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

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1. Introduction

The term "UV bright stars" was introduced by Zinn et al. (1972) for stars in globular clusters that lie above the horizontal branch (HB) and are bluer than red giants. The name resulted from the fact that in the U band these stars were brighter than all other cluster stars. Further investigations showed that this group of stars consists of post-AGB stars, evolving away from the asymptotic giant branch (AGB) to the white dwarf domain (Sweigart et al., 1974; de Boer 1987), and of so-called Supra-HB stars that evolve from the blue HB (BHB) towards higher luminosities (Dorman et al., 1993) but do not have enough mass to ascend the AGB. Both evolutionary stages are immediate precursors to white dwarfs.

Up to now detailed analyses have been performed mainly for post-AGB stars in the field of the Milky Way (McCausland et al., 1992; Napiwotzki et al. 1994), for which the population membership is difficult to establish. The summarized result of these analyses is that the abundances of N, O, and Si are roughly 1/10 of the solar values, while Fe and C are closer to 1/100 solar. McCausland et al. (1992) and Conlon (1994) interpret the observed abundances as the results of dredge-up processes on the AGB, i.e. the mixing of processed material from the stellar interior to the surface. Although this hypothesis contradicts stellar evolution theories (Renzini & Voli 1981; Iben & Renzini 1984; Vassiliadis & Wood 1993), which do not predict any dredge-up processes for the low-mass precursors of these objects, compelling evidence that such dredge-up processes do occur is provided by K 648, the central star of the Planetary Nebula Ps 1 in M 15. Its atmosphere is strongly enriched in carbon when compared to the cluster carbon abundance (Heber et al., 1993) pinpointing the dredge-up of triple [FORMULA] processed material to the stellar surface.

Napiwotzki et al. (1994) discuss another possible explanation: The photospheric abundances of the Pop II central star [FORMULA] (and also of the objects analysed by McCausland et al., 1992) can be understood as the results of gas-dust separation towards the end of the AGB phase, which leads to a depletion of certain elements. This process has already been proposed by Bond (1991) for some cooler post-AGB stars with extreme metal deficiencies. A distinction between the two scenarios is hampered by the fact that the original metallicities remain unknown. Iron is one of the elements which are most sensitive to depletion. Thus the knowledge of its abundance in the UV bright stars and a comparison with the known cluster metallicity allows an unambiguous distinction between both scenarios. Unfortunately, for hot metal poor stars iron is not accessible for a spectroscopic analysis from optical spectra due to the lack of lines in this wavelength region. In the ultraviolet, however, a large number of spectral lines can be used. Hence the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope was used to measure the iron line spectra of ROA 5701 and Barnard 29 .

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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