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

1. Introduction

The oldest galactic stellar clusters provide us with useful fossil records of the time when our Galaxy was still forming. As a consequence, the age and chemical composition of the globular cluster system has been the target of several studies aimed to the analysis of the chronology of the early galactic evolution (Larson 1990).

Recent results obtained by different groups (Buonanno et al. 1989, Straniero & Chieffi 1991, Chaboyer et al. 1992, Carney et al. 1992, Salaris et al. 1993, Sarajedini et al. 1995) roughly agree in showing that most of the best studied globular clusters are coeval and old (17 Gyr). This fact implies that the clusters of the (inner) halo system were formed rapidly, perhaps on a time scale no larger than the rotation period of the initial protogalaxy, as in the model devised by Eggen et al. (1962). However there exist some exceptions, as for instance NGC 362 and Palomar 12, which appear to be significantly younger (4-5 Gyrs). A further complication of this simple view arises from a number of clusters such as 47 Tucanae or Ruprecht 106, clusters for which different age indicators lead to contradictory results (see for instance Salaris et al. 1993). In this context, the most metal-rich globular clusters may be used to trace the halo-disk transition. Within the sample considered by Zinn (1985) and Armandroff (1989) to analyse the system of stellar clusters in our Galaxy, there is a small number of clusters lying near the boundary value of metallicity which separate the halo and disk subsystems of globular clusters (i.e. disk: [Fe/H] -0.8, scale height of 1 Kpc, and a substantial rotation velocity, and halo: [Fe/H] -0.8, and a spatial distribution and kinematic properties that resemble those of the stars of the galactic halo). Among them, NGC 6366 (C1725-050; =17 25.1, =-05 02; l=18.41, b=16.04) is a heavily reddened globular cluster located at approximately Rgc=5 Kpc, and z=1 Kpc toward the north galactic pole (Webbink 1985). Recent determinations of the metallicity of this cluster range between [Fe/H]=-0.67 and [Fe/H]=-0.99. Despite of its chemical composition and spatial position in the Galaxy, its kinematical parameters (Da Costa & Seitzer 1989) are incompatible with disklike rotation, and therefore it belongs to the halo system according to the above mentioned classification for the system of globular clusters. For this reason, the comparison of the photometric study of NGC 6366 with those of typical disk globular clusters such as 47 Tucanae and M71 may be used to analyse possible correlations among the various parameters which characterize the Color Magnitude Diagrams (i.e. age, chemical composition and the like).

The previous works on NGC 6366 (Pike 1976, Harris 1993) were mainly devoted to the study of the Red Giant Branch (RGB) and Horizontal Branch (HB) stars (and also to the identification of blue-Stragglers in the latter case), therefore the derived CMDs are not deep enough to properly define the Turn-Off (TO) and Main Sequence (MS). In this paper, we present new BV photometry for NGC 6366 down to V 21.

The plan of the paper is as follows: in Sect. 2, we describe the observations, the procedure of reduction and the calibration of the photometric data; in Sect. 3 analyse and discuss the CMD of the cluster. We present our conclusions in Sect. 4.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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