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

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

2002-03-22

arxiv.org/abs/astro-ph/0203393v1

Astronomy and Astrophysics

Astrophysics

19 pages, 15 figures, emulateapj5/apjfonts style. Astronomical Journal, in press. This version contains some very low-resoluti

Scientific paper

10.1086/340804

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 dex 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.

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