VLT observations of the peculiar globular cluster NGC 6712. III. The evolved stellar population

We present extensive UBV R photometry of the Galactic globular cluster (GGC) NGC 6712 obtained with the ESO Very Large Telescope (VLT) which reaches down to 2 mag below the main-sequence turn-off and allows us for the first time to determine the age of this cluster. By using the apparent luminosity of the zero age horizontal branch (ZAHB), and the stellar main-sequence (MS) V-ZAHB =16.32 +/- 0.05 turn-off (TO) magnitude V-TO = 19.82 +/-0.10 we obtain DeltaV(TO)(HB) = 3.5 +/- 0.1 (a value fully compatible with that derived for other clusters), which suggests that, at an age of similar to 12 Gyr, NGC 6712 is coeval with other GGC of similar metallicity. We derive interstellar reddening by comparing the position and morphology of the red giant branch (RGB) with a wide variety of reference clusters and find E(B-V) = 0.33 +/-0.05, a value significantly lower than had been determined previously. Assuming this value for the reddening, we determine a true distance modulus of (m-M)(0) = 14.55, corresponding to a distance of similar to8 kpc. We find a population of 108 candidate blue straggler stars (BSSs), surprisingly large when compared with the typical BSS content of other low-concentration clusters. Moreover, we detect a very bright blue star in the core of NGC 6712 that might be a post-AGB star. These results, combined with those already presented in two companion papers, strongly support the hypothesis that NGC 6712 was, at some early epoch of its history, much more massive and concentrated. The continued interaction with the bulge and the disk of the Galaxy has driven it toward dissolution, and what we now observe is nothing but the remnant core of a cluster that once was probably one of the most massive in the Galaxy.