Ultraviolet Imaging of the Globular Cluster 47 Tucanae
O'Connell, Robert W.; Bohlin, Ralph C.; Landsman, Wayne B.; Dorman, Ben; Rood, Robert T.; Witt, Adolf N.; Smith, Andrew M.; Roberts, Morton S.; Stecher, Theodore P.; Neff, Susan G.; Shah, Ronak Y.
Abstract
We have used the Ultraviolet Imaging Telescope to obtain far-UV (1620 Å), wide field (40' diameter) images of the prototypical metal-rich globular cluster 47 Tucanae. We find a population of about 20 hot (Teff>9000 K) objects near or above the predicted UV luminosity of the hot horizontal branch (HB) and lying within two half-light radii of the cluster center. We believe these are normal hot HB and post-HB objects. lUE spectra of two of the post-HB candidates, lying ∼1.5 mag above the theoretical ZAHB, imply temperatures of 14 000 and ≥3 000 K. The radial distribution of the detected stars parallels the distribution of the V-band light. These observations, and recent HST photometry of two other metal-rich clusters, demonstrate that systems with populous, cool HBs can nonetheless produce hot HB and post-HB stars, probably through normal single-star evolutionary processes rather than from binary or other dynamical interactions. The cluster center also contains an unusual diffuse far-UV source which is more extended than its V-band light. It is possible that this is associated with an intracluster medium, for which there was earlier infrared and x-ray evidence.