Age-luminosity relations for low-mass metal-poor stars

We present a grid of evolutionary calculations for metal-poor low-mass stars for a variety of initial helium and metal abundances. The intention is mainly to provide a database for deriving directly stellar ages of halo and globular cluster stars for which basic stellar parameters are known, but the tracks can also be used for isochrone or luminosity function construction, since they extend to the tip of the red giant branch. Fitting formulae for age-luminosity relations are provided as well. The uncertainties of the evolutionary ages due to inherent shortcomings in the models and due to the unclear effectiveness of diffusion are discussed. A first application to field single stars is presented.Comment: accepted for publication by Astron. Astrophys. Suppl. Series; Appendix (tables) include

Fine-tuning the basic forces of nature through the triple-alpha process in red giant stars

We show that the synthesis of carbon and oxygen through the triple-alpha process in red giant stars is extremely sensitive to the fine details of the nucleon-nucleon (N-N) interaction. A +/-0.5% change in the strength of the N-N force would reduce either the carbon or oxygen abundance by as much as a factor of 30-1000. This result may be used to constrain some fundamental parameters of the Standard Model.Comment: 3 pages with 2 figures. Proceedings of the Nuclei in the Cosmos Conference, Aarhus, Denmark, June 27-July 1, 2000. To be published in Nuclear Physics A. The postscript file and more information are available at http://matrix.elte.hu/~csoto http://info.tuwien.ac.at/e142/ and http://www.MPA-Garching.MPG.DE/~schlattl

Stellar production rates of carbon and its abundance in the universe

The bulk of the carbon in our universe is produced in the triple-alpha process in helium-burning red giant stars. We calculated the change of the triple-alpha reaction rate in a microscopic 12-nucleon model of the C-12 nucleus and looked for the effects of minimal variations of the strengths of the underlying interactions. Stellar model calculations were performed with the alternative reaction rates. Here, we show that outside a narrow window of 0.5 and 4% of the values of the strong and Coulomb forces, respectively, the stellar production of carbon or oxygen is reduced by factors of 30 to 1000.Comment: 6 pages with 1 figure. Science, 2000 July 7 issue. The postscript file and more information are available at http://info.tuwien.ac.at/e142/, http://nova.elte.hu/~csoto and http://www.MPA-Garching.MPG.DE/~schlattl

First full evolutionary computation of the He-flash induced mixing in Population II stars

The core helium-flash in low-mass stars with extreme mass loss occurs after the tip of the RGB, when the H-rich envelope is very thin. The low efficiency of the H-shell source enables the He-flash driven convective zone to penetrate H-rich layers and trigger a thermonuclear runaway, resulting in a subsequent surface enrichment with He and C. In this work we present the first full computations of Population II low-mass stellar models through this phase. Models experiencing this dredge-up event are significantly hotter than their counterparts with H-rich envelopes, which makes them promising candidates for explaining the existence of stars observed beyond the canonical blue end of the horizontal branch ("blue hook stars"). Moreover, this temperature difference could explain the observed gap in M_V between extreme blue horizontal-branch and blue hook stars. A first comparison with spectroscopic observations of blue hook stars in the globular cluster omega Cen is also presented.Comment: 4 pages, 2 figures, uses emulateapj.sty; accepted for publication in ApJ