oMEGACat V: Helium Enrichment in $\ddot{I}‰$ Centauri as a Function of Metallicity

Abstract

Constraining the helium enhancement in stars is critical for understanding the formation mechanisms of multiple populations in star clusters. However, measuring helium variations for many stars within a cluster remains observationally challenging. We use Hubble Space Telescope photometry combined with MUSE spectroscopic data for over 7,200 red-giant branch stars in \omc\ to measure helium differences between distinct groups of stars as a function of metallicity separating the impact of helium enhancements from other abundance variations on the pseudo-color (chromosome) diagrams. Our results show that stars at all metallicities have subpopulations with significant helium enhancement ($\hat{I}”Y_{min}\gtrsim$ 0.11). We find a rapid increase in helium enhancement from low metallicities ($[Fe/H]\simeq -2.05$ to $[Fe/H]\simeq -1.92)$, with this enhancement leveling out at \deltay\ $=0.154$ at higher metallicities. The fraction of helium-enhanced stars steadily increases with metallicity ranging from 10\% at $[Fe/H]\simeq -2.04$ to over $90\%$ at $[Fe/H]\simeq -1.04$. This study is the first to examine helium enhancement across the full range of metallicities in \omc{}, providing new insight into its formation history and additional constraints on enrichment mechanisms.