The rotation of horizontal-branch stars. II. Members of the globular clusters M 3, M 5 and M 13.

Broadened lines evident in echelle spectra obtained at the Multiple Mirror Telescope suggest that rotation at a level of v sin i greater than 15 km/s is present in seven of 17 horizontal-branch (HB) stars belonging to the globular clusters M3, M5, and M13. The maximum size of the broadening, corresponding to v sin i approximately 30 km/s, resembles that seen in field HB stars. Radial velocities and cluster densities mitigate against binary association as its cause. Instead, the discovery implies the presence of internal rotation among the main-sequence members of at least some globular clusters; this affects the lifetime on the main sequence and therefore the age attributed to the cluster from a comparison of the color of the main-sequence turnoff with stellar evolutionary calculations. Indeed, since the high frequency and the large values of rotation found in M13 stars are distinctly different from what is observed in other clusters and the field, this work supports the idea that the anomalously blue horizontal branch of M13, and the large star-to-star variations in carbon and nitrogen abundances seen among its low-luminosity giants, might be due to a more frequent occurrence of larger stellar rotational velocities.