The rotation of early B-type stars and the problem of star formation

Distributions of projected rotational velocities V sin i are derived for B0–B5 stars in young and older subgroups of OB associations and for field stars. The overall distribution of V sin i for stars in young subgroups is consistent with a Maxwellian distribution of V and random orientations. The underlying distribution of V sin i for stars in older subgroups is similar except for a prominent excess of slow rotators. Since known braking mechanisms fail to account for this excess, the slow rotators may have been formed later when the subgroups began to disperse. Field stars of luminosity class V resemble the stars in older subgroups as regards rotation and probably originated in subgroups. These results are considered with previous work on late B-type stars which revealed a striking difference between the rotation of cluster and field stars. Stars in tightly bound clusters may have their spin axes roughly perpendicular to the galactic plane and were probably formed through the collapse and fragmentation of molecular clouds. However, the formation of stars with random orientations in loose clusters and associations may have been through the collision of turbulent elements within the clouds, and protostars in the denser regions of the clouds may acquire angular momentum through gravitational interactions.