Analysis of BD+33 2642 - a newly detected planetary nebula in the galactic halo and its central star
R.Napiwotzki, U.Heber, J.Köppen
ABSTRACT An analysis of the central star BD+33 2642 and its recently discovered planetary nebula (PN) is presented. An analysis of the stellar spectrum yields Teff=20200 K, log g =2.9 and a stellar mass of 0.560 Msun. It is shown that the nebular data is in agreement with these stellar parameters. Both the analysis of the stellar and the nebular spectrum give evidence for strongly subsolar metallicity of this object. Thus BD+33 2642 is one of the few known Pop. II central stars. Iron is depleted in the atmosphere of BD+33 2642 relative to other metals (C, N, O, Mg, and Si). The photospheric abundance pattern of BD+33 2642 resembles that found in atmospheres of B-type post-AGB stars in the galactic halo and some peculiar A- and F-type post-AGB stars. The chemical mixture observed in central stars and post-AGB stars is commonly explained by dredge-up processes during the previous stellar evolution. We will discuss this scenario and demonstrate that a gas-dust-separation scenario, recently proposed for the interpretation of chemical peculiarities in some cooler post-AGB stars, can explain the observed atmospheric mixture as well. We try to estimate the influence of a stellar wind on the chemical evolution. We emphasize that gas-dust-fractionation is potentially important for chemical abundances in Pop. I central stars too.