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Institut für Astronomie und Astrophysik

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Preprint D/03

Spectroscopic Analyses of the Blue Hook Stars in NGC 2808: A More Stringent Test of the Late Hot Flasher Scenario

S. Moehler (1), A.V. Sweigart (2), W.B. Landsman (3), N.J. Hammer (4), S. Dreizler (4,5)

(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
(3) SSAI, NASAGoddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
(4) Institut für Astronomie und Astrophysik der Universität Tübingen, Sand 1, Abteilung Astronomie, 72076 Tübingen, Germany
(5) Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, D-37083 Göttingen

To be published in: A&A

Abstract. Recent UV observations of the globular cluster NGC2808 (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 (HB) stars and should have He- and C-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 NGC2808 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 NGC2808. The blue hook stars in these two clusters are both hotter (Teff > 35,000 K) and more helium-rich than canonical extreme HB stars in agreement with the late hot flasher scenario. Moreover, we find indications for C enhancement in the three most He-enriched stars in NGC2808. However, the blue hook stars still show some H in their atmospheres, perhaps indicating that some residual H survives a late hot flash and then later diffuses to the surface during the HB 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 HB morphology.
Key words: Stars: horizontal branch - Stars: evolution - globular cluster: individual: NGC2808 - globular cluster: individual: NGC5139

Preprint (380 kb PDF file including figures)

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