Rotation velocities of white dwarfs - III. DA stars with convective atmospheres

The sharp H alpha NLTE line cores of hydrogen- rich ( DA) white dwarfs allow their projected rotational velocities to be determined. High resolution optical spectra of 22 stars obtained with the Keck I telescope are matched by synthetic spectra computed from a grid of NLTE model atmospheres. In this paper, the third in a series on white dwarf rotation, we concentrate preferentially on DA white dwarfs with convective atmospheres, i. e. with T-eff < 14 000 K. Previous analyses found DA white dwarfs hotter than 14 000 K to be very slow rotators and rarely show any spectroscopically detectable rotation. For 19 of our programme stars we were able to derive projected rotational velocities or upper limits. No rotation could be detected for seven stars in our sample. However twelve stars show significant line broadening. In the case of the ZZ Ceti star G 117- B15A, the observed H alpha line profile cannot be matched by a rotationally broadened profile as its line core is too narrow. Combining our results with those from two similar studies, we have obtained information on the rotation or other line broadening mechanisms ( such as caused by magnetic fields) of 56 DA white dwarfs. The fraction of rotating DA white dwarfs whose line profiles can be matched for a vanishing projected rotation velocity is high for hot white dwarfs with radiative atmospheres ( 25 out of 28). Amongst the cool white dwarfs with presumedly convective atmospheres, only for a few stars ( 8 out of 22) has no additional line broadening to be invoked to explain their observed H alpha line profiles. We conjecture that the physics of H alpha line formation in convective DA white dwarf atmospheres is not yet sufficiently well understood and additional observational and theoretical efforts have to be made.