EDP Sciences
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Volume 395, Number 1, November III 2002
Page(s) 37 - 43
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20021248

A&A 395, 37-43 (2002)
DOI: 10.1051/0004-6361:20021248

Spectroscopic analyses of the "blue hook" stars in $\omega$ Centauri: A test of the late hot flasher scenario

S. Moehler1, 2, A. V. Sweigart3, W. B. Landsman4 and S. Dreizler5

1  Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
2  Institut für Theoretische Physik und Astrophysik der Universität Kiel, Abteilung Astrophysik, 24098 Kiel, Germany
    e-mail: moehler@astrophysik.uni-kiel.de
3  NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
    e-mail: sweigart@bach.gsfc.nasa.gov,
4  SSAI, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
    e-mail: landsman@mpb.gsfc.nasa.gov
5  Astronomisches Institut der Universität Tübingen, Sand 1, 72076 Tübingen, Germany
    e-mail: dreizler@astro.uni-tuebingen.de

(Received 3 June 2002 / Accepted 26 August 2002 )

$\omega$ Cen contains the largest population of very hot horizontal branch (HB) stars known in a globular cluster. Recent UV observations (Whitney et al. 1998; D'Cruz et al. 2000) show a significant population of hot stars below the zero-age horizontal branch ("blue hook" stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these "late hot flashers" should show higher temperatures than the hottest canonical HB stars and should have helium- and carbon-rich atmospheres. We obtained and analysed medium resolution spectra of a sample of blue hook stars to derive their atmospheric parameters. The blue hook stars are indeed both hotter ( $T_{\rm eff} \ge 35\,000$ K) and more helium-rich than classical extreme HB stars. In addition we find indications for a large enhancement of the carbon abundance relative to the cluster abundance.

Key words: stars: horizontal-branch -- stars: evolution -- Galaxy: globular clusters: individual: NGC 5139

Offprint request: S. Moehler, moehler@astrophysik.uni-kiel.de

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