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Astron. Astrophys. 348, L25-L28 (1999)

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1. Introduction

Hot subluminous B stars (sdB) form a homogeneous group dominating the population of faint blue stars ([FORMULA]). Following ideas outlined by Heber (1986) the sdB stars can be identified with models for extreme HB (EHB) stars, which differ markedly from those for normal HB stars. An EHB star bears great resemblance to a helium main-sequence star of half a solar mass and its further evolution should proceed similarly (i.e. directly to the white dwarf graveyard, Dorman et al. 1993).

Recently, several sdB stars have been found to be pulsating (termed EC14026 stars after the prototype, see O'Donoghue et al. 1999 for a review), defining a new instability strip in the HR-diagram. The study of these pulsators offers the possibility of applying the tools of asteroseismology to investigate the structure of sdB stars. The existence of pulsating sdB stars was predicted by Charpinet et al. (1996), who uncovered an efficient driving mechanism due to an opacity bump associated with iron ionization in EHB models. However, in order to drive the pulsations, iron needed to be enhanced in the appropriate subphotospheric layers, possibly due to diffusion. Subsequently, Charpinet et al. (1997) confirmed this assumption by detailed diffusion calculations. Even more encouraging was the agreement of the observed and predicted instability strip.

Thirteen pulsating sdB stars are well-studied photometrically (O'Donoghue et al. 1999). A precise knowledge of effective temperature, gravity, element abundances and rotation is a prerequisite for the asteroseismological investigation.

PG 1605+072 was selected for a detailed quantitative spectral analysis because it displays the richest frequency spectrum amongst the EC 14026 stars ([FORMULA]50 periods have been identified, Kilkenny al. 1999). Recently, Kawaler (1999) predicted from his modelling of the pulsations that PG 1605+072 should be rotating.

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999