CCD Photometry of the Globlular 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 5 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 M_V(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 (LF) of such yellow PAGB stars is a promising standard candle. We suggest that the LF is sharply peaked at M_V(PAGB) = -3.28 +- 0.07.

accepted for publication in The Astronomical Journal, 25 pages, 5 figures

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