VLT Observations of the Peculiar Globular Cluster NGC 6712. III. The Evolved Stellar Population

We present extensive UBVR photometry of the Galactic globular cluster (GGC)NGC6712 obtained with the ESO Very Large Telescope (VLT) which reach down totwo magnitudes below the main sequence turn-off and allows us for the firsttime to determine the age of this cluster. By using the apparent luminosity ofthe zero age horizontal branch (ZAHB), $V_{ZAHB}=16.32\pm0.05$ and the stellarmain sequence (MS) turn--off (TO) magnitude $V_{TO}=19.82\pm0.10$, we obtain$\Delta V_{TO}^{HB}=3.5\pm0.1$ (a value fully compatible with that derived forother clusters) which suggests that, at an age of $\sim 12$ Gyr, NGC 6712 iscoeval with other GGC of similar metallicity. We derive interstellar reddening by comparing the position and morphology ofthe red giant branch (RGB) with a wide variety of reference clusters and find$E(B-V)=0.33\pm0.05$, a value significantly lower than had been determinedpreviously. Assuming this value for the reddening, we determine a true distancemodulus of $(m-M)_0 = 14.55$, corresponding to a distance of $\sim 8$ kpc. We find a population of 108 candidate blue straggler stars (BSS),surprisingly large when compared with the typical BSS content of other lowconcentration clusters. Moreover, we detect a very bright blue star in the coreof NGC6712 that might be a post-AGB star. These results, combined with thosealready presented in two companion papers, strongly support the hypothesis thatNGC6712 was, at some early epoch of its history, much more massive andconcentrated. The continued interaction with the bulge and the disk of theGalaxy has driven it toward dissolution, and what we now observe is nothing butthe {\it remnant} core of a cluster that once was probably one of the mostmassive in the Galaxy.Comment: 31 pages, 12 figures, Astronomical Journal in pres