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Astron. Astrophys. 340, 476-482 (1998)


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Spectral analysis of the low-gravity extreme helium stars LSS 4357, LS II+33o5 and LSS 99 *

C.S. Jeffery, Peter J. Hamill, Paul M. Harrison and Sandra V. Jeffers

Armagh Observatory, College Hill, Armagh BT61 9DG, UK

Received 19 May 1998 / Accepted 25 September 1998

Abstract

We have carried out quantitative analyses of three very low surface gravity extreme helium stars with very similar spectra. Their effective temperatures of [FORMULA]K 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.

Key words: stars: individual: (LS II+33O 5, LSS 4357, LSS 99) – stars: emission-line, be – stars: chemically peculiar – stars: abundances

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

Send offprint requests to: C.S. Jeffery (csj@star.arm.ac.uk)

© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998

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