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Astron. Astrophys. 339, 537-544 (1998)

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Abundances of UV bright stars in globular clusters

I. ROA 5701 in [FORMULA] Centauri and Barnard 29 in M 13 *

S. Moehler, U. Heber, M. Lemke and R. Napiwotzki

Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstrasse 7, D-96049 Bamberg, Germany

Received 18 June 1998 / Accepted 27 August 1998

Abstract

Two UV brights stars in globular clusters, ROA 5701 ([FORMULA] Cen) and Barnard 29 (M 13) are analysed from high-resolution UV and optical spectra. The main aim is the measurement of iron abundances from UV spectra obtained with the HST-GHRS. In addition atmospheric parameters and abundances for He, C, N, O, and Si are derived from optical spectra (ESO CASPEC) for ROA 5701 or taken from literature for Barnard 29 . Both stars are found to be post-asymptotic giant branch stars. Surprisingly, their iron abundances lie significantly below the cluster abundance in both cases. Barnard 29 lies 0.5 dex below the iron abundance derived for giant stars in M 13 and the iron abundance of ROA 5701 is the lowest of any star in [FORMULA] Cen analysed so far. Barnard 29 shows the same abundance pattern as the red giant stars in M 13, except for its stronger iron deficiency. The iron depletion could be explained by gas-dust separation in the AGB progenitor's atmosphere, if iron condensed into dust grains which were then removed from the atmosphere by a radiatively driven wind. The interpretation of the abundance pattern for ROA 5701 is hampered by the star-to-star abundance variations seen in [FORMULA] Cen, but its abundance pattern appears to be consistent with the gas-dust separation scenario.

Key words: stars: abundances – stars: AGB and post-AGB – globular clusters: individual: NGC 5139 – globular clusters: individual: NGC 6205 – ultraviolet: stars

* Based on observations collected a) with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, and b) at the European Southern Observatory

Send offprint requests to: S. Moehler

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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