3 The globular cluster NGC 6256

3.1 Colour-magnitude diagrams

Figures 3a, b, c show a series of V vs. (V-I) CMD spatial extractions for the cluster.

  

Figure 3: V vs. (V-I) CMD for NGC 6256: a) 9'' < r < 141'' b) extractions of r < 55'', together with mean locus of NGC 6752 overplotted, c) extraction of r < 30''

Figure 3a corresponds to a ring with 9'' < r < 141''. In the brighter features there is some field contamination but it is a deep photometry diagram where the turnoff (TO) is attained.

Figures 3b and 3c correspond to extractions of r < 55'' and r < 30'' respectively. Notice the better definition of cluster sequences due to reduced field contamination. The cluster has a blue HB and there is no evident red HB. The presence of a blue HB is made more clear in Fig. 3c where essentially no blue MS contamination is seen, and stars in the locus of the blue HB are still seen.

A quantitative check of the presence of an HB was carried out by counting stars in frames with different apertures around the cluster center. In Table 2 are given the numbers corresponding to: HB contained in the box of stars bluer than V-I < 1.6 and in the magnitude interval 18.2 < V < 19.8 and for the field the stars in a box defined along the subgiant branch (SGB) at the same magnitude level, and in the colour interval 2.8 < (V-I) < 3.3; in order to check if there is any serious selection due to colour counts were made of the blue MS disk field stars, with a selection of V < 18.2 and (V-I) < 1.6 (same colour as BHB). The numbers are self explaining. There is a big jump from the R>150 pixels (which is almost pure field) and R<100. The slow decline at R<80 could be just a stochastic fluctuation and/or a selection effect (loosing blue stars because of crowding in V). The strong relative concentration of the BHB stars relatively to the disk stars is evident as well, the ratio BHB/SGB above is not affected by colour selection.

 

Table 2: Counts of Blue HB, SGB and field MS stars in frames of different aperture

The blue HB appears to be extended, showing a similar morphology to intermediate metallicity clusters in other bulge clusters like NGC 6522 (Barbuy et al. 1994; Terndrup & Walker 1994; Shara et al. 1998) and HP 1 (Ortolani et al. 1997). Notice that the RGB of NGC 6256 is also sparsely populated.

In Fig. 3b is superimposed the mean locus of the post core-collapse globular cluster NGC 6752 from Rosino et al. (1997) of $\rm [Fe/H] = -1.54$ (Zinn & West 1984). A good agreement is obtained for the cluster sequences.

Recently, Bica et al. (1998) carried out a near-infrared integrated spectral study of this cluster, having obtained $\rm [Fe/H] = -1.01$. The cluster appears to be more metal-poor than 47 Tuc.

We adopt a compromise value for the cluster metallicity of $\rm [Fe/H] \approx -1.3$.

3.2 Reddening and distance

The brighter parts of the blue HB of NGC 6256 are located at $V_{\rm HB} = 18.0 \pm 0.15$.

We calculate the cluster reddening taking NGC 6752 as reference. By matching the overall sequences (Fig. 3b) we obtain $\Delta (V-I) = 1.40$. Adopting E(V-I)/E(B-V) = 1.31 (Dean et al. 1978), this corresponds to $\Delta (B-V) = 1.07$, and using E(B-V) = 0.03 for NGC 6752 (Harris 1996), we get E(B-V) = 1.10, which corresponds to A_V = 3.41 (R = A_V/E(B-V) = 3.1 cf. Savage & Mathis 1979). This value is somewhat higher than Alcaino's (1983) value of E(B-V) = 0.80, which was caused by the use of the 47 Tuc RGB as reference. Recently Bica et al. (1998) carried out an integrated spectroscopic study of reddened bulge clusters in the near-IR. For NGC 6256 they obtained E(B-V) = 0.95; this value is somewhat lower than the CMD value derived here, possibly due to crowding and/or contamination effects.

Adopting an absolute magnitude of the HB for NGC 6752 of M_V = 0.7 (Buonanno et al. 1989, we get the absolute distance modulus $(m-M)_0 = 13.89\pm 0.2$. This corresponds to a distance of $d_{\odot} = 6.0\pm 0.6$ kpc for NGC 6256. A slightly larger distance is obtained if the absolute distance modulus of NGC 6752 determined by Renzini et al. (1996) by means of white dwarfs is used ($d_{\odot} = 6.7$ kpc). We adopt a compromise value of $d_{\odot} = 6.4\pm 0.5$ kpc for NGC 6256. The Galactocentric coordinates of the cluster, assuming a distance of the Sun to the Galactic center of $R_{\odot} = 8.0$ kpc (Reid 1993), are X = -1.8 (X < 0 refers to our side of the Galaxy), Y = -1.3 kpc and Z = 0.4 kpc. NGC 6256 is thus located in the bulge, but does not belong to the metal-rich population.

3.3 The surrounding field

The field surrounding NGC 6256 is interesting because it is near the galactic plane ($b \approx 3.3^{\rm o}$)along the bulge major axis ($l = -12.2^{\rm o}$).

  

Figure 4: Surrounding field population (r > 188''). The mean locus of the bulge cluster NGC 6553 is overplotted

We show in Fig. 4 an extraction for r > 188'', basically corresponding to the field population. We see a bulge TO, and a cool extended RGB for (V-I) > 3.5.

The field population can be matched with the mean locus of the nearly solar metallicity globular cluster NGC 6553, which in turn is similar to that of Baade's Window (Ortolani et al. 1995). Therefore this piece of information suggests for the present field located $\approx 12^{\rm o}$ from the Galactic center along the major axis, a comparable metallicity to that of Baade's Window (which is located at $\approx 4^{\rm o}$ from the Galactic center along the minor axis).

From the fit to NGC 6553 (Fig. 4) we can estimate a reddening difference of $\Delta (V-I) = 0.5\pm 0.2$,and adopting E(B-V) = 0.7 for NGC 6553 (Guarnieri et al. 1998) we get $E(B-V) = 1.1\pm 0.2$ for the present field (thus comparable to that of NGC 6256 itself). Since we assumed that the field is as metal-rich as NGC 6553, there might be an additional error of $\sim 0.2$ in E(B-V) if the metallicity of the field were as low as that of NGC 6752, due to a blanketing difference relative to NGC 6553; however such low metallicity for the bulge is unlikely.

This adds a new point in the reddening mapping of different directions in the bulge, as deduced from CMDs of clusters and fields (Barbuy et al. 1998 and references therein).

Finally, we note that a peculiarity of this field is the lack of a defined HB, probably due to low statistics and differential reddening effects.