the H-alpha and the CaII infrared triplet (8498, 8542) lines in these stars to the lines in inactive stars of similar spectral type. A correlation was found between the excess H-alpha lime emission and V sin(i) and between the excess H-alpha line emission and X-ray luminosity. No correlation was found between the infrared line emission and any other measured quantity.

Takalo, L. O.↗
NASA NTRS1990

Massive soliton stars

The structure of nontopological solutions of Einstein field equations as proposed by Friedberg, Lee, and Pang (1987) is examined. This analysis incorporates finite temperature effects and pair creation. Quarks are assumed to be the only species that exist in interior of soliton stars. The possibility of primordial creation of soliton stars in the incomplete decay of the degenerate vacuum in early universe is explored. Because of dominance of pair creation inside soliton stars, the luminosity of soliton stars is not determined by its radiative transfer characteristics, and the surface temperature of soliton stars can be the same as its interior temperature. It is possible that soliton stars are intense X-ray radiators at large distances. Soliton stars are nearly 100 percent efficient energy converters, converting the rest energy of baryons entering the interior into radiation. It is possible that a sizable number of baryons may also be trapped inside soliton stars during early epochs of the universe. In addition, if soliton stars exist they could assume the role played by massive black holes in galactic centers.

Chiu, Hong-Yee↗
NASA NTRS1990

(C-12)O emission from the envelopes of cool stars in the solar neighborhood

Results are presented on observations of the CO J = 1-0 line emission from all M giants, S stars, and C stars listed in the Two-Micron Sky Survey having strong FIR emission and lying north of delta = -10 deg. The data from this survey and other data for C and S stars show that the line profiles of these stars look like flattened parabolas and have roughly the same shape for different stars. In contrast, the shapes of the spectral lines from giant M stars are diverse, ranging from triangular to spiked and asymmetric, suggesting that the envelopes of M stars have complex kinematics and structure. The outflow velocities inferred from the line profiles of the stars surveyed span a range of more than an order of magnitude, with the velocities of C stars correlating with IR color.

Margulis, M.↗
NASA NTRS1991

Gravitational microlensing of the Galactic bulge stars

The optical depth to microlensing on the ordinary disk stars is about 4 x 10 to the -7th for the Galactic bulge stars visible in Baade's window. This means that at any given time one out of about 2.5 x 10 to the 6th bulge stars must be brightened by at least 0.3 mag. The duration of events is proportional to the square root of mass of the lensing star, and it is in the range of 1 week to 1 month for the lensing stars of 0.1-1.0 solar mass. The average number of events with the amplitude exceeding 0.3 mag is expected to be about 4 per year per million bulge stars. If there are brown dwarfs in the Galactic disk with masses in the range 0.01-0.1 solar mass, with the total mass density equal to the total mass density of the ordinary stars, then the combined optical depth to microlensing of the bulge stars is about 8 x 10 to the -7th, the average number of events with the amplitude exceeding 0.3 mag is expected to be about 17 per year per million bulge stars, and the time scales of events are in the range of 3-20 days.

Paczynski, B.↗
NASA NTRS1991

Dynamical capture of field stars by interstellar clouds

Evidence suggesting the existence of old stars in young galactic clusters, has motivated an investigation of a model in which such stars are captured from the field during the gas cloud collapse phase of star formation. A hyperbolic field star cannot be gravitationally captured by a static cloud since the star's total energy is conserved. However, if the cloud's potential is an explicit function of time, the energy of a penetrating star can be decreased. As a first step in modeling the dynamical capture of field stars, capture by freely falling, homogeneous spherical cloud is considered. This simplified model is solved, and a prediction is obtained for number of captured field stars in terms of the cloud parameters and dispersion velocities of the field stars.

Whitman, P. G.↗