Spectroscopic evidence of velocity fields at the surface of ZZ Ceti stars

Abstract

Aims. High resolution spectra of DA and DAZ white dwarfs show peculiar line profiles for objects in or near the ZZ Ceti instability strip. The lines are broader and shallower than in slightly hotter or cooler objects. Rotational broadening could produce a similar effect but several of the objects are known to be very slow rotators. We therefore study here whether these profiles could be caused by velocity fields on the surface originating from the pulsation of the stars. Methods. Theoretical line profiles are simulated by dividing the projected stellar disk into many small area elements. The intensity at each point in the direction of the observer is calculated from accurate theoretical atmosphere models, and the contributions are summed up, taking into account Doppler shifts from the velocity field. Results. We obtain predicted light curves, variation of the average line-of-sight velocity, of the distribution of radial velocity over the visible surface, and of spectral line profiles for the Ca II K resonance line. Conclusions. The predicted line profiles are very similar to those observed. It is thus very plausible that indeed the pulsations and associated velocity fields are at the origin of these peculiarities.