Rotation Velocities of DA White Dwarfs with Convective Atmospheres

The sharp Hα 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. We concentrate preferentially on hydrogen-rich white dwarfs with convective atmospheres, i.e. with Teff < 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 program stars we were able to derive projected rotational velocities or upper limits. Combining our results with those from two similar studies (Heber et al 1997, paper I and Koester et al. 1998, paper II), we have obtained information of 56 DA white dwarfs. The fraction of rotating DA white dwarfs whose line profiles can be matched for a vanishing projected rotation velocities is high for hot white dwarfs with radiative atmospheres (25 out of 28) with upper limits ranging from 1 kms to 24 kms, whereas amongst the cool white dwarfs with presumably convective atmospheres only for a few stars (8 out of 22) no additional line broadening has to be invoked to explain their observed Hα line profiles resulting in upper limits to v sin i of 7 kms to 35 kms .