Stellar Initial Mass Function Varies with Metallicities and Time

TL;DR

This study uses a large sample of M-dwarf stars to demonstrate that the stellar initial mass function varies with metallicity and age, challenging the notion of a universal IMF and impacting galaxy evolution models.

Contribution

It provides the first large-scale star-counting evidence of IMF variability with metallicity and age in the Solar neighborhood, using spectroscopic data of 93,000 stars.

Findings

Early-time stellar populations have fewer low-mass stars than the canonical IMF.

In current populations, low-mass star proportion increases with metallicity.

The IMF variability affects models of star formation and galaxy evolution.

Abstract

Most structural and evolutionary properties of galaxies strongly rely on the stellar initial mass function (IMF), namely the distribution of the stellar mass formed in each episode of star formation. As the IMF shapes the stellar population in all stellar systems, it turns out to become one of the most fundamental concepts of modern astronomy. Both constant and variable IMFs across different environments have been claimed despite a large number of theoretical and observational efforts. However, the measurement of the IMF in Galactic stellar populations has been limited by the relatively small number of photometrically observed stars, leading to high uncertainties. Here we report a star-counting result based on ~93,000 spectroscopically observed M-dwarf stars, an order of magnitude more than previous studies, in the 100--300 parsec (pc) Solar neighbourhood. We find unambiguous evidence of a variable IMF that depends on both metallicity and stellar age. Specifically, the stellar population formed at the early time contains fewer low-mass stars compared to the canonical IMF, independent of stellar metallicities. In present days, on the other hand, the proportion of low-mass stars increases with stellar metallicity. The variable abundance of low-mass stars in our Milky Way establishes a powerful benchmark for models of star formation and can heavily impact results in Galactic chemical enrichment modelling, mass estimation of galaxies, and planet formation efficiency.