1995A&A...300..481B

We describe a non-LTE analysis of seven subdwarf O stars and four central stars of planetary nebulae. The analysis yields metal abundances (or upper limits to these) of C, N, O, Mg, Al, and Si for most of these stars. Also effective temperature, surface gravity and helium abundance were determined for those programme stars for which no reliable estimates of these parameters were available. The derived abundance patterns prove to be disparate. We also compare the sdO stars in our sample with other sdO analyses (18 object in total) and find that this stellar class is not homogeneous with respect to metal abundances. A tentative subclassification according to helium content and luminosity-to-mass ratio indicates that the compact, high-gravity sdO stars have similar metal abundances with a strong signature of hydrogen burning in the CNO cycle. They only differ in the amounts of helium and carbon present in the photosphere, probably caused by varying efficiencies of mixing or surface erosion. Luminous sdOs, on the other hand, show markedly different metallicities, suggesting different origins. The star CPD-31 1701 is identified as a peculiar object. As compared with other O subdwarfs with nearly identical parameters, its He II line profiles appear shallow and rounded, lacking a well defined core. They can also not be fitted by theoretical profiles. No explanation for this discrepancy could be found yet. Our analysis yields microturbulence velocities in the range 0-10km/s for all stars with a sufficient number of metal lines. No indication was found for higher values like the 20km/s reported by Hubeny et al. (1991) for the sdO star BD+75 325.