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

1998; EDP Sciences; Volume: 335; Issue: 1 Linguagem: Inglês

1432-0746

D. Queloz, S. Allain, J. C. Mermilliod, J. Bouvier, M. Mayor,

Astronomical Observations and Instrumentation

We present new v sin i 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 1.2M. Applying a numerical inversion tech- nique on the v sin i 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 M have rotation rates lower than 10 km s 1 . Comparison of the rotational distributions of low-mass mem- bers between IC 2602/2391 ( 35 Myr) and the Pleiades ( 100 Myr) suggests that G dwarfs behave like solid-bodies and follow Skumanich's law during this time span. However, com- parison between Pleiades and older clusters -M34 ( 200 Myr) and Hyades ( 600 Myr)- indicates that the braking of slow ro- tators 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 ro- tators 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 be- tween 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 pro- vides an excellent description of the X-ray activity for all stars in the mass domain studied. The Pleiades data for slow and moder- ate rotators fills the gap between the X-ray-rotation correlation found for slow rotators and the X-ray plateau ob- served for young fast rotators. The transition between increas- ing X-ray flux with rotation and X-ray saturation is observed at