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  • ASTROPHYSICS  (9)
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  • 1
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    Stellar evolution from the zero-age main sequence (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: A consistent set of 247 evolutionary sequences extending from the ZAMS to the red-giant branch is presented for Y from 0.10 to 0.40, Z from 0.00001 to 0.10, and masses of 0.55 to 6.90 solar masses. Each sequence is started from a homogeneous ZAMS model, and almost all are evolved to the base of the red-giant branch. It is shown that: (1) the relative position of the main sequence can be determined as a function of composition; (2) theoretical luminosity functions can be derived from the relative evolutionary time scales; (3) a dip in luminosity sometimes occurs at the base of the red-giant branch and is most pronounced at larger Z values; (4) metal-poor stars evolve farther up along the main sequence before turning off toward the red-giant branch; and (5) the onset of helium burning halts the evolution across the Hertzsprung gap for the most massive and most metal-poor models, so that the star remains blue during its phase of core-helium burning.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series; 40; Aug. 197
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  • 2
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    Evolutionary models of halo stars with rotation. II - Effects of metallicity on lithium depletion, and possible implications for the primordial lithium abundance (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Models of metal-poor stars with rotation were computed and their lithium depletion was compared with observations of halo stars. The models that have turn-off ages compatible with the observations have a nearly flat Li-T(eff) relationship in the region of the Spite lithium 'plateau'. Depending on the initial angular momentum, the models have a depletion factor ranging between a factor of 5 and a factor of 10 at fixed T(eff), implying a maximum initial lithium abundance of 3.1. Both the dispersion and the overall depletion factor are much smaller for metal-poor models than for solar metallicity ones. The factors that determine lithium depletion in rotational models are discussed and the different depletion patterns in solar metallicity and metal-poor models are traced to differences in their structure and evolution. The dependence of the lithium depletion on age, mass, initial angular momentum, and metallicity is also discussed. The dispersion predicted from these models is not inconsistent with the observations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 78; 179-203
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  • 3
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    Evolution and seismology of Procyon (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We have calculated an array of stellar models for Procyon A that are based on the best physics available to us, including the latest opacities and nuclear cross sections. The array of models spans the error space centered on Procyon's mass, chemical composition, effective temperature, and luminosity. We find that with OPAL opacities no convective overshoot is needed at the edge of the convective core to match Procyon's position in the H-R diagram. We discuss the discrepancy between the astrophysical mass and the astrometric mass, which is reduced by the more up-to-date physics in the models, and describe how oscillation data can be used to distinguish among the possible models and help resolve the discrepancy. We have calculated oscillation frequencies for the l = 0, 1, 2, and 3 p-modes and oscillation periods for the l = 1, 2, and 3 g-modes for several of these models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 405; 1; p. 298-306.
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  • 4
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    The metal-rich globular cluster NGC 6553 - Clump morphology and age (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Stellar evolutionary tracks for metal-rich HB stars have been used to construct synthetic population models for the peculiar HB clump in NGC 6553. Although the uncertainties are large, it is possible to derive an age for NGC 6553 in the range 8-13 Gyr, which supports the idea that the disk globular cluster system is younger than the oldest stars in the Galactic halo and bulge.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 265; 1; p. 40-42.
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  • 5
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    Li-7 abundances in halo stars: Testing stellar evolution models and the primordial Li-7 abundance (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A large number of stellar evolution models with (Fe/H) = -2.3 and -3.3 have been calculated in order to determine the primordial Li-7 abundance and to test current stellar evolution models by a comparison to the extensive database of accurate Li abundances in extremely metal-poor halo stars observed by Thorburn (1994). Standard models with gray atmospheres do a very good job of fitting the observed Li abundances in stars hotter than approximately 5600 K. They predict a primordial. Li-7 abundance of log N(Li) = 2.24 +/- 0.03. Models which include microscopic diffusion predict a downward curvature in the Li-7 destruction isochrones at hot temperatures which is not present in the observations. Thus, the observations clearly rule out models which include uninhibited microscopic diffusion of Li-7 from the surface of the star. Rotational mixing inhibits the microscopic diffusion and the (Fe/H) = -2.28 stellar models which include both diffusion and rotational mixing provide an excellent match to the mean trend in T(sub eff) which is present in the observations. Both the plateau stars and the heavily depleted cool stars are well fit by these models. The rotational mixing leads to considerable Li-7 depletion in these models and the primordial Li-7 abundance inferred from these models is log N(Li) = 3.08 +/- 0.1. However, the (Fe/H) = -3.28 isochrones reveal problems with the combined models. These isochrones predict a trend of decreasing log N(Li) with increasing T(sub eff) which is not present in the observations. Possible causes for this discrepancy are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 433; 2; p. 510-519
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  • 6
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    Rotation of low-mass stars - A new probe of stellar evolution (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Models of stars of various masses and rotational parameters were developed and compared with observations of stars in open clusters of various ages in order to analyze the evolution of rotating stars from the early premain sequence to an age of 1.7 x 10 to the 9th yrs. It is shown that, for stars older than 10 to the 8th yrs and less massive than 1.1 solar mass, the surface rotation rates depend most strongly on the properties of the angular momentum loss. The trends of the currently available observations suggest that the rotation periods are a good indicator of the field-star ages.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 74; 501-550
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  • 7
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    Stellar models with microscopic diffusion and rotational mixing. 2: Application to open clusters (1995)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: Stellar models with masses ranging from 05.5 to 1.3 solar mass were constructed for comparison with young cluster observations of Li and of rotation velocities. The amount of Li depletion in cool stars is sensitive to the amount of overshoot at the base of the surface convection zone, and the exact metallicity of the models. Even when this is taken into account, the Li observations are a severe constraint for the models and rule out standard models and pure diffusion models. Stellar models which include diffusion and rotational mixing in the radiative regions of stars are able to simultaneously match the Li abundances observed in the Pleiades, the UMa Group, The Hyades, Praesepe, NGC 752, and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to simultaneously explain the presence of the rapidly rotating late G and K stars in the Pleiades and the absence of rapidly rotating late F and early G stars.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 441; 2; p. 876-885
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  • 8
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    The evolution of high-metallicity horizontal-branch stars and the origin of the ultraviolet light in elliptical galaxies (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 388; L53-L56
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  • 9
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    The evolution of red clump stars - Theoretical sequences (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Two grids of horizontal-branch evolutionary sequences are presented which extend the grid of Sweigart and Gross (1976) to higher masses and higher metallicities. These two grids were constructed specifically for the study of red clump stars observed in intermediate-age star clusters in the Magellanic Clouds and in the Galaxy. An additional evolutionary track, with a thick hydrogen-burning shell which simulates the effect of a modest amount of mixing from the helium core into the shell at the time of the helium-core flash, is also presented. It is found that a small amount of mixing does not alter in a significant way the standard course of horizontal-branch evolution.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 63; 917-945
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