Astron. Astrophys. 323, 374-381 (1997)

4. Summary and conclusions

We have presented new CCD photometry for the inner halo globular NGC 6366 which extends the results of the previous works. We have constructed the CMD and obtained the fiducial line of the cluster. Its comparison with the theoretical isochrones computed by Straniero & Chieffi (1991) yield an age of 18 Gyr, a reddening and a distance moduli . A small differential reddening effect, which can be explained by the position of the cluster near the disk of the Galaxy, is found.

On the basis of the comparison of the CMD of NGC 6366 and those of other representative clusters of different systems of the Galaxy, we extract the following conclusions:

(1) The absolute age derived for NGC 6366 is very similar to those of the bulk of well studied globular clusters of the Galaxy (M68, M30, M92, M3, M10, etc) obtained by fitting their CMD to the same set of isochrones considered in this work (i.e. Straniero & Chieffi 1991). Therefore, there exist in the Galaxy coeval clusters ( Gyr) which cover a wide range of metallicity (). In contrast, there exist halo clusters with a similar range of chemical abundances which appear younger by 4-5 Gyr than NGC 6366 (e.g. Pal 12 and NGC 362).

(2) Since the vertical age indicator () and the horizontal one () provide the same age for NGC 6366, the assumed helium abundance used in our analysis (Y=0.23) should be correct. This point has been confirmed by estimating fraction. As a consequence NGC 6366, one of the most metal-rich cluster in our Galaxy, also has the same helium abundance as the bulk of halo globular clusters.

(3) By comparing the overall CMD of NGC 6366 with those of other clusters with similar [Fe/H] and spatial position, we find a bimodal distribution, i.e. on the one hand, old globular clusters with kinematical parameters typical of halo system (such as NGC 6366 or M107) have a helium abundance Y 0.23, on the other, younger clusters with disk kinematical properties (such as 47 tuc or M71) have a helium abundance Y 0.28. In particular, the larger helium content is necessary to eliminate the discrepancy between the vertical and the horizontal age indicators as obtained when the helium content is settled at 0.23. Also in those cases, the occurrence of a larger helium abundance has been confirmed by deriving R'. Because of this fact, a revision of the ages derived for globular clusters using the -age relation is necessary, since the proper application of this method requieres in addition to the knowledge of [Fe/H], an adequated estimation of Y.

In summary, we suggest that the helium abundance should be considered a parameter of alike relevance together with age, metal abundance, spatial position, and kinematics, in order to classify the system of globular clusters of the Galaxy.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

helpdesk.link@springer.de