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

Abstract

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 turno † and allows us for the Ðrst 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-o † (TO) magnitude we obtain (a value fully compatible with V TO \ 19.82^0.10, *V TO HB \ 3.5^0.1 that derived for other clusters), which suggests that, at an age of D12 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 Ðnd E(B[V ) \ 0.33^0.05, a value signiÐcantly lower than had been determined previously. Assuming this value for the reddening, we determine a true distance modulus of corresponding to a distance of D8 kpc. (m[M) 0 \ 14.55, We Ðnd 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.

VLT observations of the peculiar globular cluster NGC 6712

“…Although we have limited our analysis to stars for which photometric completeness is always >85%, severe crowding and the concentration of many saturated stars in the innermost regions could make our determination of the core radius r c uncertain. Using shorter FORS1 exposures of the central ∼2 radius of this cluster, however, Paltrinieri et al (2001) also find a value of r c 1 , in excellent agreement with that estimated here.…”

“…Instrumental (F1) magnitudes were finally transformed to the standard Johnson system, using the stars in common with the bright portion of the CMD which has been properly calibrated using 10 standard stars (see Paltrinieri et al 2001). Figure 3 shows the total CMD of the central region of NGC 6712 (field F1).…”

“…The figure is obtained by merging the deep (180 s long exposure) and the bright (10 s exposure) data covering the core of the cluster. In the following, we deal exclusively with the properties of the cluster MS (from the TO at R 19 to R 23; see Table 1) while the bright portion of the CMD and the properties of the evolved stellar population are discussed elsewhere (Ferraro et al 2000;Paltrinieri et al 2001). …”

“…5, in units of absolute R-band magnitude. To convert our measurements from R to M R we have adopted a distance modulus in the R band of (m − M ) R = 15.33 (see Paltrinieri et al 2001), which differs only marginally from the value of (m − M ) R = 15.31 used by De Marchi et al (1999). An inspection of this figure confirms that for NGC 6712 the LFs obtained at different radial distances from the centre have a similar shape and, over the magnitude range common to all of them, they are consistent with one another within the quoted errors in the region from 60 to 135 .…”

“…In this paper, we report on the results of the analysis of our VLT data set on NGC 6712 with an emphasis on the cluster MS. The analysis of the evolved part of the cluster CMD is the subject of other papers (Ferraro et al 2000;Paltrinieri et al 2001).…”