UBV Photometry of the Metal-Rich Globular Cluster NGC 6171

Abstract

Three-colour ( UBV ) photographic photometry of stars in the globular cluster NGC 6171 is reported. The photometry is calibrated with a previously published photoelectric sequence and consists essentially of a revision and extension of an earlier colour–magnitude (C–M) diagram by Sandage and Katem, together with new ultra-violet measurements. The general character of the C–M diagram is unchanged, confirming the strong population of horizontal branch stars on the red side of the instability region, the large spread in colour on the red giant branch, possibly caused by the presence of asymptotic branch stars, and a small value of Δ V = 2.07 mag. However, the red horizontal branch is shifted redwards by about 0.15 mag from its position in the earlier C–M diagram, which did not take account of colour equation, and there is a clear separation of about 0.1 mag between the red end of the horizontal branch and the subgiant branch. Other non-uniformities in the stellar distribution along the horizontal branch are discussed, together with the extremely small dispersion in V magnitude found there. The photographic ultra-violet colours confirm the previously found small ultra-violet excess of the giant stars, with a final best value of δ ( U–B ) = 0.09 ± 0.03 mag at ( B−V ) 0 = 1.04. Some of the stars probably belonging to the asymptotic branch have smaller ultra-violet excesses than subgiant stars of the same colour. The red giant stars appear to be more concentrated towards the cluster centre than stars on the horizontal branch. In an Appendix, it is shown that such differences could be ascribed to a lower mass for stars on the horizontal branch, since the time of relaxation of stellar velocities near the centre of NGC 6171 is of the same order as the evolutionary time spent on the horizontal branch.