Revisiting the bound on axion-photon coupling from Globular Clusters

TL;DR

This paper establishes a new, stronger upper limit on the axion-photon coupling by analyzing globular cluster star populations and accounting for helium abundance uncertainties, improving previous constraints.

Contribution

The study provides the most stringent bound on axion-photon coupling from globular clusters by incorporating recent helium abundance measurements and stellar modeling.

Findings

Upper bound on $g_{a heta}$: < 0.66×10^{-10} GeV^{-1} at 95% CL

Helium abundance significantly affects the axion-photon coupling estimate

Results improve upon previous astrophysical constraints

Abstract

We derive a strong bound on the axion-photon coupling $g_{a\gamma}$ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e. the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for $g_{a\gamma}$. We show that the estimated value of $g_{a\gamma}$ substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as benchmark for our study the most recent determinations of the He abundance in H II regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound $g_{a\gamma}<0.66\times 10^{-10}$ GeV$^{-1}$ at 95$\%$ confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.