Subluminous O stars

Abstract

We report results of spectral analyses of sdO stars selected from the Supernova la Progenitor Survey, the Hamburg Quasar Survey and the Sloan Digital Sky Survey and based on state-of-the-art NLTE model atmospheres. By combining the sdO with the sdB samples we discuss trends of the atmospheric parameters in order to search for evidence for possible evolutionary linkage. The He-sdO stars are found to cluster near T-eff = 45 000 K, log g = 5.5, whereas the number of sdO stars in this area is very small. The "cooler" sdO stars seem to form an extension to the sdB sequences. A couple of sdO stars are obviously evolved from the extended horizontal branch and reach temperatures as high as 80000 K. We conjecture that the He-sdO stars and sdO/sdB stars have a different evolutionary origin. This is corroborated by the much lower binary frequency of the former. Strong enrichments of iron group elements are discovered for hydrogen-rich sdO stars as well as for sdB stars from high resolution UV spectra and have severe implications for the temperature scale. We finally highlight the discovery of a hyper-velocity He-sdO star with a radial velocity of 708 km s(-1) unbound to the Galaxy.