B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?

Abstract

Projected rotational velocities (v sin i) have been measured for 216 B0–B9 stars in the rich, dense h and χ Persei double cluster and compared with the distribution of rotational velocities for a sample of field stars having comparable ages (t ~ 12–15 Myr) and masses (M ~ 4–15 M⊙). For stars that are relatively little evolved from their initial locations on the zero-age main sequence (ZAMS) (those with masses M ~ 4–5 M⊙), the mean v sin i measured for the h and χ Per sample is slightly more than 2 times larger than the mean determined for field stars of comparable mass, and the cluster and field v sin i distributions differ with a high degree of significance. For somewhat more evolved stars with masses in the range 5–9 M⊙, the mean v sin i in h and χ Per is 1.5 times that of the field; the v sin i distributions differ as well, but with a lower degree of statistical significance. For stars that have evolved significantly from the ZAMS and are approaching the hydrogen exhaustion phase (those with masses in the range 9–15 M⊙), 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 χ Per from their field analogs were likely imprinted during the star formation process rather than a result of angular momentum evolution over the 12–15 Myr cluster lifetime. 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, (1) higher initial rotation speeds; (2) higher initial radii along the stellar birth line, resulting in greater spin-up between the birth line and the ZAMS; and (3) a more pronounced maximum in the birth line radius-mass relationship that results in differentially greater spin-up for stars that become mid- to late-B stars on the ZAMS.