We address the origin and evolutionary status of hot subdwarf stars by
studying the optical spectral properties of 58 subdwarf O (sdO) stars.
Combining them with the results of our previously studied subdwarf B (sdB)
stars, we aim at investigating possible evolutionary links. We analyze
high-resolution ESO VLT UVES spectra from the ESO Supernova Ia Progenitor
Survey (SPY). Effective temperatures, gravities, and helium abundances are
determined simultaneously by fitting the profiles of H and He lines using
dedicated synthetic spectra in NLTE. Evidence for cool companions to 8 sdOs as
well as a binary consisting of two sdO stars is found. A correlation between He
abundances and the presence of carbon and/or nitrogen lines emerges: below
solar He abundance, no sdO shows C or N lines. In contrast, C and/or N lines
are present in ALL sdOs with super- solar He abundance. We thus use the solar
He abundance to divide our sample into He-deficient and He-enriched sdOs. While
He-deficient sdOs are scattered in a wide range of the Teff-log(g)-diagram,
most of the He-enriched sdOs cluster in a narrow region at Teff = 40,000 ...
50,000K and log(g)=5.5 ... 6.0. An evolu- tionary link between sdBs and sdOs
appears plausible only for the He-deficient sdOs indicating that they are the
likely successors to sdBs. The properties of He-enriched sdOs cannot be
explained with canonical single star evolutionary models. Alternative scenarios
(late hot flasher) as well as for binary evolution (white dwarf merger;
post-RGB evolution) are tested. While we regard the post-RGB scenario as
inappropriate, the white dwarf merger and the late hot flasher scenarios remain
viable to explain the origin of He-enriched sdOs.Comment: 14 pages, 10 figures, Astronomy & Astrophysics accepte
