On the origin of the peak of the stellar initial mass function: exploring the tidal screening theory

Abstract

Classical theories for the stellar initial mass function (IMF) predict a peak mass which scales with the properties of the molecular cloud. In this work, we explore a new theory proposed by Lee & Hennebelle (2018). The idea is that the tidal field around first Larson cores prevents the formation of other collapsing clumps within a certain radius. The protostar can then freely accrete the gas within this radius. This leads to a peak mass of roughly 10 M 1LC , independent of the parent cloud properties. Using simple analytical arguments, we derive a collapse condition for clumps located close to a protostar. We then study the tidal field and the corresponding collapse condition using a series of numerical simulations. We find that the tidal field around protostars is indeed strong enough to prevent clumps from collapsing unless they have high enough densities. For each newly formed protostar, we determine the region in which tidal screening is dominant. We call this the tidal bubble. The mass within this bubble is our estimate for the final mass of the star. Using this formalism, we are able to construct a very good prediction for the final IMF in our simulations. Not only do we correctly predict the peak, but we are also able to reproduced the high and low mass end of the IMF. We conclude that tidal forces are important in determining the final mass of a star and might be the dominant effect in setting the peak mass of the IMF.

STARFORGE: Towards a comprehensive numerical model of star cluster formation and feedback

“…(iv) Scale up to massive GMCs: Current star formation simulations that do both (i) and (ii) have focused upon lower mass systems, simulating gas masses of 100-1000 M (Federrath 2015;Cunningham et al 2018;Jones & Bate 2018;Lee & Hennebelle 2018;Li, Klein & McKee 2018;Wurster, Bate & Price 2019;Colman & Teyssier 2020), producing ∼10-100 M in stars. Low-mass clusters are important to model, as they can be readily compared to well-studied sites of star formation in the Solar neighbourhood (e.g.…”

STARFORGE: the effects of protostellar outflows on the IMF

“…This transition from near-isothermal to adiabatic behaviour was originally proposed to be responsible for setting the peak of the IMF (see Low & Lynden-Bell 1976;Rees 1976), but the corresponding mass scale (∼0.008 M ) was too low to explain observations. The idea has recently been revived by taking into account the tidal screening effect around the first Larson core (Colman & Teyssier 2020), which increases the relevant mass scale to be comparable to the observed IMF peak.…”

“…have shown significant temperature differences between low-density regions (∼10 2 cm −3 , T ∼ 30 K) and highdensity regions where collapse occurs (∼10 5 cm −3 , T ∼ 10 K). Even at high densities, the isothermal assumption inevitably breaks down completely at high densities, when the cloud becomes opaque to its own cooling radiation, leading to an increase in temperature and thus a suppression of fragmentation (for the original idea, see Low & Lynden-Bell 1976;Rees 1976; for modern interpretations, see Colman & Teyssier 2020).…”

The IMF and multiplicity of stars from gravity, turbulence, magnetic fields, radiation and outflow feedback

“…Here our sink particles have radii of 𝑟 sink = 250 AU, and the high computational cost of these simulations currently prevents us from going higher in numerical resolution, because at the same time, we want to produce a statistically converged sample of stars (here built up from 10 independent simulations with different random seeds of the turbulence). Recent studies by Hennebelle et al (2019) and Colman & Teyssier (2020) suggest that the origin of the peak of the IMF may lie in the tidal screening of the first hydrostatic core or the Larson core. The characteristic mass of the IMF would then be determined by the typical mass within the radius in which the tidal force by the Larson core prevents formation of any fragments, in which case a resolution of ∼ 10-20 AU would be necessary to obtain the peak mass.…”