Free access article

A&A 379, 235-244 (2001)
DOI: 10.1051/0004-6361:20011334

Early type stars at high galactic latitudes

II. Four evolved B-type stars of unusual chemical composition

M. Ramspeck, U. Heber and H. Edelmann

Dr. Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

(Received 26 July 2001 / Accepted 10 September 2001 )

Abstract
We present the result of differential spectral analyses of a further four apparently normal B-type stars. Abundance anomalies (e.g. He, C, N enrichment), slow rotation and/or high gravities suggest that the programme stars are evolved low-mass B-type stars. In order to trace their evolutionary status several scenarios are discussed. Post-AGB evolution can be ruled out. PG 0229+064 and PG 1400+389 could be horizontal branch (HB) stars, while HD 76431 and SB 939 have already evolved away from the extreme HB (EHB). The low helium abundance of HD 76431 is consistent with post-EHB evolution. The enrichment in helium, carbon and nitrogen can be explained either by deep mixing of nuclearly processed material to the surface or by diffusion processes modified by magnetic fields and/or stellar winds. A kinematic study of their galactic orbits indicates that the stars belong to an old disk population.

Key words: Galaxy: halo -- stars: early-type -- stars: abundances -- stars: kinematics -- stars: evolution

Offprint request: M. Ramspeck, ramspeck@sternwarte.uni-erlangen.de

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