Effects of the environment and feedback physics on the initial mass function of stars in the STARFORGE simulations

Abstract

One of the key mysteries of star formation is the origin of the stellar initial mass function (IMF). The IMF is observed to be nearly universal in the Milky Way and its satellites, and significant variations are only inferred in extreme environments, such as the cores of massive elliptical galaxies and the Central Molecular Zone. In this work we present simulations from the STARFORGE project that are the first cloud-scale radiation-magnetohydrodynamic simulations that follow individual stars and include all relevant physical processes. The simulations include detailed gas thermodynamics, as well as stellar feedback in the form of protostellar jets, stellar radiation, winds and supernovae. In this work we focus on how stellar radiation, winds and supernovae impact star-forming clouds. Radiative feedback plays a major role in quenching star formation and disrupting the cloud, however the IMF peak is predominantly set by protostellar jet physics. We find the effect of stellar winds is minor, and supernovae ‘occur too late’ to affect the IMF or quench star formation. We also investigate the effects of initial conditions on the IMF. We find the IMF is insensitive to the initial turbulence, cloud mass and cloud surface density, even though these parameters significantly shape the star formation history of the cloud, including the final star formation efficiency. Meanwhile, the characteristic stellar mass depends weakly on metallicity and the interstellar radiation field, which essentially set the average gas temperature. Finally, while turbulent driving and the level of magnetization strongly influences the star formation history, they only influence the high-mass slope of the IMF.

The most massive stars in very young star clusters with a limited mass: Evidence favours significant self-regulation in the star formation processes

“…For example, in preferential attachment phenomena and diffusion-limited aggregation, a large number of random processes (instead of a single stochastic event) determine the outcome, which becomes highly deterministic. The state-of-art star cluster formation simulation, STAR-FORGE, appears to support this view (Guszejnov et al 2022). Despite the chaotic and complex physical processes involved, the simulated maximum stellar masses in embedded star clusters 1 presented by these latter authors seem to have too little scatter to be explained by stochastic sampling schemes and to show strong evidence of a correlation between the embedded star cluster mass and the mass of the most massive star (m max −M ecl relation), which is consistent with optimal sampling.…”

“…Kirk & Myers (2012) find that massive stars can be better described if they assume an older cluster age (2 Myr instead of 1 Myr). This suggests that stars with different masses in the same star cluster may have different average ages and a single age assumption may introduce bias to the mass estimation of these pre-main sequence stars; see also STARFORGE simulations where more massive stars in a star cluster form at a later time (Guszejnov et al 2022). In addition, the statistical uncertainty on individual stars due to different extinction estimation errors and multiplicity are neglected.…”

The statistical properties of stars at redshift, z = 5, compared with the present epoch

“…This result is at odds with the results of Bate (2019) who find that under present-day conditions the characteristic stellar mass is quite independent of the metallicity (ranging from 1/100 to 3 times the solar value). This may be due to the limited resolution of the Guszejnov et al (2022) study -as mentioned above, Bate (2019) found that at low metallicity the effects of intermediatedensity gas being hotter were offset by enhanced cooling at high densities that led to enhanced small-scale fragmentation. On the other hand, Guszejnov et al (2022) found that subjecting solar-metallicity clouds to a stronger ISRF increased the characteristic stellar mass, which is in qualitative agreement with the results we present here.…”

“…In another recent paper, Guszejnov et al (2022) studied how environment, initial conditions, and feedback may alter the stellar IMF. Among other results, they find that the characteristic stellar mass varies with metallicity and with the level of the ISRF.…”

“…In addition to comparing the results of star cluster formation cal-E-mail: M.R.Bate@exeter.ac.uk culations to observed Galactic stellar populations, the ability to perform numerical simulations that produce clusters of stars gives us the ability to directly predict how the statistical properties of stellar systems depend on environment and initial conditions, and to investigate the roles of various physical processes. Thus far, hydrodynamical calculations that either include radiative transfer and/or radiative heating from protostars have been used to explore the dependence of stellar properties on metallicity (Myers et al 2011;Bate 2014Bate , 2019He, Ricotti & Geen 2019;Guszejnov et al 2022), molecular cloud density (Bate 2009b;Jones & Bate 2018b;Tanvir et al 2022;Guszejnov et al 2022), protostellar outflows (Krumholz, Klein & McKee 2012;Mathew & Federrath 2021;Tanvir, Krumholz & Federrath 2022;Grudić et al 2021Grudić et al , 2022Guszejnov et al 2021Guszejnov et al , 2022, variations in turbulence (Lomax, Whitworth & Hubber 2015;Nam, Federrath & Krumholz 2021;Mathew, Federrath & Seta 2022;Guszejnov et al 2022), and magnetic fields (Myers et al 2013(Myers et al , 2014Krumholz et al 2016;Cunningham et al 2018;Grudić et al 2021;Guszejnov et al 2022). Some of the most recent calculations have even begun exploring how the non-ideal magnetohydrodynamic (MHD) effects of ambipolar diffusion, Hall conduction, and Ohmic resistivity affect the formation of stellar groups, albeit only for small numbers of stars as yet (Wurster et al 2019).…”