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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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NASA NTRS1991

A line identification study of the IUE SWP high dispersion spectrum of the UV-bright star Barnard 29 of Messier 13

A line identification study was performed of the coaddition of three SWP high dispersion IUE exposures of the UV-bright star Barnard 29 of Messier 13. One of these images took two IUE shifts. The previous study by de Boer and Savage (1983) was extended and stellar lines of C I, C II, C III, C IV, N I, N III, O I, O III, O IV,Al III, Si II, Si III, Si IV, P III, S II, and S III and interstellar lines of C I, C II, C IV, N I, N III, N IV, N V, O I, Al II, Al III, Si II, Si IV, S II, and Fe II were found. These lines are the strongest expected lines in a hot Population II star. Apparent intensity minima corresponding to interstellar features are noted, especially those matching a high velocity cloud found by de Boer and Savage.

Adelman, S. J.↗
NASA NTRS1972

Enrichment of intergalactic matter.

The primordial gas out of which the Galaxy condensed may have been significantly enriched in heavy elements. A specific mechanism of enrichment is described, in which quasi-stellar sources eject enriched matter into the intergalactic medium. This matter is recycled through successive generations of these sources, and is progressively enriched. The enriched intergalactic matter is accreted by the protogalaxy and we find, for rates of mass ejection by quasi-stellar sources equal to about one solar mass per year in heavy elements, that this mechanism can account for the heavy-element abundances in the oldest Population II stars. Expressions are given for the degree of enrichment of the intergalactic gas as a function of redshift, and we show that our hypothesis implies that the present density of intergalactic gas must be at least a factor 3 larger than the mean density in galaxies at the present epoch.

Silk, J.↗
NASA NTRS1988

Primordial lithium - New reaction rates, new abundances, new constraints

Newly measured nuclear reaction rates for H-3(alpha, gamma)Li-7 (higher than previous values) and Li-7(p, alpha)He-4 (lower than previous values) are shown to increase the Li-7 yield from big band nucleosynthesis for lower baryon-to-photon ratio (less than about 4 x 10 to the 10th). Recent revisions in the He-3(alpha, gamma)Be-7 and the D(p, gamma)He-3 rates enhance the high (greater than 4 x 10 to the 10th) Li-7(Be) production. New, independent determinations of Li abundances in extreme population II stars are in excellent agreement with the work of Spites and give continued confidence in the use of Li-7 in big bang baryon density determinations.

Kawano, Lawrence↗
NASA NTRS1992

Light-element abundances in Population II dwarfs

This paper reviews the abundances of the light elements lithium, beryllium, and boron in main-sequence and subgiant, Population II stars. Li is important to cosmology because it is synthesized in the Big Bang, but is also used to study stellar structure. Beryllium is useful for studying galactic chemical evolution because its formation in the interstellar medium involves different physics to stellar nucleosynthesis, and it thus provides independent data on the evolution of the halo. Some (though not all) inhomogeneous Big Bang nucleosynthesis codes predict a significant primordial component to this element, so its observed abundance may constrain such models. Boron, observations of which became feasible with the operation of the Hubble Space Telescope, provides complementary data to Be, helping check the element ratios predicted by calculations of spallation reactions in the interstellar medium, and will indicate whether the observed Be abundance has an excess over the expected spallation component, indicating a possible primordial component.

Ryan, Sean G.↗
NASA NTRS1992

Astrophysical Li-7 as a product of big bang nucleosynthesis and galactic cosmic-ray spallation

The astrophysical Li-7 abundance is considered to be largely primordial, while the Be and B abundances are thought to be due to galactic cosmic ray (GCR) spallation reactions on top of a much smaller big bang component. But GCR spallation should also produce Li-7. As a consistency check on the combination of big bang nucleosynthesis and GCR spallation, the Be and B data from a sample of hot population II stars is used to subtract from the measured Li-7 abundance an estimate of the amount generated by GCR spallation for each star in the sample, and then to add to this baseline an estimate of the metallicity-dependent augmentation of Li-7 due to spallation. The singly reduced primordial Li-7 abundance is still consistent with big bang nucleosynthesis, and a single GCR spallation model can fit the Be, B, and corrected Li-7 abundances for all the stars in the sample.

