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Paper: Hot Subdwarfs from SDSS and SPY
Volume: 392, Hot Subdwarf Stars and Related Objects
Page: 131
Authors: Hirsch, H.A.; Heber, U.; O’Toole, S.J.
Abstract: We present the results of quantitative spectral analysis of subdwarf O stars (sdO) obtained from the SDSS database. By visual inspection, 112 sdO stars could be identified in a set of spectra of photometrically selected faint blue stars. Fitting the Balmer, He I and He II lines to state-of-the-art non-LTE model spectra, we derived their effective temperatures, surface gravities and helium abundances. We find the helium-enriched sdO stars to rally in a small intervall of Teff = 40 000K. . . 50 000K and log g = 5.5 . . . 6.0, whereas the helium-deficient ones are showing a wide variance both in temperature and log g. A puzzling feature is a significant number of helium rich subdwarfs below the helium main sequence. These stars would not be able to burn helium in their cores and pose a serious problem to subdwarf evolution theories. We conclude, that the sdB stars are linked with the helium-deficient sdO stars, i.e. the sdBs are predecessors for the helium-deficient sdOs. To explain the helium-enriched sdO stars in their entirety, we regard two scenarios as most promising: The merging of two helium-core white dwarfs and the late hot flashers. To investigate this further, we started an NLTE analysis of carbon abundances. So far, we find carbon slightly enriched above solar, but no trends can be seen yet.
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