EDP Sciences
Free Access
Volume 415, Number 1, February III 2004
Page(s) 313 - 323
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361:20034505
Published online 03 February 2004

A&A 415, 313-323 (2004)
DOI: 10.1051/0004-6361:20034505

Spectroscopic analyses of the blue hook stars in NGC 2808: A more stringent test of the late hot flasher scenario

S. Moehler1, A. V. Sweigart2, W. B. Landsman3, N. J. Hammer4 and S. Dreizler4, 5

1  Institut für Theoretische Physik und Astrophysik der Universität Kiel, Abteilung Astrophysik, 24098 Kiel, Germany
2  NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
    e-mail: Allen.V.Sweigart@nasa.gov
3  SSAI, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
    e-mail: landsman@mpb.gsfc.nasa.gov
4  Institut für Astronomie und Astrophysik der Universität Tübingen, Sand 1, 72076 Tübingen, Germany
    e-mail: hammer@astro.uni-tuebingen.de
5  Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany
    e-mail: dreizler@astro.physik.uni-goettingen.de

(Received 13 October 2003 / Accepted 4 November 2003 )

Recent UV observations of the globular cluster NGC 2808 (Brown et al. 2001) show a significant population of hot stars fainter than the zero-age horizontal branch ("blue hook" stars), which cannot be explained by canonical stellar evolution. Their results suggest that stars which experience unusually large mass loss on the red giant branch and which subsequently undergo 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 horizontal branch stars and should have helium- and carbon-rich atmospheres. As a test of this late hot flasher scenario, we have obtained and analysed medium resolution spectra of a sample of blue hook stars in NGC 2808 to derive their atmospheric parameters. Using the same procedures, we have also re-analyzed our earlier spectra of the blue hook stars in $\omega$ Cen (Moehler et al. 2002) for comparison with the present results for NGC 2808. The blue hook stars in these two clusters are both hotter ( $T_{\rm eff} \ge 35\,000$ K) and more helium-rich than canonical extreme horizontal branch stars in agreement with the late hot flasher scenario. Moreover, we find indications for carbon enhancement in the three most helium-enriched stars in NGC 2808. However, the blue hook stars still show some hydrogen in their atmospheres, perhaps indicating that some residual hydrogen survives a late hot flash and then later diffuses to the surface during the horizontal branch phase. We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its horizontal branch morphology.

Key words: stars: horizontal branch -- stars: evolution -- Galaxy: globular clusters: individual: NGC 2808 -- Galaxy: globular clusters: individual: NGC 5139

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

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© ESO 2004

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