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Astron. Astrophys. 339, 61-69 (1998)

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1. Introduction

The formation of the Galactic halo is presently at the center of an open debate. Stetson et al. (1996) state that, apart from a handful of anomalous clusters that may well have been captured from a satellite dwarf galaxy, there is no strong evidence for a significant spread in age among clusters of a given metal abundance, while Chaboyer et al. (1996) support an age spread of 5 Gyr among the bulk of the Galactic globular clusters (GGCs) (which is increased to 9 Gyr, if the youngest clusters are considered).

One of the largest underlying sources of uncertainty is the heterogeneity of the data used in these studies, which prevents "large scale" tests. This is the main reason that prompted our group to gather an homogeneous photometric data base of GGC, in V and I, as discussed in Saviane et al. (1997). To date, we have observed about 80% of the closest GGC's ([FORMULA]) with 1m class telescopes, and our data set allows us to obtain color-magnitude diagrams (CMDs) from the RGB tip down to a few magnitudes below the turn-off (TO).

Several young (or suspect young) GGCs were included in our program: Palomar 12 (Gratton & Ortolani 1988, hereafter GO88), Ruprecht 106 (Buonanno et al.1990), Arp 2 (Buonanno et al. 1995a), Terzan 7 (Buonanno et al. 1995b) and Palomar 1 (Rosenberg et al. 1998, hereafter Paper I). Their age is suspected to significantly deviate from the general distribution. A precise determination of this deviation within our homogeneous data set is particularly valuable in the general framework of the GGC ages and of great importance in order to decide on the models of Galactic formation. For this reason, we will present and discuss in separate papers of this series the photometric data for the clusters whose age is significantly different from the average age of the GGC in our sample.

In Paper I we have discussed the case of Palomar 1, which resulted to be the youngest GGC in our Galaxy. In this paper we concentrate on Pal 12 (C2143-214, [FORMULA], [FORMULA]; l[FORMULA], b[FORMULA]) discovered by Harrington & Zwicky (1953) on the Palomar Sky Survey plates. Indeed, Pal 12 has been the first cluster to be classified as younger than the bulk of GGCs. However, the age determination in previous studies of its CMD, namely Stetson et al. (1989, hereafter S89), and Da Costa & Armandroff (1990, hereafter DA90) was affected by large uncertainties in its metal content (more than 0.3 dex in [Fe/H]). Since then, new low- and high-resolution spectroscopy have been used in order to estimate the metallicity of Palomar 12 (Armandroff & Da Costa 1991, AD91; Brown et al. 1997, B97).

The observations and data reduction are presented in Sect. 2 and the resulting CMD is discussed in Sect. 3. The relative age determination is carried out in Sect. 4, and in Sect. 5 the structural parameters of the cluster are established.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998