Abundance analysis of the hot post-AGB star Barnard 29.

Abstract

We present a model atmosphere analysis for the relatively bright (V~13) globular cluster post-AGB star, Barnard 29, using IUE and high resolution (0.06A FWHM) optical spectra. The derived atmospheric parameters are T_eff_=20000+/-1000K and logg=3.0+/-0.1dex, which imply a current mass of ~0.55M_sun_. The chemical composition provides important constraints on the post-AGB evolutionary stage. We find an overall metal deficiency of 1.46+/-0.11dex, which is compatible with the mean [Fe/H] obtained from previous M13 studies. The most interesting feature is the severe carbon deficiency of more than 2.0dex which has also been observed in a number of high latitude B-type low gravity stars. Hence the present results supports the recent classification of these objects as hot post-AGB stars. Relative to the overall metallicity, the observed carbon deficiency of Barnard 29 coupled with a significant nitrogen overabundance indicates that this star has left the AGB before the third dredge-up has occurred. The implications for AGB and post-AGB evolution are briefly discussed.