Early type stars at high galactic latitudes II. Four evolved B-type stars of unusual chemical composition

TL;DR

This study analyzes four high galactic latitude B-type stars with unusual chemical compositions, exploring their evolutionary status and suggesting they are evolved low-mass stars with complex surface abundance patterns.

Contribution

It provides detailed spectral analyses of four B-type stars, proposing their evolutionary stages and mechanisms behind their chemical peculiarities, including deep mixing and diffusion processes.

Findings

Stars are evolved low-mass B-type stars.

Chemical anomalies suggest deep mixing or diffusion.

Stars belong to an old disk population.

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 of the remaining stars 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.