“Hot Helium Flashers” The Road to Extreme Horizontal Branch Stars

Details “Hot Helium Flashers” The Road to Extreme Horizontal Branch Stars “Hot Helium Flashers” The Road to Extreme Horizontal Branch Stars

Oct 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008iaus..252..261y&link_type=abstract

The Art of Modeling Stars in the 21st Century, Proceedings of the International Astronomical Union, IAU Symposium, Volume 252, p

Astronomy and Astrophysics

Astronomy

1

Stars: Evolution, Stars: Horizontal-Branch, Stars: Mass Loss

Scientific paper

Observational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in Extreme Horizontal Branch (EHB) and subsequent evolutionary phases.
Using our new stellar evolution code a code that follows through complete evolutionary tracks, Pre-MS to cooling WD without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs.
HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 - 1.1 M&sun;) and for metallicities covering both populations I and II (Z = 0.01 - 0.001). The [M,Z] combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr.

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