Analysis of BD+33 2642: a newly detected planetary nebula in the galactic halo and its central star.

Abstract

The spectra of the star BD+33 2642 and its surrounding planetary nebula (PN) are analyzed. The stellar parameters are found to be T_eff_=20200K, logg=2.9; the stellar mass is 0.560M_sun_. In the stellar atmosphere, He is solar, C, N, O, Mg, Si are depleted by about 1dex, and Fe is depleted by 2dex relative to the sun. This abundance pattern is similar to 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 of these stars is commonly explained by dredge-up processes during the previous stellar evolution. It is shown that separation of gas and dust, proposed for the interpretation of chemical peculiarities in some cooler post-AGB stars, can explain the observed atmospheric mixture as well. The nebular emission is well interpreted by photoionization from the star, with the parameters given by the stellar atmospheric analysis. The metal abundances in the nebula are found to be as strongly depleted as they are in the star. Thus BD+33 2642 is one of the few known population II PNe.