Theoretical vs . semi - empirical relative ages of globular clusters

Theoretical relative ages of galactic globular clusters have recently been challenged by a semi-empirical relation. It was used to point out that tested sets of iso-chrones were unable to reproduce the relation and yield internally inconsistent relative ages. We find that differential cluster ages derived with the isochrones by Salaris & Weiss (1998) are reliable and internally consistent. We also show that this consistency depends on using the lower absolute ages determined by SW98, which therefore receive more empirical support. Moreover, we discuss the effect of the clusters absolute age on the evaluation of their differential ages, and its connection with the question of their age dispersion. 1. Two opponent views of relative cluster ages The determination of galactic globular cluster (GC) ages has recently come into a state of flux, first due to updated stellar physics and lately due to the HIPPARCOS results. Theoretical isochrones were challenged by Buo-nanno et al. (1998, hereafter BCP), who have presented a self-consistent method for the determination of relative cluster ages, which as much as possible makes use of observational properties and tries to minimize the input from theoretical isochrones. They argue that three tested sets of isochrones fail to reproduce the empirical relation between △(B−V) RGB TO (the (B-V) difference between the TO and the base of the RGB) and [Fe/H] at a given age. Furthermore , relative ages based on brightness or colour differences are inconsistent in all cases. In this Letter we will confront the isochrones by Salaris & Weiss (1997, 1998; hereafter SW97 & SW98) with the observational results and show that this set passes the test to a great extent. SW97 have determined GC ages by means of the traditional theoretical approach of comparing theoretical iso-chrones with an observed CMD. The differences with respect to related works are (i) that they use the very lat-est (canonical) input physics, (ii) that also α-element enhancement in the GC composition is taken into account (e.g. in the opacity tables) and (iii) that they determined absolute ages only for the very few clusters where both CMD morphology and available data guarantee an accurate dating. The brightness difference between Turn-Off (TO) and Zero-Age Horizontal Branch (ZAHB), △V HB TO , was used as the absolute age indicator. The imposed constraints result in a rather small sample of 7 clusters of all metallicities (including disk GC in SW98) suitable for the determination of absolute …