## 4. AgeThe first evidence of a relatively young age for Pal 12 was given by GO88, who estimated that Pal 12 must be 30% younger than 47 Tuc on the basis of an atypically small value of the magnitude difference between the HB and the TO. Indeed, this was the first clear identification of a young GGC. Almost at the same time, S89 presented an independent In both studies, the 47 Tuc fiducial lines were taken from
Hesser et al. (1987, hereafter H87). These fiducials were constructed
by merging The heterogeneity of the data base for the comparison clusters and
the high uncertainty in the metal content do not allow us to quantify
the error associated to the results by GO88 and S89. In the following
we will attempt a new, independent determination of the Pal 12
Since no GGCs with metallicity have been
observed to date in the The best The metallicities and abundance ratios have been taken from Table 2 of Carney (1996): and for 47 Tuc, and and for M5. Fig. 5 shows the fiducial points of M5, 47 Tuc and
Pal 12 registered to a common TO point. It is clear that, while
the RGBs of 47 Tuc and M5 are almost overlapping, the RGB of
Pal 12 is significantly redder. The modest color shift between
the RGB of M5 and that of 47 Tuc shows that metallicity
differences have small influence on the RGB-TO color-difference, in
the
The position of the Pal 12 RGB cannot therefore be explained by a simple metallicity effect. The observed difference in the location of the RGB of Pal 12 with respect to 47 Tuc and M5 must be due either to a different element abundance or to an age effect. We begin by examining the first possibility. According to Salaris
et al. (1993), an enhancement by a factor An age difference is the only remaining explanation. In order to make an estimate of the Pal 12 relative age, we have measured between the TO and the RGB for different (fixed) values in the models of B94, Straniero et al. (1997, hereafter S97), and V98). The first two sets of models are , while the third one is. Fig. 6 displays the for mag as a function of the logarithm of age. With a good approximation, linearly depends on the logarithm of age. The -2.2 mag level has been chosen after an analysis of the behavior of the TO-RGB color difference with respect to the age. We have repeated our measurements at the RGB levels marked by dotted lines in Fig. 5 and found that, if a value is taken, the SGB plays an important role, making relative measurements difficult to interpret. The same occurs for , where the slope of the RGB becomes very sensitive to the clusters metallicity. Conversely, for in the range and age older than 8 Gyrs, the seems to be almost independent of metallicity. We simply chose a mean value -2.2. The linear relations in Fig. 5 have the same slopes for , while for the B94 and V98 models give the same slope, which is slightly different from that obtained from S97. The zero points are different, but this does not affect the relative age determination. We will therefore obtain the same relative ages when using either the B94, V98 or S97 models at , while the S97 isochrones give age differences larger by than B94 or V98 at Z=0.003.
From Fig. 5 we have =0.280 for M5, =0.265 for 47 Tuc, and =0.330 for Pal 12. Assuming Z=0.003, from Fig. 6, we obtain that Pal 12 is 34%, 34%, or 30% younger than 47 Tuc on the basis of V98, B94 and S97 models, respectively. As discussed above, adopting Z=0.001 we have quite similar results: formally, Pal 12 is 33%, 32%, or 32% younger than M5. Taking into account the errors in measuring the parameter (estimated assuming an uncertainty of mag and mag in the magnitude and color of the TO) for both Pal 12 and the reference clusters, the uncertainties in the relative ages is of the order of 10%. We conclude that Pal 12 has an age that of a typical GGC, assuming that 47 Tuc and M5 age are representative of the ages of the bulk of the GGC population (Buonanno et al 1998). © European Southern Observatory (ESO) 1998 Online publication: September 30, 1998 |