Spectral analysis of extremely helium rich subdwarfs O-stars.

Abstract

The NLTE analyses of four extremely helium rich sdO stars based on high resolution, high S/N CASPEC spectra are presented. Effective temperatures, gravities and the abundances of H, He, C, N, O, Ne, Mg, and Si have been deter- mined. It is found that LS_IV+1O°9, LSS 1274, and UV 0832-01 have very similar parameters: T_eff = 44500±1000 K, log g = 5.55±0.15. H is undetected in all programme stars resulting in a upper limit of n_H/n_He ≤ 0.1. C, N, and Ne are enriched, while Mg and Si are solar and O is slightly depleted. Microturbulent velocity is found to be lower than to km/s. The positions of these stars in the (log T_eff,log g) diagram indicate, that they are post-EHB (Extended Horizontal Branch) stars. Their atmospheric composition can be understood as a mixture of CNO and 3 - α processed material. UV 0904-02 is slightly hotter, T_eff = 46500±1000 K, log g = 5.55±0.15, and is as well a post-EHB star. The atmosphere also displays a mixture of CNO and 3-α processed material, the primordial metallicity however, was possibly lower.