The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods

TL;DR

This study examines how unrecognized chemical abundance differences in globular clusters affect their relative age estimates, highlighting the robustness of the MS-fitting method over horizontal and vertical methods.

Contribution

It provides a theoretical analysis of the impact of chemical variations on globular cluster age-dating techniques, emphasizing the reliability of the MS-fitting method.

Findings

Horizontal and vertical methods are sensitive to He and heavy element variations.

Unrecognized chemical differences can cause age errors up to ~6 Gyr.

MS-fitting method is less affected by chemical abundance variations.

Abstract

The impact that unrecognised differences in the chemical patterns of Galactic globular clusters have on their relative age determinations is studied. The two most widely used relative age-dating methods, horizontal and vertical, together with the more recent relative MS-fitting method, were carefully analyzed on a purely theoretical basis. The BaSTI library was adopted to perform the present analysis. We find that relative ages derived using the horizontal and vertical methods are largely dependent on the initial He content and heavy element distribution. Unrecognized cluster-to-cluster chemical abundance differences can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr if an age of 12.8 Gyr is adopted for normalization), and even larger for some extreme cases. It is shown that the relative MS-fitting method is by far the age-dating technique for which undetected cluster-to-cluster differences in the He abundance have less impact. Present results are used in order to pose constraints on the maximum possible spread in the He and CNONa elements abundances on the basis of the estimates - taken from the literature - of the Galactic globular clusters relative age dispersion obtained with the various relative age-dating techniques. Finally, it is shown that the age-metallicity relation found for young Galactic globular clusters by the GC Treasury program is a real age sequence and cannot be produced by variations in the He and/or heavy element distribution.