VI photometry of the post-core-collapse globular cluster NGC 6558 and the adjacent bulge field population

Abstract

We present V I color-magnitude diagrams (CMDs) of the globular cluster NGC 6558 and its surrounding field, obtaining cluster parameters. The cluster has a blue extended horizontal branch and a depleted red giant branch, characteristics already observed in other post-core-collapse clusters in the bulge, such as NGC 6522, NGC 6540, and HP 1. These clusters do not belong to the metal-rich stellar population, and they may define a distinct class in the bulge. We derive a reddening of E(B−V )=0.50 and a distance dʘ ≈ 6.3 kpc. The CMD morphology is consistent with a metallicity range of −1.6<[Fe/H]<−1.2. We also report photometry of the bulge field population (b = −6° on the minor axis) in which NGC 6558 is imbedded. As is the case for other bulge fields, there is a prominent red clump and a strong descending red giant branch, similar to that seen in lower latitude bulge fields; we estimate the metallicity of the bulge population to lie between that of 47 Tuc and the nearly solar-metallicity cluster NGC 6553 (i.e., approximately −0.3 dex).