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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Formation, Evolution, and Population Synthesis of Binary Systems Containing Collapsed Stars

During the period September 1, 2002 through August 31, 2003 we have been supported in part by a small NASA 'bridge" grant NAG5-12522 to continue our theoretical investigations of the "Formation, Evolution, and Population Synthesis of Binary Systems Containing Collapsed Stars". This research includes theoretical studies of the formation and evolution of several different types of interacting binary systems containing collapsed stars. Four papers were completed under the auspices of this grant: 1. Theoretical Consideration on the Properties of Accreting Millisecond Pulsars. 2. Accretion Onto Fast X-Ray Pulsars. 3. The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star.

West, Donald↗
NASA NTRS1987

Basic properties and variability

Giant and supergiant M, S, and C stars are discussed in this survey of research. Basic properties as determined by spectra, chemical composition, photometry, or variability type are discussed. Space motions and space distributions of cool giants are described. Distribution of these stars in our galaxy and those nearby is discussed. Mira variables in particular are surveyed with emphasis on the following topics: (1) phase lag phenomenon; (2) Mira light curves; (3) variations in color indices; (4) determination of multiple periods; (5) correlations between quantities such as period length, light-curve shape, infrared (IR) excess, and visible and IR color diagram; (6) semiregular (SR) variables and different time scales in SR light variations; (7) irregular variable Lb and Lc stars; (8) different time-scale light variations; (9) hydrogen-deficient carbon (HdC) stars, in particular RCB stars; and (10) irreversible changes and rapid evolution in red variable stars.

Querci, Francois R.↗
NASA NTRS1986

Bolometric luminosity evolution

The observations made by IRAS are capable of detecting evolution in the OB star formation rates of galaxies at cosmologically interesting redshifts if that evolution takes the form of star formation rates ten or more times higher than seen in nearby galaxies. Spectroscopic follow-up observations of IRAS-selected galaxies, and IRAS source counts, indicate that there may indeed exist a rare population of galaxies with such high star formation rates. The most luminous found so far approach quasars in luminosity.

Lonsdale, Carol J.↗
NASA NTRS1992

The evolution of high-metallicity horizontal-branch stars and the origin of the ultraviolet light in elliptical galaxies

Evolutionary calculations of high-metallicity horizontal-branch stars show that for the relevant masses and helium abundances, post-HB evolution in the HR diagram does not proceed toward and along the AGB, but rather toward a 'slow blue phase' in the vicinity of the helium-burning main sequence, following the extinction of the hydrogen shell energy source. For solar and twice solar metallicity, the blue phase begins during the helium shell-burning phase (in agreement with the work of Brocato and Castellani and Tornambe); for 3 times solar metallicity, it begins earlier, during the helium core-burning phase. This behavior differs from what takes place at lower metallicities. The implications for high-metallicity old stellar populations in the Galactic bulge and for the integrated colors of elliptical galaxies are discussed.

Horch, E.↗
NASA NTRS1989

The ages of globular cluster stars - Effects of rotation on pre-main-sequence, main-sequence, and turnoff evolution

Evolutionary sequences for low-metallicity stars (Z ranging from 0.001 to 0.0001) to study the effects of internal stellar rotation on the evolutionary time scales in the pre-main sequence, the main sequence (MS), and around the MS turnoff. Although a substantial amount of angular momentum remains in the interior, rotation is only a minor perturbation on the structure and ages of globular cluster stars. Even models with large initial angular momenta have MS lifetimes that are within 1 percent of those of standard models of the same mass and composition. Therefore, rotation does not affect age estimates of globular clusters from isochrone fitting. Furthermore, the models suggest that because rotation is not likely to affect horizontal-branch (HB) morphology, it does not affect significantly age estimates from the Delta-V method. Nevertheless, the internal angular momentum in the models is consistent with observations of surface rotational velocities on the HB, which require the preservation of a large reservoir of internal angular momentum.

