Studies of Stellar Rotation. V. The Dependence of Rotation on Age among Solar-Type Stars

Abstract

The rotational velocities of a large sample of field stars with spectral types between F2 IV, V and G3 IV, V have been obtained from coude' spectrograms of resolution about 6 km/sec. It is shown that the average rotational velocity is higher among those with Ca ii emission than among those without. Since there is strong evidence that stars with Ca ii emission are younger, on the average, than most solar-type stars, a picture is advanced in which rotation declines with advancing age. It is proposed that magnetically coupled winds, similar to the solar wind, are responsible for this deceleration even after a star takes up residence on the main sequence. The winds are presumably generated only in stars which have subsurface hydrogen convection zones and concomitant chromospheres. Estimates of rotational velocity among solar-type stars in the Hyades and Pleiades are given which confirm this general picture. It is shown that the mean rotation in the Pleiades is highest of all groups considered and averages about 40 km/sec for stars of mass 1.20 o. The time scale for reduction by a factor of 2 of the rotational velocities of Pleiades stars is equal to the age of the Hyades, about 4 X 108 years.