Previous abstract Next abstract
Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
The UIT on Astro-2 in March 1995 obtained deep far-UV exposures of the clusters NGC 362 and NGC 104 (= 47 Tuc). Both of these clusters are famous for having red horizontal branches. We report the detection of UV sources in both of these clusters, some of which appear to be blue HB stars that are almost certain cluster members. The NGC 362 field contains a clump of 9 stars within a 13\arcsec\ radius (R) of the optical center of the cluster, as well as at least 13 other sources that have R < 13\arcmin. The outer set of stars has been identified with blue stars [(B-V) < 0.13] in the recent optical data of K. Montgomery and K. Janes (1995, private communication). In 47 Tuc, 12 stars are detected with luminosities within 1 mag of the predicted ZAHB and with R \gtrsim 2\arcmin, i.e. unconcentrated. Three of these stars have high (\sim 97 %) membership probability. For a fourth, we have subsequently obtained an IUE SWP spectrum; its temperature appears to be about 50000K, implying that it is a P-AGB or sdO star within the cluster. The center itself additionally contains a number of sources too faint to be HB stars, which we suspect to be blue stragglers and whose correspondence with HST/FOC detections is being investigated.
In both cases, the interpretation depends on reliably accounting for background contamination from the nearby SMC. However, preliminary analysis suggests that the blue stars in NGC 362 are only marginally more concentrated than the optical light profile. The significance of NGC 362 is that it is a paradigm `second parameter' cluster, whose turnoff color (as well as HB morphology) indicates an age \sim 11 Gyr\: (Y__ZAMS \sim 0.24). The presence of blue HB stars implies that they may be found in populations young compared to most Galactic globulars. For 47 Tuc, the presence of blue HB stars in this metal-rich cluster ([Fe/H] \sim -0.7 is important for the study of HB morphology in disk clusters.
Program listing for Wednesday