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Astron. Astrophys. 317, L83-L86 (1997)

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Letter to the Editor

Hot HB stars in globular clusters - physical parameters and consequences for theory

IV. sdB candidates in M 15

S. Moehler * 1, 2, U. Heber 3 and P.R. Durrell 4

1 Landessternwarte, Königstuhl, D-69117 Heidelberg, Germany
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA (e-mail: smoehler@stsci.edu)
3 Dr. Remeis-Sternwarte, Sternwartstr. 7, D-96049 Bamberg, Germany
4 Dept. of Physics & Astronomy, McMaster University, Hamilton, ON, L8S 4M1, Canada

Received 12 August 1996 / Accepted 17 September 1996


Quantitative spectroscopic analyses of two faint blue stars [FORMULA] - [FORMULA] in the globular cluster M 15 are presented. Their derived [FORMULA], gravities and absolute magnitudes [FORMULA], log g=5.2, [FORMULA] ; [FORMULA], log g=5.9, [FORMULA], 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.

Key words: Stars: early-type – Stars: subdwarfs – Stars: Population II – globular clusters: M 15

* Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie jointly with the Spanish National Commission for Astronomy

Send offprint requests to: U. Heber

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998