"Hot helium flashers" - The road to extreme horizontal branch stars

Research output: Contribution to journalArticlepeer-review

Abstract

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) 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.

Original languageEnglish
Pages (from-to)261-262
Number of pages2
JournalProceedings of the International Astronomical Union
Volume4
Issue numberS252
DOIs
StatePublished - 2008

Keywords

  • Stars: evolution
  • Stars: horizontal-branch
  • Stars: mass loss

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