Variations in the Milky Way's Stellar Mass Function at [Fe/H] < -1

TL;DR

This study measures the stellar mass function of low-mass stars in the Milky Way at metallicities below -1, revealing that the distribution of low-mass stars varies significantly with metallicity, challenging the notion of a universal initial mass function.

Contribution

First determination of the Galactic low-mass stellar mass function at [Fe/H] < -1 using Gaia DR3 data calibrated with SDSS-V spectra, showing metallicity-dependent variations.

Findings

MF below 0.5 M_sun is bottom-heavy at -1.5 < [Fe/H] < -1

MF becomes very bottom-light at lower metallicities with a flat slope

Results indicate the low-mass MF is not universal across metallicities

Abstract

We present the first determination of the Galactic stellar mass function (MF) for low-mass stars (0.2-0.5 M_sun) at metallicities [Fe/H] < -1. A sample of ~53,000 stars was selected as metal-poor on the basis of both their halo-like orbits and their spectroscopic [Fe/H] from Gaia DR3 BP/RP (XP) spectra. These metallicity estimates for low-mass stars were enabled by calibrating Gaia XP spectra with stellar parameters from SDSS-V. For -1.5 < [Fe/H] < -1, we find that the MF below 0.5 M_sun exhibits a "bottom-heavy" power-law slope of alpha ~ -1.6. We tentatively find that at even lower metallicities, the MF becomes very bottom-light, with a near-flat power-law slope of alpha ~ 0 that implies a severe deficit of low-mass stars. This metallicity-dependent variation is insensitive to the adopted stellar evolution model. These results show that the Galactic low-mass MF is not universal, with variations in the metal-poor regime. A further calibration of XP metallicities in the regime of M < 0.5 M_sun and [Fe/H] < -1.5 will be essential to verify these tentative low-metallicity trends.