CCD Photometry of the Globular Cluster NGC 5986 and Its Post–Asymptotic Giant Branch and RR Lyrae Stars

Abstract

We have obtained new CCD BV photometry of the little-studied southern Galactic globular cluster NGC 5986, including light curves of five of its RR Lyrae variables. The cluster's red giant branch bump is detected for the first time, at V = 16.47 ± 0.03. We derive a reddening and true distance modulus of E(B-V) = 0.29 ± 0.02 and (m - M)0 = 15.15 ± 0.10, respectively. The cluster's color-magnitude diagram reveals a mostly blue horizontal branch, like that of M13 or M2, and quite unlike M3; yet all of these clusters have nearly identical metallicities ([Fe/H]CG97 = -1.35). We show that the RR Lyrae variables in NGC 5986 are about 0.2 mag brighter on average than those in M3, an important exception to the often-employed, universal MV(RR)-[Fe/H] relation. Finally, we note that NGC 5986 contains two luminous stars with spectral types A–F, which are likely to be post–asymptotic giant branch (PAGB) objects. The V-band luminosity function of such yellow PAGB stars is a promising standard candle. We suggest that the luminosity function is sharply peaked at MV(PAGB) = -3.28 ± 0.07.