COYOTES-III - OBSERVATIONAL EVIDENCE FOR A LOWER LIMIT TO THE ROTATIONAL VELOCITY OF ZAMS-G AND ZAMS-K DWARFS

In order to provide additional constraints to current models of angula r momentum evolution of solar-type stars, we have conducted a period-d etermination program in the Alpha Perseus cluster. We have selected a sample of 22 Alpha Persei G and K dwarfs on the basis of their seeming ly slow rotation rate (vsini less than or equal to 10 km.s(-1)). We se arched for periodic modulation induced by stellar spots by performing a photometric CCD monitoring campaign in the V band over a baseline of 14 days. We detected 10 periods ranging from 1.3 to 7.1 days. A perio d was found for 7 out of the 10 K stars of our sample but for only 3 o ut of the 12 G and F stars. This bias is due to the existence of a cle ar relationship between spectral type and the amplitude of modulation by spots. We show that our study is not biased against the detection o f very slow rotators and conclude to the existence of a lower limit of about 6 km.s(-1) for the rotational velocity of Alpha Per G and K dwa rfs. A significant fraction of solar-type stars in young clusters thus appear to have equatorial velocities distributed in a very narrow ran ge between about 6 and 10 km.s(-1), a result which poses a severe cons traint to current models of angular momentum evolution.