Armagh Observatory Preprint Series No. 304

Spectroscopic analyses of the low-gravity helium stars LSS 4357, LS II+33 5 and LSS 99

Authors: C.S.Jeffery, P.J.Hamill, P.M.Harrison, S.V.Jeffers

Journal: 1998, A&A 340, 476-482

Abstract. We have carried out quantitative analyses of three very low surface gravity extreme helium stars with very similar spectra. Their effective temperatures of ~ 16 000K fill a gap in a nearly continuous sequence of extreme helium stars all having similar luminosity-to-mass ratios, but extending from effective temperatures around 12 000 K to more than 20 000 K. Because of the low surface gravities and extremely rich line spectra, the model atmosphere calculations have been reviewed, and large-scale spectral synthesis techniques have been introduced to the analyses for the first time. In addition to the high carbon and nitrogen abundances usually seen in extreme helium stars, two of the programme stars have extremely high oxygen abundances, comparable with or greater than their carbon abundances.

Based on observations obtained at the European Southern Observatory, La Silla, Chile.

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