Spectral analysis of four multi mode pulsating sdB stars

Abstract

Four members of the new class of pulsating sdB stars are analysed from Keck HIRES spectra using NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, log(He/H)), metal abundances and rotational velocities are determined. Balmer line fitting is found to be consistent with the helium ionization equilibrium for PG1605+072 but not so for PG1219+534 indicating that systematic errors in the model atmosphere analysis of the latter have been underestimated previously. All stars are found to be helium deficient probably due to diffusion. The metals are also depleted with the notable exception of iron which is solar to within error limits in all four stars, confirming predictions from diffusion calculations of Charpinet et al. (1997). While three of them are slow rotator's (vsini < 10km/s), PG1605+072 displays considerable rotation (vsini = 39km/s, P<8.7h) and is predicted to evolve into an unusually fast rotating white dwarf. This nicely confirms a prediction by Kawaler (1999) who deduced a rotation velocity of 130km/s from the power spectrum of the pulsations which implies a low inclination angle of the rotation axis.