B Star Rotational Velocities in h and chi Persei: A Probe of Initial Conditions During the Star-Formation Process

Abstract

Projected rotational velocities have been measured for 216 B0--B9 stars in the rich, dense h and chi Persei double cluster, and compared with the distribution of rotational velocities for a sample of field stars having comparable ages and masses. For stars that are relatively little evolved from their initial locations on the Zero Age Main Sequence (3-5 solar masses) the mean vsini measured for the h and chi Per sample is slightly more than 2 times larger than the mean determined for field stars of comparable mass, and the cluster and field vsini distributions differ with a high degree of significance. For somewhat more evolved stars with masses in the range 5-9 solar masses, the mean vsini in h and chi Per is 1.5 times that of the field; the vsini distributions differ as well, but with a lower degree of statistical significance. For stars that have evolved significantly from the ZAMS (those with masses in the range 9-15 solar masses), the cluster and field star means and distributions are only slightly different. We argue that both the higher rotation rates and the pattern of rotation speeds as a function of mass that differentiate main sequence B stars in h and chi Per from their field analogs were likely imprinted during the star formation process. We speculate that these differences may reflect the effects of the higher accretion rates that theory suggests are characteristic of regions that give birth to dense clusters, namely: (a) higher initial rotation speeds; (b) higher initial radii along the stellar birthline, resulting in greater spinup between the birthline and the ZAMS; and (c) a more pronounced maximum in the birthline radius-mass relationship that results in differentially greater spinup for stars that become mid- to late- B stars on the ZAMS.