Milky Way Globular Cluster Metallicity and Low-Mass X-ray Binaries: The Red Giant Influence

TL;DR

This study investigates whether the density of red giant branch stars in Milky Way globular clusters correlates with metallicity, aiming to explain the higher likelihood of metal-rich clusters hosting low-mass X-ray binaries.

Contribution

It provides the first detailed analysis linking RGB star density and metallicity in GCs, suggesting dynamical history influences LMXB formation.

Findings

Weak correlation between RGB star density and metallicity (p < 0.001)

No correlation found in GCs hosting quiescent LMXBs

Dynamical history may influence LMXB formation

Abstract

Galactic and extragalactic studies have shown that metal-rich globular clusters (GCs) are approximately three times more likely to host bright low-mass X-ray binaries (LMXBs) than metal-poor GCs. There is no satisfactory explanation for this metallicity effect. We tested the hypothesis that the number density of red giant branch (RGB) stars is larger in metal-rich GCs, and thus potentially the cause of the metallicity effect. Using Hubble Space Telescope photometry for 109 unique Milky Way GCs, we investigated whether RGB star density was correlated with GC metallicity. Isochrone fitting was used to calculate the number of RGB stars, which were normalized by the GC mass and fraction of observed GC luminosity, and determined density using the volume at the half-light radius $r_{h}$. The RGB star number density was weakly correlated with metallicity [Fe/H], giving Spearman and Kendall Rank test $p$-values of 0.00016 and 0.00021 and coefficients $r_{s} = 0.35$ and $\tau = 0.24$ respectively. This correlation may be biased by a possible dependence of $r_{h}$ on [Fe/H], although studies have shown that $r_{h}$ is correlated with Galactocentric distance and independent of [Fe/H]. The dynamical origin of the $r_{h}$-metallicity correlation (tidal stripping) suggests that metal-rich GCs may have had more active dynamical histories, which would promote LMXB formation. No correlation between the RGB star number density and metallicity was found when using only the GCs that hosted quiescent LMXBs. A complete census of quiescent LMXBs in our Galaxy is needed to further probe the metallicity effect, which will be possible with the upcoming launch of eROSITA.