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 HST/WFPC2 camera as part of the snapshot program GO-7470. A B, V color–magnitude diagram (CMD) for ∼28,000 stars is presented and discussed. The most prominent 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 distribution and a significant gap. The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system. From this comparison we conclude that all the globular 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 suggests that at least some of these gaps may be occuring at a particular value of the stellar mass, common to a number of different clusters. From 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 reddening spanning a ∆E(B − V ) ∼ 0.2 magnitude Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy; [email protected], [email protected] Astronomy Department, University of California, Berkeley, CA 94720-3411; [email protected] California Institute of Technology, MS 105-24, Pasadena, CA 91125; [email protected] Astronomy Department, University of California, Berkeley, CA 94720-3411; [email protected] Department of Physics and Astronomy, Division of Astronomy and Astrophysics, University of California, Los Angeles, CA 90095-1562; [email protected] European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany; [email protected]