Rotation velocities of white dwarf stars

The narrow Her line cores observed with the Keck high resolution spectrograph of thirteen hydrogen-rich (DA) white dwarfs are analysed using line blanketed Non-LTE model atmospheres. The synthetic line profiles reproduce the observed line cores very well. Hence no evidence for additional line broadening due to rotation or to magnetic fields can be found. Statistical upper limits to the projected rotational velocities are derived by also taking into account inaccuracies of the atmospheric parameters. v sin i range from <8 km/s (40 Eri B) to <43 km/s (LB 5893). The programme stars sample almost the entire DA mass distribution indicating that the DA stars are slow rotators irrespective of their mass. Eight of our programme stars are members of the Hyades or Praesepe open clusters and have evolved from 2.5 M. to 3 M. mass stars. Our results add to a growing evidence that isolated white dwarfs are slowly rotating stars and that the angular momentum transfer from the core to the envelope of their precursors must have been efficient during post main sequence evolution (at least for masses of the order of 2.5 M. to 3.0 M. i.e. for Hyades and Praesepe stars).