What Physics Determines the Peak of the IMF? Insights from the Structure of Cores in Radiation-Magnetohydrodynamic Simulations

TL;DR

This study uses radiation-magnetohydrodynamic simulations to identify thermal pressure from stellar radiation as the main factor limiting fragmentation, explaining the observed peak of the initial mass function at around 0.2 solar masses.

Contribution

It reveals that thermal pressure from stellar radiation, rather than the opacity limit, primarily determines the IMF peak in star-forming regions.

Findings

Thermal pressure from stellar radiation limits fragmentation near stars.

Objects around 0.01 M_sun tend to accrete more gas, explaining their rarity.

The IMF peak at 0.2 M_sun results from thermal feedback effects.

Abstract

As star-forming clouds collapse, the gas within them fragments to ever-smaller masses. Naively one might expect this process to continue down to the smallest mass that is able to radiate away its binding energy on a dynamical timescale, the opacity limit for fragmentation, at $\sim 0.01$ $M_\odot$. However, the observed peak of the initial mass function (IMF) lies a factor of $20-30$ higher in mass, suggesting that some other mechanism halts fragmentation before the opacity limit is reached. In this paper we analyse radiation-magnetohydrodynamic simulations of star cluster formation in typical Milky Way environments in order to determine what physical process limits fragmentation in them. We examine the regions in the vicinity of stars that form in the simulations to determine the amounts of mass that are prevented from fragmenting by thermal and magnetic pressure. We show that, on small scales, thermal pressure enhanced by stellar radiation heating is the dominant mechanism limiting the ability of the gas to further fragment. In the brown dwarf mass regime, $\sim 0.01$ $M_\odot$, the typical object that forms in the simulations is surrounded by gas whose mass is several times its own that is unable to escape or fragment, and instead is likely to accrete. This mechanism explains why $\sim 0.01$ $M_\odot$ objects are rare: unless an outside agent intervenes (e.g., a shock strips away the gas around them), they will grow by accreting the warmed gas around them. In contrast, by the time stars grow to masses of $\sim 0.2$ $M_\odot$, the mass of heated gas is only tens of percent of the central star mass, too small to alter its final mass by a large factor. This naturally explains why the IMF peak is at $\sim 0.2$ $M_\odot$.