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Astron. Astrophys. 335, 183-198 (1998)


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The rotational velocity of low-mass stars in the Pleiades cluster * **

D. Queloz 1, S. Allain 2, J.-C. Mermilliod 3, J. Bouvier 2 and M. Mayor 1

1 Observatoire de Genève, 51 ch. des Maillettes, CH-1290 Sauverny, Switzerland
2 Laboratoire d"Astrophysique, Observatoire de Grenoble, Université Joseph Fourier, B.P. 53, F-38041 Grenoble Cedex 9, France
3 Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-bois, Switzerland

Received 13 January 1998 / Accepted 9 March 1998

Abstract

We present new [FORMULA]   measurements for 235 low-mass stars in the Pleiades. The differential rotational broadening has been resolved for all the stars in our sample. These results, combined with previously published measurements, provide a complete and unbiased rotation data set for stars in the mass range from 0.6 to 1.2 [FORMULA]. Applying a numerical inversion technique on the [FORMULA]   distributions, we derive the distributions of equatorial velocities for low-mass Pleiades members. We find that half of the Pleiades dwarfs with a mass between 0.6 to 1 [FORMULA]   have rotation rates lower than 10 km s-1.
Comparison of the rotational distributions of low-mass members between IC 2602/2391 ([FORMULA] Myr) and the Pleiades ([FORMULA] Myr) suggests that G dwarfs behave like solid-bodies and follow Skumanich's law during this time span. However, comparison between Pleiades and older clusters -M34 ([FORMULA] Myr) and Hyades ([FORMULA] Myr)- indicates that the braking of slow rotators on the early main sequence is weaker than predicted by an asymptotical Skumanich's law. This strongly supports the view that angular momentum tapped in the radiative core of slow rotators on the zero age main sequence (ZAMS) resurfaces into the convective envelope between Pleiades and Hyades age. For the G-dwarfs, we derive a characteristic coupling time scale between the core and the envelope of about 100-200 Myr, which accounts for the observed evolution of surface rotation from the ZAMS to the Hyades.
The relationship between rotation and coronal activity in the Pleiades is in agreement with previous observations in other clusters and field stars. We show that the Rossby diagram provides an excellent description of the X-ray activity for all stars in the mass domain studied. The Pleiades data for slow and moderate rotators fills the gap between the X-ray-rotation correlation found for slow rotators and the X-ray "saturation plateau" observed for young fast rotators. The transition between increasing X-ray flux with rotation and X-ray saturation is observed at [FORMULA]. These results strengthen the hypothesis that the "saturation" of the angular momentum loss process depends on the stellar mass.

Key words: stars: activity – stars: evolution – stars: pre-main sequence – Galaxy: open clusters and associations: Pleiades

* Based on observations collected at the Observatoire de Haute-Provence with ELODIE at the 193cm telescope and with CORAVEL at the 1m-swiss telescope
** Tables 3, 4, 5 are also available in electronic form at the CDS via anonymous ftp 130.79.128.5

Present address: Jet Propulsory Laboratory, Mail-Stop: 306-473, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Send offprint requests to: Didier.Queloz@obs.unige.ch

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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