I. Saviane

GALACTIC GLOBULAR CLUSTER RELATIVE AGES

We present accurate relative ages for a sample of 55 Galactic globular clusters. The ages have been obtained by measuring the difference between the horizontal branch and the turnoff in two internally photometrically homogeneous databases. The mutual consistency of the two data sets has been assessed by comparing the ages of 16 globular clusters in common between the two databases. We have also investigated the consistency of our relative age determination within the recent stellar model framework. All clusters with [Fe/H] -0.8 are ~1 Gyr younger than the most metal-poor ones, with a relatively small age dispersion, although the metal-rich sample is still too small to allow firmer conclusions. There is no correlation of the cluster age with the galactocentric distance. We briefly discuss the implication of these observational results for the formation history of the Galaxy.

Photometric catalog of nearby globular clusters - II. A large homogeneous

In this paper we present the second and nal part of a large and photometrically homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands. The catalog has been collected using only two tele- scopes (one for each hemisphere). The observed clus- ters represent 75% of the known Galactic globulars with (m M)V 16:15 mag, cover most of the globular clus- ter metallicity range ( 2:2 (Fe=H) 0:4), and span Galactocentric distances from 1: 2t o 18: 5k pc. In particular, here we present the CMDs for the 13 GGCs observed in the Northern hemisphere. The remaining 39 Southern hemisphere clusters of the catalog have been presented in a companion paper (Rosenberg et al. 2000). We present the rst CCD color magnitude diagram for NGC 6779 (M 56). All the CMDs extend from at least 2 magnitudes below the turn-o (i.e. Vlim 22) to the tip of the red giant branch. The calibration has been done using a large number of standard stars, and the absolute calibration is reliable to a 0:02 mag level in both lters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

(V,I)

?????Deep V and I CCD images of the loosely populated Galactic globular cluster Pal 1 and the surrounding field have been obtained with the Isaac Newton Telescope. A color-magnitude diagram (CMD) down to Vlim ~ 24 and a luminosity function down to Vlim = 23.25 have been constructed. Adopting a reddening E(V - I) = 0.20 ? 0.04 and comparing the CMD of Pal 1 with that of 47 Tucanae, we obtain a distance modulus (m - M)0 = 15.25 ? 0.25, indicating that Pal 1 is located 3.7 ? 0.4 kpc above the Galactic disc and 17.3 ? 1.6 kpc from the Galactic center. Comparison with 47 Tuc and M71 shows that Pal 1 must be significantly younger. The best-fitting isochrones, as listed by Bertelli et al., yield an age between 6.3 and 8 Gyr, which would make Pal 1 the youngest Galactic globular cluster identified thus far, casting some doubt on the real nature of this object. The luminosity function shows some evidence of mass segregation, consistent with the very short relaxation time. The global mass function can be fitted with a power law of slope x = -1.4 ? 0.7. This mass function is anomalously flat, suggesting either a strong dynamical evolution or an initial mass function significantly different from most other halo globular clusters. A discussion of the possible nature of Pal 1 is presented.

color-magnitude diagram data-base

Palomar 1 is a peculiar Galactic globular cluster suspected of being younger than the bulk of Galactic halo objects. However, such a low age can be confirmed only after a reliable determination of the metallicity. In the present paper, we use the equivalent widths, W, of the Ca II triplet on medium-resolution spectra to determine the metal content of Pal 1. From the comparison of the luminosity-corrected Ws in four stars of Pal 1 with those of a sample of stars in each of three calibration clusters (M2, M15, and M71), we derive [Fe/H] = -0.6 ± 0.2 on the Zinn-West scale or [Fe/H] = -0.7 ± 0.2 on the Carretta-Gratton scale. We also obtain a radial velocity vr = -82.8 ± 3.3 km s-1 for Pal 1.

Palomar 1: Another Young Galactic Halo Globular Cluster?

We present deep VLT/FORS1 observations of the two distant, isolated Local Group dwarfs Phoenix and Antlia. Our results provide further evidence for the presence of old stars in these star-forming dwarf galaxies. Old stellar populations are known in all of the Local Group dwarf spheroidal galaxies and in some dwarf irregulars, implying that dwarf galaxies started forming stars at a sharply defined early epoch irrespective of their subsequent star formation histories (e.g., Held et al., 2000; Saviane et al., 2000; and references therein).The new color-magnitude diagrams of Phoenix confirm the presence of a spatially extended blue HB population, indicating a conspicuous old component (Held et al., 1999;Martinez-Delgado et al., 1999). A preliminary analysis of stellar variability has led to the discovery of several tens RR Lyrae variables, which can provide clue information on the earliest star formation episode (see, e.g., Siegel and Majewski, 2000). The young main sequence extends down to the limit of our photometry (V=25.5 mag), which suggests that Phoenix underwent nearly continuous star formation in the last 2 Gyr. Our deep color-magnitude diagrams of Antlia have been used to investigate the gradient in the stellar populations of this dwarf irregular/spheroidal galaxy. While the young stars appear to be concentrated in a round central region(Aparicio et al. 1997; Sarajedini et al. 1997), the spatial distribution of the red giant stars defines an extended flattened halo (or disk) 2-3 kpc across.

The Metallicity of Palomar 1

Based on two new large, homogeneous photometric databases of 35 (from V and I groundbased data) and 15 (fromBand V HST data) Galactic globular clusters, a set of distance and reddening independent relative age indicators has been measured. The observed(V —i)02 5,8(B —V)@2.5, and AVM vs. metallicity relations have been compared to the relations predicted by two recent updated libraries of isochrones. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for our GGC samples. The main results are: (a) there is no age-metallicity trend; (b) there is no evidence of an age spread for clusters with [Fe/H]< —1.2, all the clusters of our sample in this range being old and coeval; (c) for the more metal rich clusters there are indications of a larger age dispersion, of the order of 10-15%; (d) there is no trend of the age with the distance from the Galactic center out to 25 Kpc.