The rotation of horizontal-branch stars. III. Members of the globularcluster M 4.

Radial velocities and line breadths are measured for nine blue horizontal-branch stars in the globular cluster M4. The mean velocity is 72.3 + or - 0.9 km/s, and the intrinsic velocity dispersion is 1.9 + or - 0.3 km/s, considerably smaller than expected if the mass-to-light ratio is solar. No line broadening was detected, which indicates the complete absence of rotation as large as v sin i = 15 km/s in all of the nine stars. This establishes a level of rotation in M4 lower than that found either among blue-horizontal-branch stars of the field or among those in the globular clusters M3, M5, and M13. In one M4 star, the oxygen triplet at 6155 A was observed. The deduced oxygen abundance is one-sixth solar or less, similar to that of other clusters with both blue and red stars on the horizontal branch. The idea remains viable but unproven that rotation, by encouraging mass loss, is largely responsible for anomalous increases in the proportion of blue stars on the horizontal branch of some clusters.