On the Axial Rotation of the Brighter O and B Stars.

Abstract

Rotational velocities have been determined for a total of 123 stars, comprising the brighter 0, B2-B5, and B2e-B5e stars. The method involves the comparison of the observed line contours of He i 4026 with theoretical line contours computed by means of a graphical method for various equatorial rotational velocities. An attempt has been made to take into account the effects of limb darkening, gravity darken- ing, and differential rotation on the computed line contours. Values of v sin i in excess of 400 km/sec were found for several stars in each of the three groups studied. The Be stars have the most rapid axial rotation observed for any class of stars; their mean rotational velocity was found to be at least 150 km/sec larger than the mean rotational velocity for the correspond- ing group of B stars. Among the Be stars the shell stars have the largest rotational velocities and the pole-on stars the smallest, which is interpreted as an inclination effect. The observed rotation in the Be stars is somewhat lower than would be expected under the assumption that these stars may be represented by Roche models at the point of rotational instability. It is suggested that radiation pressure and tur- bulence may be responsible for the discrepancy. Rapid rotation appears to be principally confined to stars on or slightly above the main sequence