Olive, Keith A.↗
NASA NTRS1994

Production of Lithium, Beryllium, and Boron from Baryon inhomogeneous primordial nucleosynthesis

We investigate the possibility that inhomogeneous nucleosynthesis may eventually be used to explain the abundances of Li-6, Be-9, and B in Population II stars. The present work differs from previous studies in that we have used a more extensive reaction network. It is demonstrated that in the simplest scenario the abundances of the light elements with A less than or = 7 constrain the separation of inhomogeneities to sufficently small scales that the model is indistinguishable from homogeneous nucleosynthesis and that the abundnace of Li-6, Be-9, and B are then below observations by several orders of magnitude. This conclusion does not depend on the Li-7 constraint. We also examine alternative scenarios which involve a post-nucleosynthesis reprocessing of the light elements to reproduce the observed abundances of Li and B, while allowing for a somewhat higher baryon density (still well below the cosmological critical density). Future B/H measurements may be able to exclude even this exotic scenario and further restrict primirdial nucleosynthesis to approach the homogeneous model conclusions.

Thomas, David↗
NASA NTRS2004

FUSE Cycle 3 Program CO22: Chromospheric Activity in Population II Giants

One of the mysteries of Population II giants is that they still show chromospheric emission despite their great age. The global dynamo which was active during their main-sequence lifetimes is expected to become extremely weak through magnetic rotational braking. The nature of the observed emission is not understood; although acoustic shock waves might provide the heating, acoustic waves are not predicted to drive the observed mass loss - which in turn requires the dissipation of magneto-hydrodynamic waves. This program was designed to search for the faint stellar H Ly(beta) emission wings and the fluorescent Fe II and H2 emission from one of the brightest, metal poor, Population II stars. These FUSE diagnostics, when combined with existing UV and optical spectra, help determine the major radiative cooling channels for the chromosphere. This observation was to complement that previously planned for the mildly metal deficient giant alpha Boo (K2 III). However, a Boo has yet to be observed with FUSE.

Harper, Graham M.↗
NASA NTRS2004

Chromospheric Activity in Population II Giants

One of the mysteries of Population II giants is that they still show chromospheric emission despite their great age. The global dynamo which was active during their main-sequence lifetimes is expected to become extremely weak through magnetic rotational braking. The nature of the observed emission is not understood; although acoustic shock waves might provide the heating, acoustic waves are not predicted to drive the observed mass loss - which in turn requires the dissipation of magneto-hydrodynamic waves. This program was designed to search for the faint stellar H Ly beta emission wings and the fluorescent Fe II and H2 emission from one of the brightest, metal poor, Population II stars. These FUSE diagnostics, when combined with existing UV and optical spectra, help determine the major radiative cooling channels for the chromosphere. This observation was to complement that previously planned for the mildly metal deficient giant alpha Boo (K2 III). However, alpha Boo has yet to be observed with FUSE.

Harper, Graham M.↗
NASA NTRS1988

Spectral synthesis in the ultraviolet. II - Stellar populations and star formation in blue compact galaxies

An initial attempt to apply optimizing spectral synthesis techniques to the far-UV spectra of blue compact galaxies (BCGs) is presented. The far-UV absorption-line spectra of the galaxies are clearly composite, with the signatures of the main-sequence types between O3 and mid-A. Most of the low-ionization absorption lines have a stellar origin. The Si IV and C IV features in several objects have P Cygni profiles. In Haro I the strength of Si IV indicates a significant blue supergiant population. The metal-poor blue compact dwarf Mrk 209 displays weak absorption lines, evidence that the stellar component has the same low metallicity as observed in the ionized gas. Good fits to the data are obtained the technique of optimizing population synthesis. The solutions yield stellar luminosity functions which display large discontinuities, indicative of discrete star formation episodes or bursts. The amount of UV extinction is low.

