4 IC 1276 (Palomar 7)

4.1 Colour-magnitude diagrams

In Fig. 5a we show a V vs. (B-V) diagram for the whole Danish frame (1986 run). The TO region is detected but at the limit of the photometry and will be used to estimate the age-related magnitude difference between TO and HB (Sect. 5).

  

Figure 5: V vs. (B-V) CMD of IC 1276. a) June 1986 run (Danish data), where four variables given in Kinman & Rosino (1962) are identified; b) May 1985 run, where the solid line is a fit of the mean locus of 47 Tuc

The horizontal branch (HB) is red, suggesting that, like in the case of Terzan 3, it is metal-rich. Again, like in the Terzan 3 case, the best fit with mean loci of globular clusters templates of different metallicity occurs with that of 47 Tuc (mean locus from Desidera & Ortolani 1997), as shown in Fig. 5b, where is given a deeper diagram corresponding to slightly offcenter frames taken in the 1985 run. In conclusion, IC 1276 presents [Fe/H] $\approx -0.7$ similarly to 47 Tuc, as well as Terzan 3 discussed above.

The cluster HB is located at $V \approx$ 17.7 $\pm$ 0.1 and the (B-V) colour of the RGB at the HB level is $(B-V)_{\rm g}$ $\approx$ 2.07 $\pm$ 0.07 (Fig. 5). Likewise for Terzan 3, we adopt 47 Tuc as reference, obtaining E(B-V) = 1.16.

Using the same prescription as for Terzan 3, the derived observed distance modulus is (m-M) _V = 16.83. Using R = 3.3, we get A _V = 3.83. The true distance modulus is then $(m-M)_{\rm 0}$ = 13.0 and the distance to the Sun $d_{\odot}$ $\approx$ 4.0 kpc. The Galactocentric coordinates are X = -4.3, Y = 1.5 kpc and Z = 0.4 kpc.

IC 1276 is therefore a bulge cluster located near the plane, nearly half way between the Sun and the Galactic center, which is also the case of Terzan 5 and NGC 6553 (Ortolani et al. 1996; Guarnieri et al. 1998).

4.2 Variables

Five variable stars were identified in the cluster area by Kinman & Rosino (1962), four of them indicated in Fig. 5a (variable number 5 is outside the field).

In order of brightest to faintest, the stars identified in Fig. 5a correspond to numbers 4, 2, 1 and 3. Number 2 was suggested to be an RR Lyrae by Kinman & Rosino, which corresponds very well to its location in our diagram. The other three ones are semiregular or long-period variables. Number 4 probably is a cluster long-period variable. Stars number 1 and 3 are in peculiarly reddened location in the diagram - these are more probably non-members.