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

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1. Introduction

The group of extreme helium stars (EHes) at present comprises some 20 objects. These stars have early-type spectra dominated by strong neutral helium, singly-ionized carbon and almost undetectable Balmer lines. Most show high luminosity-to-mass ratios. Indicators such as galactic location and pulsation point to masses in the range 0.7 to 1.0 [FORMULA]. Their surface gravities imply luminosities typical of post-asymptotic giant branch stars ([FORMULA]). The question posed is how they come to have typical photospheric hydrogen abundances of less than one part in [FORMULA] or, in other words, what is their evolutionary status?

Previous studies of surface abundances in extreme helium stars (Jeffery 1996) have demonstrated that, in addition to hydrogen, their photospheres show anomalous proportions of carbon, nitrogen and oxygen, and other elements. These are indicative that in general the surface helium is the product of CNO cycling, and that there is a significant contamination by material that has been converted to carbon and possibly oxygen by [FORMULA]burning processes. The origin of other abundance anomalies, including those of neon and phosphorous remains undetermined.

The simultaneous presence of traces of primordial stellar material and CNO- and [FORMULA]- processed waste in the stellar photosphere points to a history that includes considerable mixing between layers of a highly evolved star. The principle hypotheses proposed to explain this mixing involve either a late helium flash in a cooling white dwarf or the coalescence of a helium and carbon-oxygen white dwarf. More precisely, these are the late thermal pulse (LTP) model of Iben et al. (1983) or the very late thermal pulse (VLTP) model of Blöcker & Schönberner (1997) and the merged binary white dwarf (MBWD) model of Webbink (1984) and Iben & Tutukov (1985).

The question of the origin of the extreme helium stars is closely related to that of the R Coronae Borealis stars. These cooler stars have photospheric abundances very similar to those of the extreme helium stars, although the carbon abundance has proved difficult to establish (Asplund 1997). A major study of abundances in these objects (Lambert et al. 1997) prefers the LTP model as origin.

Although a general picture of the photospheric abundances in helium stars can be given in a few sentences, individual stars show large deviations from the mean (Jeffery 1996). Thus their evolutionary origin (or origins) must be capable of generating a wide range of outcomes. A study of abundances in extreme helium stars has been in progress for several years in an effort to delimit this range and analyses of some ten EHes have been completed. This study presents results for the remaining EHes with effective temperatures between 14 000 and 30 000 K.

The extreme helium stars LSS 4357, LS II[FORMULA] and LSS 99 were all discovered by Drilling during his surveys of OB+ stars in the Milky Way (Drilling & Hill 1986, Vijapurkar & Drilling 1993). A preliminary analysis of these stars was made by Heber et al. (1986), together with the extreme helium star LSE 78, who concluded that their effective temperatures were in the range 14 000 to 20 000 K and that their surface gravities were lower than covered by their grid of LTE model atmospheres. This study refines that earlier conclusion by applying the method of fine analysis to high resolution optical spectra. As a consequence of the rich absorption line content of these spectra, conventional line-by-line techniques have been superceded by large-scale spectral synthesis methods introduced here.

These analyses provide abundance measurements for most of the astrophysically important light elements and are consistent with previous studies of other extreme helium stars. Together with work on four cooler helium stars nearing completion (Pandey et al., in preparation), this contribution will establish a comprehensive picture of the abundances in helium stars and will be reviewed in a concluding paper.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998