Fanelli, Michael N.↗
NASA NTRS1981

IUE observations of blue halo high luminosity stars

Two high luminosity population II blue stars of high galactic latitude, BD+33 deg 2642 and HD 137569 were observed at high resolution. The stellar spectra show the effect of mass loss in BD+33 deg 2642 and abnormally weak metallic lines in HD 137569. The interstellar lines in the direction of BD+33 deg 2642, which lies at a height z greater than or equal to 6.2 kpc from the galactic plane, are split into two components. No high ionization stages are found at the low velocity component; nor can they be detected in the higher velocity clouds because of mixing with the corresponding stellar/circumstellar lines.

Hack, M.↗
NASA NTRS2003

HST Snapshot Study of Variable Stars in Globular Clusters: Inner Region of NGC 6441

We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the metal-rich globular cluster NGC 6441 for its variable stars. A total of 57 variable stars was found including 38 RR Lyrae stars, 6 Population II Cepheids, and 12 long period variables. Twenty-four of the RR Lyrae stars and all of the Population II Cepheids were previously undiscovered in ground-based surveys. Of the RR Lyrae stars observed in h s survey, 26 are pulsating in the fundamental mode with a mean period of 0.753 d and 12 are first-overtone mode pulsators with a mean period of 0.365 d. These values match up very well with those found in ground-based surveys. Combining all the available data for NGC 6441, we find mean periods of 0.759 d and 0.375 d for the RRab and RRc stars, respectively. We also find that the RR Lyrae in this survey are located in the same regions of a period-amplitude diagram as those found in ground-based surveys. The overall ratio of RRc to total RR Lyrae is 0.33. Although NGC 6441 is a metal-rich globular cluster and would, on that ground, be expected either to have few RR Lyrae stars, or to be an Oosterhoff type I system, its RR Lyrae more closely resemble those in Oosterhoff type II globular clusters. However, even compared to typical Oosterhoff type II systems, the mean period of its RRab stars is unusually long. We also derived I-band period-luminosity relations for the RR Lyrae stars. Of the six Population II Cepheids, five are of W Virginis type and one is a BL Herculis variable star. This makes NGC 6441, along with NGC 6388, the most metal-rich globular cluster known to contain these types of variable stars. Another variable, V118, may also be a Population II Cepheid given its long period and its separation in magnitude from the RR Lyrae stars. We examine the period-luminosity relation for these Population II Cepheids and compare it to those in other globular clusters and in the Large Magellanic Cloud. We argue that there does not appear to be a change in the period-luminosity relation slope between the BL Herculis and W Virginis stars, but that a change of slope does occur when the RV Tauri stars are added to the period-luminosity relation.

Pritzl, Barton J.↗
NASA NTRS1987

Intrinsically variable stars

The characteristics of intrinsically variable stars are examined, reviewing the results of observations obtained with the IUE satellite since its launch in 1978. Selected data on both medium-spectral-class pulsating stars (Delta Cep stars, W Vir stars, and related groups) and late-type variables (M, S, and C giants and supergiants) are presented in spectra, graphs, and tables and described in detail. Topics addressed include the calibration of the the period-luminosity relation, Cepheid distance determination, checking stellar evolution theory by the giant companions of Cepheids, Cepheid masses, the importance of the hydrogen convection zone in Cepheids, temperature and abundance estimates for Population II pulsating stars, mass loss in Population II Cepheids, SWP and LWP images of cold giants and supergiants, temporal variations in the UV lines of cold stars, C-rich cold stars, and cold stars with highly ionized emission lines.

