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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The very high rotators in the late-B and early-A stars: Shell stars with Si IV and C IV features the case of HD 119921

Study of several stars in the late B and early A spectral types shows that very high rotators are associated with shell characteristics (sometimes not detected at all in the visible spectra) and also with C IV and some Si IV spectral absorption features which can be explained by circumstellar phenomena superimposed over stellar metallic blends. These particularities are evidenced by comparison with other spectra of low and high rotators in the same spectral range. HD 119921, a star with similar characteristics to the other ones of the sample, is given special attention. A possible scenario is suggested to explain the observed superionization features.

Freireferrero, R.↗
NASA NTRS1991

CO fundamental bands in late-type stars. II - Spectrum simulations for F-K stars

The procedure of Ayres and Wiedemann (1989) was applied to a range of stellar-atmosphere models to study the CO Delta-v = 1 spectrum and to establish its use as a remote sensor of thermal conditions in late-type stars. Spectra were computed to examine the sensitivity of the CO Delta-v = 1 to fundamental stellar parameters and to assess the errors introduced into the spectrum intepretation by uncertain input parameters and non-LTE effects. Results of the sensitivity study demonstrate that CO fundamental spectra are useful probes for the temperature structure of the outer layers of cool stellar atmospheres, but that their value is limited by the uncertainties introduced by non-LTE effects. However, in stars with surface gravities of log g of about 1.5 and greater, the values of these uncertainties are reasonably small.

Wiedemann, Guenter↗
NASA NTRS1991

Evolutionary models of halo stars with rotation. I - Evidence for differential rotation with depth in stars

Evolutionary models of metal-poor stars are computed including the effects of rotation, and their properties are compared with observations. The models rotate slowly at the surface, in agreement with the observed upper limits on the rotation velocity at main-sequence turnoff; they also have substantial differential rotation with depth. This differential rotation preserves a sufficient amount of internal angular momentum to explain the rapid rotation of evolved horizontal-branch stars. These results hold for a wide range of angular momentum loss and transport parameter values. Differences and similarities between the surface and internal rotation of solar metallicity and metal-poor models are discussed. Rigidly rotating models are found to be incompatible with the observations once giant branch mass loss is taken into account. Horizontal-branch rotation velocity measurements as a function of color are proposed as a test of the rotation law enforced in convection zones, and their dependence on cluster age and metallicity are discussed.

Pinsonneault, Marc H.↗
NASA NTRS1991

Wolf-Rayet stars of type WN/WC and mixing processes during core helium burning of massive stars

Consequences of the recent finding that most WN/WC spectra probably originate from individual Wolf-Rayet stars for the internal structure of massive stars are discussed. Numerical models including the effect of slow-down or prevention of convective mixing due to molecular weight gradients are presented, in which a transition layer with a composition mixture of H- and He-burning ashes is formed above the convective He-burning core. These models are able to qualitatively account for the observed WN/WC frequency and agree quantitatively with the only WN/WC-composition determination so far. It is argued that the same transition layer may be responsible for the final blue loop which the SN 1987 A progenitor performed some 10,000 yr before explosion. These results indicate that composition barriers may be efficient in restricting convection during central helium burning, in contrast to computations relying on the Schwarzschild criterion for convection, with or without overshooting.

Langer, N.↗
NASA NTRS1993

The chromospheres of late-type stars. II - An atlas of chromospheric lines for selected early-K stars

High-resolution spectra of the chromospheric Na I D lines, Ca I 4227, Mg I 4571, 5167, and 5172, the Ca II H and K resonance and IR 8542.144 and 8662.170 lines, and H-alpha and H-beta, all observed simultaneously at the AAT, are presented. These data are presented as the observational basis for the self-consistent, semiempirical modeling of the outer photospheres and chromospheres of the target stars. Stellar activity is found in the low-chromosphere lines as core-filling and, in some stars, as line-broadening. Integrated fluxes are derived from the difference spectra, formed by subtracting quiet from active spectra, in the Ca II H and K resonance and IR 8542 and 8662 lines and in H-alpha, which are presented as chromospheric activity indicators. All the activity indices exhibit strong correlations, with the exception of the equivalent widths of H-alpha and H-beta, which are found to be relatively poor activity discriminators.

Thatcher, John D.↗
NASA NTRS2010

The Fermi Gamma-Ray Space Telescope, Exploding Stars, Neutron Stars, and Black Holes

Since August, 2008, the Fermi Gamma-ray Space Telescope has been scanning the sky, producing a full-sky image every three hours. These cosmic gamma-rays come from extreme astrophysical phenomena, many related to exploding stars (supernovae) or what these explosions leave behind: supernova remnants, neutron stars, and black holes. This talk uses sample Fermi results, plus simple demonstrations, to illustrate the exotic properties of these endpoints of stellar evolution.

Thompson, David J.↗
NASA NTRS2016

The Star Formation Rate Efficiency of Neutral Atomic-Dominated Hydrogen Gas in the Ooutskirts of Star-Forming Galaxies From z approx. 1 to z approx. 3

Current observational evidence suggests that the star formation rate (SFR)efficiency of neutral atomic hydrogen gas measured in damped Ly(alpha) systems (DLAs) at z approx. 3 is more than 10 times lower than predicted by the Kennicutt-Schmidt (KS)relation. To understand the origin of this deficit, and to investigate possible evolution with redshift and galaxy properties, we measure the SFR efficiency of atomic gas at z approx. 1, z approx. 2, and z approx. 3 around star-forming galaxies. We use new robust photometric redshifts in the Hubble Ultra Deep Field to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies' outskirts. We find that the SFR efficiency of H I gas at z > 1 is approx. 1%-3% of that predicted by the KS relation. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency with redshift. Our analysis instead suggests that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity playing a secondary effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies and local dwarf galaxies.

Rafelski, Marc↗
DOE OSTI2025

Connecting GRBs from binary neutron star mergers to nuclear properties of neutron stars

The fate of the binary neutron star (NS) merger remnants hinges sensitively upon the NS equation of state and the threshold mass, M ls , that separates a long-lived from a short-lived NS remnant. The nature of the electromagnetic counterparts is also influenced by the remnant type, particularly in determining whether a gamma-ray burst from a compact binary merger (cbGRB) is of short or long duration. We propose a novel approach to probe Mls by linking it to the estimated observed ratio of long to short cbGRBs. We find that current observations broadly favor a relatively high value for this transition, M ls ≃ 1.3M TOV , for which M TOV ≲ 2.6M ⊙ , consistent with numerical simulations, as also shown here. Furthermore, our results disfavor nuclear physics scenarios that would lead to catastrophic pressure loss at a few times nuclear density and temperatures of tens of MeV, leading to a rapid gravitational collapse of binaries with total mass M ≲ 1.3M TOV . Future individual gravitational wave events with on-axis cbGRBs can further bound Mls.

79 ASTRONOMY AND ASTROPHYSICS↗