Deliyannis, Constantine P.↗
NASA NTRS1990

What we learn from eclipsing binaries in the ultraviolet

Recent results on stars and stellar physics from IUE (International Ultraviolet Explorer) observations of eclipsing binaries are discussed. Several case studies are presented, including V 444 Cyg, Aur stars, V 471 Tau and AR Lac. Topics include stellar winds and mass loss, stellar atmospheres, stellar dynamos, and surface activity. Studies of binary star dynamics and evolution are discussed. The progress made with IUE in understanding the complex dynamical and evolutionary processes taking place in W UMa-type binaries and Algol systems is highlighted. The initial results of intensive studies of the W UMa star VW Cep and three representative Algol-type binaries (in different stages of evolution) focused on gas flows and accretion, are included. The future prospects of eclipsing binary research are explored. Remaining problems are surveyed and the next challenges are presented. The roles that eclipsing binaries could play in studies of stellar evolution, cluster dynamics, galactic structure, mass luminosity relations for extra galactic systems, cosmology, and even possible detection of extra solar system planets using eclipsing binaries are discussed.

Guinan, Edward F.↗
NASA NTRS1998

Recurrent X-ray Emission Variations of Eta Carinae and the Binary Hypothesis

Recent studies suggest that, the super-massive star eta Carinae may have a massive stellar companion (Damineli, Conti, and Lopes 1997), although the dense ejecta surrounding the star make this claim hard to test using conventional methods. Settling this question is critical for determining the current evolutionary state and future evolution of the star. We address this problem by an unconventional method: If eta Carinae is a binary, X-ray emission should be produced in shock waves generated by wind-wind collisions in the region between eta Carinae and its companion. Detailed X-ray monitoring of eta Carinae for more that) 2 years shows that the observed emission generally resembles colliding-wind X-ray emission, but with some significant discrepancies. Furthermore, periodic X-ray "flaring" may provide an additional clue to determine the presence of a companion star and for atmospheric pulsation in eta Carinae.

Ishibashi, K.↗
NASA NTRS2001

SWIRE: The SIRTF wide-area infrared extragalactic survey

SWIRE is a wide-area, high latitude, imaging survey to be undertaken with SIRTF to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and AGN, as a function of environment from z~2.5 to the current epoch. Building on ISO's heritage, SWIRE complements smaller, deeper SIRTF GTO (Guaranteed Time Obeserver) surveys, and paves the way for the Herschel (FIRST) observatory to be launched in 2007.

SWIRE imaging survey, star-forming galaxies, galax↗
NASA NTRS1993

The effect of Livermore OPAL opacities on the evolutionary masses of RR Lyrae stars

We have investigated the effect of the new Livermore OPAL opacities on the evolution of horizontal-branch (HB) stars. This work was motivated by the recent stellar pulsation calculations using the new Livermore opacities, which suggest that the masses of double-mode RR Lyrae stars are 0.1-0.2 solar mass larger than those based on earlier opacities. Unlike the pulsation calculations, we find that the effect of opacity change on the evolution of HB stars is not significant. In particular, the effect of the mean masses of RR Lyrae stars is very small, showing a decrease of only 0.01-0.02 solar mass compared to the models based on old Cox-Stewart opacities. Consequently, with the new Livermore OPAL opacities, both the stellar pulsation and evolution models now predict approximately the same masses for the RR Lyrae stars. Our evolutionary models suggest that the mean masses of the RR Lyrae stars are about 0.76 and about 0.71 solar mass for M15 (Oosterhoff group II) and M3 (group I), respectively. If (alpha/Fe) = 0.4, these values are decreased by about 0.03 solar mass. Variations of the mean masses of RR Lyrae stars with HB morphology and metallicity are also presented.

Yi, Sukyoung↗
NASA NTRS1995

Gravitational radiation from rapidly rotating nascent neutron stars

We study the secular evolution and gravitational wave signature of a newly formed, rapidly rotating neutron star. The neutron star may arise from core collapse in a massive star or from the accretion-induced collapse of a white dwarf. After a brief dynamical phase, the nascent neutron star settles into an axisymmetric, secularly unstable state. Gravitational radiation drives the star to a nonaxisymmetric, stationary equilibrium configuration via the bar-mode instability. The emitted quasi-periodic gravitational waves have a unique signature: the wave frequency sweeps downward from a few hundred Hertz to zero, while the wave amplitude increase from zero to a maximum and then decays back to zero. Such a wave signal could detected by broadband gravitational wave interferometers currently being constructed. We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency (f greater than or approximately = 1000 Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.

