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Astron. Astrophys. 346, L1-L4 (1999)

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Letter to the Editor

Physical parameters of hot horizontal-branch stars in NGC 6752: deep mixing and radiative levitation

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

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

Received 20 April 1999 / Accepted 28 April 1999


Atmospheric parameters ([FORMULA], [FORMULA] and [FORMULA]) are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars MgII and FeII lines are detected indicating an iron enrichment by a factor 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster metallicity. This finding adds to the growing evidence that radiative levitation plays a significant role in determining the physical parameters of blue HB stars. Indeed, we find that iron enrichment can explain part, but not all, of the problem of anomalously low gravities along the blue HB. Thus the physical parameters of horizontal branch stars hotter than about [FORMULA] K in NGC 6752, as derived in this paper, are best explained by a combination of helium mixing and radiative levitation effects.

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-145, 61.E-0145, 61.E-0361)
** Hubble Fellow.
*** Visiting Scientist, Universities Space Research Association.

Send offprint requests to: S. Moehler

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999