5 The globular cluster NGC 6453

NGC 6453 (GCl B1747-345, ESO 393-SC36) is located at $\alpha_{1950}=17^{\rm h}$ 47$^{\rm m}$ 32.0$^{\rm s}$,$\delta_{1950}=-34^\circ$ $35'08\hbox{$^{\prime\prime}$}$ ($l=355.72^\circ$, $b=-3.87^\circ$). Bica & Pastoriza (1983) derived E(B-V)=0.60 and ${\rm [Fe/H]}=-1.28$. The spectroscopy of Zinn & West (1984) gave E(B-V)=0.60 and ${\rm [Fe/H]}=-1.53$. The visible and near-infrared spectra of Bica & Alloin (1986, 1987) were consistent with ${\rm [Fe/H]}=-1.4$ and E(B-V)=0.61. For this little studied cluster the compilations of Webbink (1985) and Harris (1996) list E(B-V)=0.61, ${\rm [Fe/H]}=-1.52$, $V_{\rm HB}=17.7$ and $d_{\odot}=10.6$ kpc. No CMD is available for this cluster in the literature.

5.1 Colour-magnitude diagrams

In Fig. 6 we give the V, I CMD for a $1000\times 1000$ pixel extraction ($6.5'\times6.5'$) centered on NGC 6453. The diagram is dominated by the metal-rich bulge population with a prominent red HB and a curved giant branch extending to very red colours, similarly to the field of Terzan 9 (Fig. 4), as well as the metal-rich globular clusters NGC 6553/NGC 6528 and the Baade Window (Ortolani et al. 1995). The blue disk MS is also important in the field of NGC 6453. The steep sequence of giants above the red HB corresponds to NGC 6453.

In order to isolate the cluster sequences we extracted a small circular area (r=150 pixels or 1'), for which the V vs. (V-I) CMD is given in Fig. 7. The spread can be explained by differential reddening and crowding. Nevertheless a blue HB and a steep RGB are clearly seen. Overimposed is the mean locus of M 30, suggesting a low metallicity ${\rm [Fe/H]}\approx -2.0$, although likewise in the case of NGC 6139 (Sect. 4), a fit with the somewhat more metal-rich template NGC 6752 cannot be ruled out within errors. Again, as a compromise, we adopt ${\rm [Fe/H]}=-1.8$ for NGC 6453. This value is again somewhat lower than those from integrated studies (Sect. 1) for this cluster, which might be accounted for by some metal-rich bulge field contamination.

  

Figure 6: V vs. (V-I) CMD of NGC 6453 plus field for a $1000\times 1000$ pixel extraction ($6.5'\times6.5'$)

5.2 Reddening and distance

The brighter parts of the blue HB of NGC 6453 are found at $V=17.53\pm0.15$. Adopting M 30 as template, we get $\Delta (V-I)=0.90$, corresponding to $\Delta (B-V)=0.68$. With the same assumptions of Sect. 4.2, E(B-V)=0.74 for NGC 6453 and A_V=2.37 (R=3.2).

Adopting M_V=0.66 for the HB level as above, we get the absolute distance modulus $(m-M)_0=14.5 \pm 0.20$. This corresponds to a distance of $d_{\odot}=7.9\pm 0.8$ kpc for NGC 6453. Alternatively, by adopting NGC 6752 as template one would get E(B-V)=0.63 and a distance of $d_{\odot}=9.4\pm 0.6$ kpc. As a compromise we adopt $E(B-V)=0.70\pm0.06$ and $d_{\odot}=8.5\pm 1$ kpc for NGC 6453.

Assuming $R_{\odot}=8.0$ kpc we derive X=0.46, Y=-0.63 and Z=-0.57. NGC 6453 would be located at $R_{\rm GC}=1$ kpc from the Galactic center.

  

Figure 7: V vs. (V-I) CMD of NGC 6453 for a radial extraction of r=150 pixels (1'). Overimposed is the mean locus of M 30

5.3 Parameters for the bulge field

Following Sect. 3.3, we calculate reddening and distance for the surrounding field of NGC 6453.

The bulge Red Horizontal Branch (RHB) is located at $V\approx 17.25$ and $(V-I)\approx 0.6$.

The template NGC 6553 is more reddened than this field under study. We derive E(B-V)=0.46 for the field of NGC 6453, which gives $A_{\rm V}=1.47$ (R=3.2). The bulge extinction in this direction ($l=-4.3^\circ$, $b=-3.9^\circ$) is comparable to that of the Baade Window (Terndrup 1988), which is at a similar Galactic latitude. The distance of the bulk of the population for this bulge field results to be at 9.2 kpc. Adopting R=3.47, it would result $d_{\odot}=8.7$ kpc. This value is consistent with the Galactic center distance estimates (Reid 1993, 1998).

Finally, we note that distances for metal-poor and metal-rich globular clusters towards the bulge, and the bulk of the field population are consistent with the picture of a common volume near the Galactic Center.