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Astron. Astrophys. 358, 1001-1006 (2000)

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1. Introduction

Pulsations in stars provide tools for researchers in several fields of astronomy. They provide standard candles for measuring Galactic and extragalactic distances and they provide methods for measuring stellar parameters, even below the directly observable photosphere. Pulsations in hydrogen-deficient, helium-rich stars such as extreme helium, R Coronae Borealis, and hydrogen-deficient carbon stars have not been studied in as much detail as those in stars with more `normal' chemical abundances. While this is partly understandable since most pulsating stars have normal compositions, studying stars with little or no hydrogen is important to allow tests of pulsation theory.

Extreme helium stars, as their name would imply, have weak or non-existent hydrogen absorption lines and very large helium abundances ([FORMULA] per cent). Two extreme helium stars, V652 Her and LSS 3184, are known to pulsate. Saio (1993) showed that in extreme helium stars with temperatures around [FORMULA] K, like V652 Her, the [FORMULA]-mechanism caused by iron-group (Z-bump) opacity can excite the observed pulsations. Saio (1994, 1995) predicted that LSS 3184 should pulsate because of its location in the Z-bump instability finger. Kilkenny & Koen (1995) discovered that LSS 3184 shows photometric variations with a period of about 0.107 d. The similarity of V652 Her and LSS 3184 in temperature, surface gravity, and pulsation period implies that they are very similar in other physical parameters.

Kilkenny et al. (1999) have recently reported an observational analysis of LSS 3184, including a determination of its photometric period (0.1066 d). In addition, they used medium resolution ([FORMULA]) spectra to measure radial velocity variations and to show that the photometric variability is caused by pulsations. Drilling et al. (1998) reported an analysis of LSS 3184 in which they found [FORMULA] K, [FORMULA] and [FORMULA]. Kilkenny et al. (1999) report that using the radius they determined and [FORMULA] from Drilling et al. (1998) gives a mass of [FORMULA], which is much smaller than [FORMULA], the mass accepted for V652 Her (Lynas-Gray et al. 1984), and small enough to imply that some input parameter or procedure is in error.

In this paper we report an analysis of LSS 3184 using high resolution optical spectra for radial velocity measurement and ultraviolet Hubble Space Telescope (HST ) spectra and ground-based BV photometry for temperature, luminosity, and angular radius measurement. The new data provide better temperature and angular radius estimates and a much cleaner radial velocity curve for the star, allowing a more reliable estimate of its radius and mass than was possible previously.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000