Hubble Space Telescope observations of Galactic globular cluster cores. II. NGC 6273 and the problem of horizontal-branch gaps

We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 as part of the snapshot program GO-7470. A BV color-magnitude diagram (CM D) for similar to 28,000 stars is presented and discussed. The most promine nt feature of the CMD, identified for the first time in this paper, is the extended horizontal-branch blue tail (EBT) with a clear double-peaked distr ibution and a significant gap. The EBT of NGC 6273 is compared with the EBT s of seven other globular clusters for which we have a CMD in the same phot ometric system. From this comparison, we conclude that all the globular clu sters in our sample with an EBT show at least one gap along the horizontal branch, which could have similar origins. A comparison with theoretical mod els suggests that at least some of these gaps may be occurring at a particu lar value of the stellar mass, common to a number of different clusters. Fr om the CMD of NGC 6273 we obtain a distance modulus (m - M)(v) = 16.27 +/- 0.20. We also estimate an average reddening E(B-V) = 0.47 +/- 0.03, though the CMD is strongly affected by differential reddening, with the relative r eddening spanning a Delta E(B-V) similar to 0.2 mag in the WFPC2 held. A lu minosity function for the evolved stars in NGC 6273 is also presented and c ompared with the most recent evolutionary models.