Rotation of Hot Horizontal-Branch Stars in the Globular Clusters NGC 1904, NGC 2808, NGC 6093, and NGC 7078

We present high resolution UVES+VLT spectroscopic observations of 56 stars inthe extended horizontal branch (EHB) of the Galactic globular clusters NGC1904, NGC 2808, NGC 6093, and NGC 7078. Our data reveal for the first time thepresence in NGC 1904 of a sizable population of fast (vsini > 20 km/s)horizontal branch (HB) rotators, confined to the cool end of the EHB, similarto that found in M13. We also confirm the fast rotators already observed in NGC7078. The cooler stars (Teff < 11,500 K) in these three clusters show a rangeof rotation rates, with a group of stars rotating at ~ 15 km/s or less, and afast rotating group at ~ 30 km/s. Apparently, the fast rotators are relativelymore abundant in NGC 1904 and M13, than in NGC 7078. No fast rotators have beenidentified in NGC 2808 and NGC 6093. All the stars hotter than Teff ~ 11,500 Khave projected rotational velocities vsini < 12 km/s, but less than 20% havevsini < 2 km/s. The connection between photometric gaps in the HB and thechange in the projected rotational velocities is not confirmed by the new data.However, our data are consistent with a relation between this discontinuity andthe HB jump. We discuss a number of possibilities for the origin of the stellarrotation distribution along the HB. We conclude that none of them can yetprovide a satisfactory explanation of the observations.Comment: 12 pages, 3 figures, ApJ Letter, accepte