HST Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps

摘要

We present observations of the center of the Galactic globular cluster NGC6273, obtained with the HST/WFPC2 camera as part of the snapshot programGO-7470. A B, V color-magnitude diagram (CMD) for ~28,000 stars is presentedand discussed. The most prominent feature of the CMD, identified for the firsttime in this paper, is the extended horizontal branch blue tail (EBT) with aclear double-peaked distribution and a significant gap. The EBT of NGC 6273 iscompared with the EBTs of seven other globular clusters for which we have a CMDin the same photometric system. From this comparison we conclude that all theglobular clusters in our sample with an EBT show at least one gap along the HB,which could have similar origins. A comparison with theoretical models suggeststhat at least some of these gaps may be occuring at a particular value of thestellar mass, common to a number of different clusters. From the CMD of NGC6273 we obtain a distance modulus (m-M)_V=16.27+-0.20. We also estimate anaverage reddening E(B-V)=0.47+-0.03, though the CMD is strongly affected bydifferential reddening, with the relative reddening spanning a 0.2 magnitudeinterval in E(B-V) in the WFPC2 field. A luminosity function for the evolvedstars in NGC 6273 is also presented and compared with the most recentevolutionary models.