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Astron. Astrophys. 338, 612-622 (1998)

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Search for rotation in white dwarfs *

D. Koester 1, S. Dreizler 2, V. Weidemann 1 and N.F. Allard 3, 4

1 Institut für Theoretische Physik und Astrophysik der Christian-Albrechts-Universität, D-24098 Kiel, Germany
2 Institut für Astronomie und Astrophysik, Waldhäuser Straße 64, D-72076 Tübingen, Germany
3 Observatoire de Paris-Meudon, Département Atomes et Molécules en Astrophysique, F-92195 Meudon Principal Cedex
4 CNRS Institut d'Astrophysique, 98 bis Boulevard Arago, F-75014 Paris, France

Received 2 June 1998 / Accepted 26 June 1998

Abstract

We report high-resolution spectroscopic observations of the NLTE [FORMULA] core in 28 DA white dwarfs. From a comparison with theoretically broadened NLTE models projected rotation velocities can be determined. The majority of objects are found to be compatible with zero or very small rotation with typical upper limits for v sin i of 15 km/s, corresponding to periods of hours or longer. The implications for the loss of angular momentum during the evolution of the progenitors is discussed. Three new magnetic white dwarfs are detected with very small tangential fields in the range of 30 to 50 kG. In two more magnetic objects known before we found field strengths significantly larger than the mean longitudinal fields determined from the circular polarization. Very puzzling is the result for three ZZ Ceti stars in the sample, which all seem to have projected rotational velocities between 30 and 45 km/s, in conflict with asteroseismological results.

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

* Based on observations collected at the European Southern Observatory, La Silla, Chile (55.D-0459, 57.D-0631)

Send offprint requests to: D. Koester

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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