Bohm-Vitense, Erika↗
NASA NTRS2004

Anchoring the Population II Distance Scale: Accurate Ages for Globular Clusters

The metal-poor stars in the halo of the Milky Way galaxy were among the first objects formed in our Galaxy. These Population II stars are the oldest objects in the universe whose ages can be accurately determined. Age determinations for these stars allow us to set a firm lower limit, to the age of the universe and to probe the early formation history of the Milky Way. The age of the universe determined from studies of Population II stars may be compared to the expansion age of the universe and used to constrain cosmological models. The largest uncertainty in estimates for the ages of stars in our halo is due to the uncertainty in the distance scale to Population II objects. We propose to obtain accurate parallaxes to a number of Population II objects (globular clusters and field stars in the halo) resulting in a significant improvement in the Population II distance scale and greatly reducing the uncertainty in the estimated ages of the oldest stars in our galaxy. At the present time, the oldest stars are estimated to be 12.8 Gyr old, with an uncertainty of approx. 15%. The SIM observations obtained by this key project, combined with the supporting theoretical research and ground based observations outlined in this proposal will reduce the estimated uncertainty in the age estimates to 5%).

Chaboyer, Brian C.↗
NASA NTRS1986

The ultraviolet IUE spectrum of the population II horizontal-branch A star HD 109995

Eight SWP and six LWP high dispersion IUE images of the field horizontal-branch A star HD 109995 have been averaged to increase the signal-to-noise ratio. The line density in the UV is greater than that of the optical region, but less than that of comparable population I A stars. Many of the identified lines are those of Fe II. Lines of C I, C II, N I, O I, Mg I, Mg II, Al II, Al III, Si I, Si II, P II, S I, Cl I, Ti II, V II, Cr II, Mn II, Fe I, Co II, Ni II, Zn II, and Ge II are also present. Definite interstellar lines of C II, Mg I, Mg II, Al II, Si II, and Fe II were found.

Adelman, S. J.↗
NASA NTRS1994

On the determination of the number of O stars in H II regions and starburst galaxies

The hot star population in H II regions, H II galaxies, and starburst galaxies is often described in terms of the number of 'equivalent' O stars of a single representative subtype and luminosity class needed to produce the ionizing luminosity deduced from the nebular recombination lines in the optical spectra. In this paper we define conversion factors eta(sub 0), eta(sub 1), and zeta(sub 5000) with which the total number of O V stars and their flux contribution at 5000 A can be derived from the number of these 'equivalent' stars. These quantities depend primarily on three parameters: the slope and upper mass limit of the stellar mass function and the metallicity of the region. Using the latest stellar atmosphere and evolution models, we calculate eta(sub 0), eta(sub 1), and zeta(sub 5000) for a large number of values of these parameters. The results are presented in tabular as well as graphical form. We apply our results to two H II regions for which the hot star population are known and find that the predicted numbers of O stars agree well the observed counts. In addition, we describe a method by which the values of eta(sub 0) and eta(sub 1) and the observed emission-line fluxes can be used to place constraints on the allowed values of the slope and upper mass limit of the stellar mass function in a region.

Vacca, William D.↗
NASA NTRS1994

Do gamma-ray bursts originate from an extended Galactic Halo of high-velocity neutron stars?

The gamma-ray burst brightness distribution is inhomogeneous and the distribution on the sky is nearly isotropic. These features argue against an association of gamma-ray bursts with those Galactic objects that are known to exhibit a strong concentration toward the Galactic center or plane. The observed statistical properties indicate a cosmological origin. Circumstantial evidence suggests that neutron stars are involved in the burst phenomenon. Here we consider Population II neutron stars in an extended Galactic Halo (EGH) as an alternative to cosmological scenarios. The Burst and Transient Source Experiment (BATSE) data indicate a small deviation from isotropy near the 2 sigma level of statistical significance. If confirmed for an increasing number of bursts, these anisotropies could rule out cosmological scenarios. On the other hand, EGH models require small anisotropies like those observed by BATSE. We consider simple distribution models to determine the generic properties such halos must have to be consistent with the observations and discuss the implications of the corresponding distance scale on burst models.

Hartmann, Dieter H.↗