THE ANGULAR-MOMENTUM OF MAIN-SEQUENCE STARS AND ITS RELATION TO STELLAR ACTIVITY

Rotational velocities are reported for intermediate-mass main-sequence stars in the field. The measurements are based on new, high-S/N CCD s pectra from the Coude Feed Telescope of the Kitt Peak National Observa tory. We analyze these rotation rates for a dependence on both mass an d age, We compare the average rotation speeds of the field stars with mean velocities for young stars in Orion, the Alpha Persei cluster, th e Pleiades, and the Hyades. The average rotation speeds of stars more massive than similar to 1.6 M. experience little or no change during t he evolutionary lifetimes of these stars on the zero age main sequence or within the main-sequence band. Less massive stars in the range bet ween 1.6 M. and 1.3 M. also show little decline in mean rotation rate while they are on the main sequence, and at most a factor of two decre ase in velocity as they evolve off the main sequence. The e-folding ti me for the loss of angular momentum by the latter group of stars is at least 1-2 billion years. This inferred characteristic time scale for spindown is far longer than the established rotational braking time fo r solar-type stars with masses below similar to 1.3 M.. We conclude fr om a comparison of the trends in rotation with trends in chromospheric and coronal activity that the overall decline in mean rotation speed along the main sequence, from similar to 2 M. down to similar to 1.3 M ., is imposed during the pre-main-sequence phase of evolution, and tha t this pattern changes little thereafter while the star resides on the main sequence. The magnetic activity implicated in the rotational spi ndown of the Sun and of similar stars during their main-sequence lifet imes must therefore play only a minor role in determining the rotation rates of the intermediate-mass stars, either because a solar-like dyn amo is weak or absent, or else the geometry of the magnetic field is a ppreciably less effective in removing angular momentum from these star s.