"Helium Enrichment Law": The Relation Between Helium and Metal Enrichment in Stars in FIRE-3 Simulations of Milky Way-mass Galaxies

Helium abundance is a key input to stellar models that are used to infer accurate ages, masses, and compositions for individually resolved stars in the Milky Way (MW), underpinning Galactic archaeology. Since spectroscopically measuring He abundances is difficult, stellar models often assume a fixed "chemical enrichment law" dY/dZ (the slope linearly relating a star's helium mass fraction Y to its metal mass fraction Z). If incorrect, this assumption can introduce significant systematics in inferring stellar properties. Using the FIRE-3 cosmological baryonic zoom-in simulations, we investigate helium versus metal enrichment in the stars of MW-mass galaxies. Given the nucleosynthetic yields in FIRE-3, overall metal production is dominated by core-collapse supernovae (CCSN) at all progenitor metallicities. He production is dominated by CCSN for progenitor metallicities of Z <= 0.1 Zsun, but by stellar winds for progenitors near Zsun. Our simulations produce a range of dY/dZ slopes slopes with best-fit values of ~0.75-2.1 depending on the age of the stars considered at present day; on average consistent with the range of values from MW observations but with no single "universal" enrichment law. The dY/dZ slope also shows galaxy-to-galaxy variance in our simulations, and to leading order increases over cosmic time within the same galaxy, based on the peculiarities of the formation histories of each MW-mass system. Despite choosing a fixed set of nucleosynthetic yields for stellar feedback processes in FIRE-3, we predict variance in dY/dZ for stars in MW-mass galaxies, and observations of MW stars must be interpreted in this context.