Astronomy and Astrophysics – Astrophysics
Scientific paper
1996-09-11
Astronomy and Astrophysics
Astrophysics
LaTeX, 4 pages, 5 figures, A&A style, A&A Letters accepted
Scientific paper
Quantitative spectroscopic analyses of two faint blue stars (V=19.5-20.0 mag) in the globular cluster M 15 are presented. Their derived Teff, gravities and absolute magnitudes (Teff=24000K, log g=5.2, Mv=4.3; Teff=36000K, log g=5.9, Mv=4.7, respectively) are matched very well by models for the Extreme Horizontal Branch (EHB). Both stars are bona-fide subdwarf B stars making M 15 only the second globular cluster (after NGC 6752) for which the existence of sdB stars has been proven spectroscopically. While the helium abundance (one tenth solar) of F1-1 is typical for sdB stars, F2-2 surprisingly turned out to be a helium rich star, the first to be reported as a member of a globular cluster. In the field population of the Milky Way such stars are rare (less than 5% of all sdB stars). From its proximity to the helium main sequence, it is speculated that F2-2 may be a naked helium core, i.e. an Extreme Horizontal Branch star which lost (almost) all of its hydrogen-rich envelope.
Durrell Patrick R.
Heber Uli
Moehler Sabine
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