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 d own to 2 mag below the main-sequence turn-off and allows us for the first t ime 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 ob tain DeltaV(TO)(HB) = 3.5 +/- 0.1 (a value fully compatible with that deriv ed 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 o f 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 deter mined previously. Assuming this value for the reddening, we determine a tru e distance modulus of (m-M)(0) = 14.55, corresponding to a distance of simi lar to8 kpc. We find a population of 108 candidate blue straggler stars (BS Ss), 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, combine d with those already presented in two companion papers, strongly support th e hypothesis that NGC 6712 was, at some early epoch of its history, much mo re massive and concentrated. The continued interaction with the bulge and t he disk of the Galaxy has driven it toward dissolution, and what we now obs erve is nothing but the remnant core of a cluster that once was probably on e of the most massive in the Galaxy.