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

Abstract

We present radiation-magneto-hydrodynamic simulations of star formation in selfgravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between m gas = 10 3 M to 3 × 10 5 M and gas densities typical of clouds in the local universe (n gas ∼ 1.8 × 10 2 cm −3 ) and 10× and 100× denser, expected to exist in high-redshift galaxies. The main results are: i) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about 40% of the mass of the sink particle, while the remaining 60% is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope 0.8, satisfy this empirical prescription. ii) The star formation law that best describes our set of simulation is dρ * /dt ∝ ρ 1.5 gas if n gas < n cri ≈ 10 3 cm −3 , and dρ * /dt ∝ ρ 2.5 gas otherwise. The duration of the star formation episode is roughly 6 cloud's sound crossing times (with c s = 10 km/s). iii) The total star formation efficiency in the cloud is f * = 2%(m gas /10 4 M ) 0.4 (1 + n gas /n cri ) 0.91 , for gas at solar metallicity, while for metallicity Z < 0.1 Z , based on our limited sample, f * is reduced by a factor of ∼ 5. iv) The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.

Shaping the structure of a GMC with radiation and winds

“…Similar substructures are found in other works featuring photoionization feedback (e.g. Walch et al 2015;Haid et al 2019;He et al 2019). Nevertheless, the quantitative results on the IMF (for simulations featuring sink particle formation), cloud dispersal time, and star formation efficiency are sensitive on differences on the initial setup.…”

“…In their simulations SFE ∼ 0.3-0.6, suggesting that radiation feedback alone is not enough to suppress the SFE to the generally observed values (∼ 1-10 per cent; Lada, Lombardi & Alves 2010;Murray, Quataert & Thompson 2010;Lada 2016;Lee et al 2016;Ochsendorf et al 2017). However, other models show that a lower SFEs can be obtained by including magnetic fields in the simulation (Geen et al 2016(Geen et al , 2018Kim, Kim & Ostriker 2018;Haid et al 2019;He, Ricotti & Geen 2019). In our paper, we make a further step in the accuracy of GMC simulations, including several novel features:…”

Simulating star clusters across cosmic time – II. Escape fraction of ionizing photons from molecular clouds

“…In this paper, the second of a series, we estimate f MC esc using a large set of realistic simulations of star cluster formation in molecular clouds. These are radiationmagneto-hydrodynamic simulations of star formation in selfgravitating, turbulent molecular clouds, presented in He, Ricotti & Geen (2019) (hereafter, Paper I). We model selfconsistently the formation of individual massive stars, including their UV radiation feedback and their lifetime.…”

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We calculate the hydrogen and helium-ionizing radiation escaping star forming molecular clouds, as a function of the star cluster mass and compactness, using a set of high-resolution radiation-magneto-hydrodynamic simulations of star formation in selfgravitating, turbulent molecular clouds. In these simulations, presented in He, Ricotti and Geen (2019), the formation of individual massive stars is well resolved, and their UV radiation feedback and lifetime on the main sequence are modelled selfconsistently. We find that the escape fraction of ionizing radiation from molecular clouds, f MC esc , decreases with increasing mass of the star cluster and with decreasing compactness.

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A Comparison of the Simulations and Observations for a Nearby Spiral Arm

“…The time-scale on which gas is removed from the embedded cluster not only affects the mass and number of stars in the surviving cluster, but also its degree of mass segregation and the density profile (Er et al, 2009). The parameters that strongly affect the outcome of the out-gassing phase are the star-formation efficiency (SFE), the population of the most massive stars, and the efficiency of the radiative coupling (He et al, 2019). The typically observed SFE of about to 0.4 can be explained by using radiative magneto-hydrodynamic simulations with self-consistent starformation and ionizing radiation (Geen et al, 2017).…”