Lai, Dong↗
NASA NTRS1975

Fossil dust shells around luminous supergiants

The observed frequency with which infrared excesses appear in F, G, and K supergiants of luminosity class Ia supports the idea that these excesses arise in a 'fossil' circumstellar dust shell that was formed during a prior M-super-giant phase of evolution. The required leftward evolution of the star on the H-R diagram would then imply that the Ledoux, rather than the Schwarzschild, criterion for convective mixing is the correct criterion to use in stellar evolution calculations.

Stothers, R.↗
NASA NTRS1982

Unsolved problems

The level of understanding of the physics of single, isolated B stars is assessed and unresolved problems are defined. The significant observational results concerning the effective temperatures, radii, masses and mantles are summarized. The results of the theory of the evolution of massive stars are confronted with the observed luminosities and effective temperatures of B stars. In addition the implications of stellar spectra theory are compared with observed spectra and a heuristic model for a mantle is developed. The chief unresolved problems for B stars concern developing detailed models for (1) the internal structure of massive stars which are beginning to evolve rapidly as they complete burning hydrogen in their cores; (2) mantles; and (3) the transfer of radiation in high temperature inhomogeneous moving bodies of gas.

Source record↗
NASA NTRS1993

Neutron star mergers and gamma-ray bursts

Under the support of grant NAG 5-1904, we have carried out research on several topics related to gamma-ray bursts (GRB's). In our proposal, we stated that we would study three topics: (1) fireball evolution; (2) neutron star mergers; and (3) statistics of bursts. We have completed a significant amount of work in each of these areas. Resulting papers from this work are presented.

Narayan, Ramesh↗
NASA NTRS1990

Models of Wolf-Rayet stars

The current status of knowledge about formation, structure and evolution of Wolf-Rayet stars is reviewed, with emphasis on a discussion of corresponding stellar models. The relevance of the LBV-scenario for WR star formation is outlined. Hydrogenless WR stars are shown to closely follow simple relations for the dependence of luminosity, radius, and surface temperature as a function of their mass. The use of these relations for simplified WR evolution calculations is demonstrated. Surface abundance predictions for the different WR types are discussed, with special emphasis to the WN + WC spectral type. Details are presented concerning the WR phase of a recent 60 solar mass evolutionary calculation, which was computed with the same input physics which reproduced the progenitor evolution of SN 1987 A in a 20 solar mass case, and which may be a representative case concerning WR stars in many respects.

Langer, Norbert↗
NASA NTRS2011

Observational Effects of Magnetism in O Stars: Surface Nitrogen Abundances

Aims. We investigate the surface nitrogen content of the six magnetic O stars known to date as well as of the early B-type star Tau Sco.. We compare these abundances to predictions of evolutionary models to isolate the effects of magnetic field on the transport of elements in stellar interiors. Methods. We conduct a quantitative spectroscopic analysis of the ample stars with state-of-the-art atmosphere models. We rely on high signal-to-noise ratio, high resolution optical spectra obtained with ESPADONS at CFHT and NARVAL at TBL. Atmosphere models and synthetic spectra are computed with the code CMFGEN. Values of N/H together with their uncertainties are determined and compared to predictions of evolutionary models. Results. We find that the magnetic stars can be divided into two groups: one with stars displaying no N enrichment (one object); and one with stars most likely showing extra N enrichment (5 objects). For one star (Ori C) no robust conclusion can be drawn due to its young age. The star with no N enrichment is the one with the weakest magnetic field, possibly of dynamo origin. It might be a star having experienced strong magnetic braking under the condition of solid body rotation, but its rotational velocity is still relatively large. The five stars with high N content were probably slow rotators on the zero age main sequence, but they have surface N/H typical of normal O stars, indicating that the presence of a (probably fossil) magnetic field leads to extra enrichment. These stars may have a strong differential rotation inducing shear mixing. Our results shOuld be viewed as a basis on which new theoretical simulations can rely to better understand the effect of magnetism on the evolution of massive stars.

Martins, F.↗