M75, a Globular Cluster with a Trimodal Horizontal Branch. I. Color-magnitude Diagram

Deep UBVI photometry for a large field covering the distant globular cluster M75 (NGC 6864) is presented. We confirm a previous suggestion (Catelan et al. 1998a) that M75 possesses a bimodal horizontal branch (HB) bearing striking resemblance to the well-known case of NGC 1851. In addition, we detect a third, smaller grouping of stars on the M75 blue tail, separated from the bulk of the blue HB stars by a gap spanning about 0.5 mag in V . Such a group of stars may correspond to the upper part of a very extended, though thinly populated, blue tail. Thus M75 appears to have a trimodal HB. The presence of the “Grundahl jump" is verified using the broadband U filter. We explore the color-magnitude diagram of M75 with the purpose of deriving the cluster’s fundamental parameters, and find a metallicity of [Fe/H] = −1.03±0.17 dex and −1.24±0.21 in the Carretta & Gratton (1997) and Zinn & West (1984) scales, respectively. We discuss earlier suggestions that the cluster has an anomalously low ratio of bright red giants to HB stars. A differential age analysis with respect to NGC 1851 suggests that the two clusters are essentially coeval. Subject headings: Stars: blue stragglers – Stars: Hertzsprung-Russell (HR) and C-M diagrams – Stars: horizontal-branch – Stars: variables: RR Lyr – Galaxy: globular clusters: individual: M75 (NGC 6864) – Galaxy: globular clusters: individual: NGC 1851