Rotation of Hot Horizontal Branch Stars in Galactic Globular Clusters

We present high resolution UVES+VLT spectroscopic observations of 61 stars in the extended blue horizontal branches of the Galactic globular clusters NGC 1904 (M79), NGC 2808, NGC 6093 (M80), and NGC 7078 M15). Our data reveal for the first time the presence 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, similar to that found in M13. We also confirm the fast rotators already observed in NGC 7078. The cooler stars (Teff < 11,500 K) in these three clusters show a range of rotation rates, with a group of stars rotating at ∼ 15 km/s or less, and a fast rotating group at ∼ 30 km/s. Apparently, the fast rotators are relatively more abundant in NGC 1904 and M13, than in NGC 7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the stars hotter than Teff ∼ 11,500 K have projected rotational velocities vsini < 12 km/s. The connection between photometric gaps in the HB and the change in the projected rotational velocities is not confirmed by the new data. However, our data are consistent with a relation between this discontinuity and the HB jump. We determined the radial velocity (vrad) and the projected rotational velocity (vsini) for each of our program stars using the cross-correlation technique described by Tonry & Davis (1979). Figure 1 shows the complete set of projected rotational velocity data for M79, NGC2808, M80 and M15, including Behr et a. (2000b, B00b) results for the last cluster. These results suggest a link between the photometric G99 jump and the discontinuity in the distribution of the HB stellar rotation rates. Radiative levitation of metals has been invoked by G99