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

Abstract

ABSTRACT Rotational velocities are reported for intermediate-mass main sequence stars it the field. The measurements are based on new, high S/N CCD spectra from the Coudé Feed Telescope of the Kitt Peak National Observatory. We analyze these rotation rates for a dependence on both mass and age. We compare the average rotation speeds of the field stars with mean velocities for young stars in Orion, the Alpha Persei cluster, the Pleiades, and the Hyades. The average rotation speeds of stars more massive than ∼1.6 M⊙\\experience little or no change during the evolutionary lifetimes of these stars on the zero age main sequence or within the main sequence band. Less massive stars in the range betwee n 1.6M⊙\\ and 1.3M⊙\\ also show little decline in mean rotation rate while they are on the main sequence, and at most a factor of 2 decrease in velocity as they evolve off the main sequence. The {e}-folding time for the loss of angular momentum b y 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 for solar-type stars with masses below ∼1.3 M⊙. We conclude from 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 ∼2 M⊙\\ down to ∼1.3 M⊙, is imposed during the pre-main sequence phase of evolution, and that this pattern changes little thereafter while the star resides on the main sequence. The magnetic activity implicated in the rotational spindown of the Sun and of similar stars during their main sequence lifetimes mus t therefore play only a minor role in determining the rotation rates of the intermediate mass stars, either because a solar-like dynamo is weak or absent, or else the geometry of the magnetic field is appreciably less effective in removing angular momentu m from these stars.