The Rotational Velocity of Helium-rich Pre-White Dwarfs

Previous investigations on hydrogen-rich white dwarfs generally yield only very small rotational velocities (vrot · sin i). We have analyzed line profiles in high-resolution optical spectra of eight hydrogen-deficient (pre-) white dwarfs and find deviations from the dominant Stark line broadening in five cases which, interpreted as an effect of stellar rotation, indicate projected rotational velocities of 40 − 70km/sec. For the three least luminous stars upper limits of vrot · sin i = 15− 25km/sec could be derived only. The resulting velocities correlate with luminosity and mass. However, since the mass-loss rate is correlated to the luminosity of a star, the observed line profiles may be affected by a stellar wind as well. In the case of RXJ2117.1+3412, this would solve discrepancies to results of pulsational modeling (vrot · sin i ≈ 0). Detailed analyses of the NLTE Balmer line cores of DA white dwarfs have revealed that for the vast majority the rotational velocities are extremely low, so that they cannot be detected, even with high-resolution high-S/N spectra (Heber et al. 1997, Koester et al. 1998). This unexpected behavior poses the question how these WDs have lost their angular momentum during previous evolutionary phases. To our best knowledge no one has ever tried a similar analysis on nonDA white dwarfs or hydrogen-deficient pre-white dwarfs. The only exception is the PG1159-type central star of NGC246, whose high rotational rate is well known since many years (Heap 1975, Rauch & Werner 1997). High-resolution optical (HIRES at KECK, EMMI at ESO NTT) spectra of PG1159 stars can in principle be used to resolve the narrow line cores (in absorption or emission) of He ii, C iv, and Ovi and to constrain the rotational velocity. Synthetic spectra of NLTE model atmospheres (see Werner et al. 1997 for parameters and references) have been convolved with rotational profiles in order to compare them to the observation. Different lines have been evaluated for the