What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

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 ∼ 0.01 M . 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, ∼ 0.01 M , 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 ∼ 0.01 M 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 ∼ 0.2 M , 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 ∼ 0.2 M .

Formation of stellar clusters in magnetized, filamentary infrared dark clouds

“…They have more low luminosity particles than we do, which we attribute to their omission of radiative feedback, which suppresses fragmentation (Bate 2009;Offner et al 2009;Krumholz et al 2016). They note that their PLF fits the observations by Kryukova et al (2012) quite well.…”

“…Their simulations did not include radiative feedback, which has been shown to be important in setting the characteristic mass of the IMF (Bate 2009;Offner et al 2009;Krumholz et al 2016), nor outflows, which also affect the IMF (e.g., Hansen et al 2012); our simulation includes both effects. On the other hand, they included the effects of heating by the interstellar radiation field, which we have not.…”

Simulating star clusters across cosmic time – I. Initial mass function, star formation rates, and efficiencies

“…Previous authors have included ideal MHD in their simulations (Myers et al 2013;Krumholz et al 2016;Cunningham et al 2018). However, since these authors only form stars up to ∼20 M , they neglect ionising radiation.…”

Mixing of metals during star cluster formation: statistics and implications for chemical tagging

“…As discussed in Section 3, thermal support enhanced by stellar radiation feedback is a key mechanism in setting the cloud fragmentation scale (e.g. Krumholz et al 2016). This step will allow us to trace a more realistic cloud fragmentation history into individual stars.…”