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Astron. Astrophys. 360, 120-132 (2000)


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

V. Radiative levitation versus helium mixing

S. Moehler  *  1, A.V. Sweigart 2, W.B. Landsman 3 and U. Heber 1

1 Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany (moehler@sternwarte.uni-erlangen.de; heber@sternwarte.uni-erlangen.de)
2 NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA (sweigart@bach.gsfc.nasa.gov)
3 Raytheon ITSS, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA (landsman@mpb.gsfc.nasa.gov)

Received 13 April 2000 / Accepted 5 June 2000

Abstract

Atmospheric parameters ([FORMULA], [FORMULA]), masses and helium abundances are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars we derive magnesium and iron abundances as well and find that iron is enriched by a factor of 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster abundance. Radiation pressure may levitate heavy elements like iron to the surface of the star in a diffusive process. Taking into account the enrichment of heavy elements in our spectroscopic analyses we find that high iron abundances can explain part, but not all, of the problem of anomalously low gravities along the blue HB. The blue HB stars cooler than about 15,100 K and the sdB stars ([FORMULA] [FORMULA] 20,000 K) agree well with canonical theory when analysed with metal-rich ([M/H] = +0.5) model atmospheres, but the stars in between these two groups remain offset towards lower gravities and masses. Deep Mixing in the red giant progenitor phase is discussed as another mechanism that may influence the position of the blue HB stars in the ([FORMULA], [FORMULA])-plane but not their masses.

Key words: stars: early-type – stars: fundamental parameters – stars: horizontal-branch – Galaxy: globular clusters: individual: NGC 6752

* Based on observations collected at the European Southern Observatory (ESO No 60.E-0145, 61.E-0145, 61.E-0361).

Send offprint requests to: S. Moehler

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000

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