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, 51 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 )
Abstract
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
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 (
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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