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Astron. Astrophys. 323, 819-826 (1997)

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Rotation velocities of white dwarf stars *

U. Heber 1, R. Napiwotzki 1 and I.N. Reid 2

1 Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, D-96049 Bamberg, Germany
2 Palomar Observatory 105-24, California Institute of Technology, CA-91125 Pasadena, USA

Received 2 September 1996 / Accepted 21 January 1997

Abstract

The narrow H [FORMULA] 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. [FORMULA] range from [FORMULA] 8 km/s (40 Eri B) to [FORMULA] 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 [FORMULA] to 3 [FORMULA] 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 [FORMULA] to 3.0 [FORMULA] i.e. for Hyades and Praesepe stars).

Key words: stars: rotation – stars: magnetic fields – stars: atmospheres – stars: white dwarfs

* Based on observations obtained at the 200" teleskope at Palomar Observatory and the W.M. Keck Observatory, which is operated by the Californian Association for Research in Astronomy for the California Institute of Technology and the University of California

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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