Astron. Astrophys. 362, 953-958 (2000)

4. NGC 6712

4.1. Colour-magnitude diagrams

In Fig. 6 we show the whole frame V vs. diagram of NGC 6712. The diagram is well populated showing a number of features. The cluster bright sequences are clear, including a RGB and a HB covering a considerable range of colours. The present CMD reaches V 19, at the cluster subgiant level. The contamination by the field is important as seen from the blue MS, which is expected since the cluster is projected close to the Galactic plane (b = ).

Fig. 6. NGC 6712: whole frame V vs. diagram.

In Fig. 7 a central extraction of r (r 120 pixels) shows that the cluster sequences become clearer. The morphology of the HB shows a striking colour range, with a red and a blue components, similar to that of NGC 6171 (M107) (see e.g. Ferraro et al. 1991), of [Fe/H] = -1.04 (H96). The red HB is more populated, with a different morphology relative to more metal-rich clusters such as 47 Tuc, where the HB is clumpy and close to the RGB (Desidera & Ortolani 1997). The brighter magnitudes at the red edge of the blue HB appear to be due to probable RR Lyrae members near their maximum light (Sect. 4.3).

Fig. 7. NGC 6712: V vs. CMD for an extraction of r 47" (r 120 pixels), where the mean loci of NGC 5904 (dashed line) and NGC 6171 (solid line) are overplotted.

4.2. Cluster parameters

4.2.1. Metallicity

In Fig. 7 the mean loci of the globular clusters NGC 5904 (Sandquist et al. 1996) of [Fe/H] = -1.29 (H96) and of the slightly more metal-rich cluster NGC 6171 (Ferraro et al. 1991) are overplotted on the cluster CMD.

The RGB as well as the HB are suitably fitted by the mean locus of NGC 6171, suggesting a comparable metallicity. With the mean locus of NGC 5904 there is no simultaneous fit of the RGB, red HB and blue HB. As shown in Fig. 7 where the RGBs are superimposed, the red HB of M5 is bluer, which suggests a higher metallicity for NGC 6712.

Therefore NGC 6712 appears to have a metallicity around [Fe/H] -1.0. This estimate agrees well with the near-infrared photometry reported by Kyeong & Byun (1998).

NGC 6712 has a wide HB colour range, similar to those of the blue tail clusters NGC 6388 and NGC 6441 (Rich et al. 1997), with the difference that the latter clusters are more metal-rich and present more strongly sloped HBs. In addition, as shown by Pritzl et al. (2000), the RR Lyrae variables in NGC 6388 and NGC 6441 have exceedingly long periods for their metallicity, unlike the case in NGC 6712 (e.g. Clement 1997). A closer analogue to NGC 6712 is thus provided by NGC 6171, except that its blue HB extends more to the blue, at the same magnitude level of the red HB. We note that a comparison with theoretical synthetic HB simulations for the cluster metallicity, surprisingly indicates that the observed blue HB of NGC 6171 is bluer and/or brighter than predicted by the models. The discrepancy is even more pronounced for NGC 6712. Observations using bluer passbands should prove very useful to further explore this result.

For the case of gently sloping HBs among the more metal-rich of the intermediate metallicity globular clusters, the reader is referred to Brocato et al. (1999) and Sweigart (1999).

Finally, we find for the HB morphology parameter (B-R)/(B+V+R) = -0.59 (where B, V and R are respectively the numbers of blue HB, RR Lyrae and red HB stars), confirming the previous estimation by Lee (1990).

4.2.2. Cluster reddening and distance

The HB is located at = 16.20.1 and the colour of the RGB at the HB level is = 1.270.05. The difference with respect to a best fit with NGC 6171 is = 0.0, implying that the reddening of NGC 6712 is the same as that of NGC 6171 (H96), estimated to be E(B-V) = 0.33. This value is somewhat lower than most estimates in the literature (Sect. 1). Adopting R = 3.1 we get = 1.02, and using = 0.7 (Buonanno et al. 1989), there results = 14.48, and the distance to the Sun = 7.91.0 kpc for NGC 6712.

Assuming the distance of the Sun to the Galaxy center to be = 8.0 kpc, the Galactocentric coordinates are X = -0.9 kpc, Y =3.4 kpc and Z = -0.6 kpc. The distance from the Galactic center is = 3.6 kpc.

4.3. Variable stars

Sawyer Hogg (1973) lists 21 variables, out of which 14 are found in the present CCD frames (13´13´). An additional RR Lyrae variable, KC 460, has been reported by Cudworth (1988) and is listed in the latest upgrade to Sawyer Hogg's catalogue (Clement 1997).

We used the charts by Sandage et al. (1966) to identify the variables, which are overplotted with circles on the CMD extraction (r ) of Fig. 8.

Fig. 8. NGC 6712: V vs. CMD extraction for r (r 150 pixels) with variable stars superimposed.

The location of 8 variables near the HB indicates that they are probable RR Lyrae members: 4 are on the blue side and the other 4 on the red side of the RR Lyrae gap. Among these, 4 are in the central extraction (open circles with dots). The two brighter blue stars indicated as variables are V17 and V6. Cudworth (1988) reports V17 as a non-variable field star, and the RR Lyrae V6 as a probable member.

NGC 6712 has been catalogued as an Oosterhoff type I (OoI) globular cluster by Castellani & Quarta (1987). A recent period shift analysis carried out for this cluster by Catelan et al. (1998a) suggests a "normal" behaviour in the period-temperature plane for its metallicity, in contrast with the case of NGC 6171 which appears to have its RR Lyrae variables shifted towards shorter periods at a fixed equilibrium temperature. More data are needed to settle this issue, especially in view of the evidence that NGC 6712 may have a bimodal HB (Catelan et al. 1998b and references therein).

The red variables near the tip of the RGB are classified by Rosino (1966) as semi-regular, irregular and Long Period variables.

From brighter to fainter in Fig. 8 they are V10 (Irr), V2 (SR), V8 (SR), V7 (LP). They may be related to the cluster or to the disk, since they are located in the outer parts of the frame. According to proper motion measurements of Cudworth (1988), V8 and V7, as well as another bright red variable (V21) in NGC 6712 are likely cluster members. NGC 6712 is thus one of the few globular clusters presenting Long Period Variables, which is an indication that it is rather metal rich (Rosino 1978).

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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