The Absolute Age of Milky Way Globular Clusters

TL;DR

This study determines the ages of eight Milky Way globular clusters using advanced stellar models and HST data, revealing ages from 11.5 to 13.5 billion years and emphasizing the impact of distance, reddening, and stellar physics uncertainties.

Contribution

The paper introduces a comprehensive method combining theoretical isochrones with full CMD fitting to accurately estimate globular cluster ages, accounting for multiple sources of uncertainty.

Findings

Globular cluster ages range from 11.5 to 13.5 Gyr.

Distance and reddening uncertainties dominate age errors.

Older ages are associated with lower metallicities.

Abstract

Globular clusters (GCs) provide statistically significant coeval populations of stars spanning various evolutionary stages, allowing robust constraints on stellar evolution model parameters and ages. We analyze eight old Milky Way GCs with metallicities between [Fe/H] $=-2.31$ and $-0.77$ by comparing theoretical isochrone sets from the Dartmouth Stellar Evolution Program to HST observations. The theoretical isochrones include uncertainties introduced by $21$ stellar evolution parameters such as convective mixing, opacity, diffusion, and nuclear reactions, capturing much of the quantifiable physics used in our code. For each isochrone, we construct synthetic color-magnitude diagrams (CMD) near the main-sequence turn-off region and apply two full-CMD-fitting methods to fit HST ACS data across a range of distance and reddening and measure the absolute age of each GC from the resulting posterior distribution, which accounts for uncertainties in the stellar models, observations, and fitting method. The resulting best-fitting absolute ages range from $\approx 11.5$ to $13.5$ Gyr, with a typical error of $0.5-0.75$ Gyr; the data show a clear trend toward older ages at lower metallicities. Notably, distance and reddening account for over $50\%$ of the uncertainty in age determination in each case, with metallicity, $\alpha$ abundance, mixing length, and helium diffusion being the most important stellar physics parameters for the error budget. We also provide an absolute age-metallicity relation for Milky Way GCs.