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4. Cluster parameters

4.1. Metallicity

We measured the slope of the cool part of the RGB, and obtained tex2html_wrap_inline1209. Comparing these values to those given in Ortolani et al. (1991) and Ortolani et al. (1996b), we would assign to Ter 6 a metallicity similar to that of NGC 6553.

An alternative indicator of metallicity is the magnitude difference between the HB level and the top (brightest stars) of the RGB. We find tex2html_wrap_inline1211 for M30 (Rosino et al. 1996), 2.3 for 47 Tuc, 2.1 for NGC 6356 (Bica et al. 1994), 1.4 for NGC 6528 and 1.3 for NGC 6553 (Ortolani et al. 1995) whereas tex2html_wrap_inline1213 for Ter 6. This would place Ter 6 with a metallicity intermediate between those of 47 Tuc and NGC 6553/NGC 6528. The cluster is probably more metallic than previously estimated by AZ88.

4.2. Reddening and distance

We calculate the cluster reddening taking NGC 6553 and 47 Tuc as references (Ortolani et al. 1995; Desidera 1996; Bica et al. 1994). The (V-I) colour of the RGB at the HB level for Ter 6 is tex2html_wrap_inline1217, while that of NGC 6553 is tex2html_wrap_inline1219 and for 47 Tuc tex2html_wrap_inline1221. The difference between NGC 6553 and Ter 6 is tex2html_wrap_inline1223; given that from HST data the revised value E(V-I) = 0.95 for NGC 6553 (Guarnieri et al. 1996), this leads to tex2html_wrap_inline1227. Relative to 47 Tuc, tex2html_wrap_inline1229, and given E(V-I) = 0.05 for 47 Tuc, tex2html_wrap_inline1233. Adopting the average value tex2html_wrap_inline1235, and using equation A1 of Dean et al. (1978) which leads to E(V-I)/E(B-V) = 1.36, we obtain E(B-V) = 2.14 for Ter 6. Assuming tex2html_wrap_inline1241 (Savage & Mathis 1979) we get tex2html_wrap_inline1243; if R = 3.6 suitable for red stars (Terndrup 1988;; Grebel & Roberts 1995), we get tex2html_wrap_inline1247. This difference dominates over all other error sources, amounting to 1.1 magnitudes.

The HB magnitude of Ter 6 is at tex2html_wrap_inline1249; assuming that the absolute magnitude of the HB for a metallicity of about [Fe/H] tex2html_wrap_inline1251 is 0.92 (Chaboyer et al. 1996), and applying a 0.14 blanketing correction which matches bolometric magnitudes (see discussion in Guarnieri et al. 1996), there results tex2html_wrap_inline1253. This value is in good agreement with values given by Buonanno et al. (1989), where however the correction due to the colour difference between the RR Lyraes and our redder stars was not included. We obtain an apparent distance modulus of (m-M) = 21.19. The true distance modulus will depend on the tex2html_wrap_inline1257 adopted, resulting tex2html_wrap_inline1259 for R = 3.1 and 13.13 for R = 3.6. This corresponds to a distance of tex2html_wrap_inline1265 kpc (R = 3.1) and tex2html_wrap_inline1269 kpc (R = 3.6). These distance values are considerably smaller than the value indicated by Webbink (1985) and more compatible with the value given by Fahlman et al. (1995).

Assuming the standard value of R = 3.1 (tex2html_wrap_inline1275 kpc), and assuming the distance of the Sun to the Galaxy center of tex2html_wrap_inline1277 kpc (Reid 1993), the Galactocentric coordinates are X = 1.0 (X>0 refers to our side of the Galaxy), Y = -0.17 and Z = -0.26 kpc.


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