DOES GOD PLAY DICE WITH STAR CLUSTERS?

When a detailed model of a stellar population is unavailable, it is most common to assume that stellar masses are independently and identically distributed according to some distribution: the universal initial mass function (IMF). However, stellar masses resulting from causal, long-ranged physics cannot be truly random and independent, and the IMF may vary with environment. To compare stochastic sampling with a physical model, we run a suite of 100 STARFORGE radiation magnetohydrodynamics simulations of low-mass star cluster formation in 2000M_⊙clouds that form ∼ 200 stars each on average. The stacked IMF from the simulated clouds has a sharp truncation at ∼ 28M_⊙, well below the typically-assumed maximum stellar mass M_up ∼ 100 − 150M_⊙ and the total cluster mass. The sequence of star formation is not totally random: massive stars tend to start accreting sooner and finish later than the average star. However, final cluster properties such as maximum stellar mass and total luminosity have a similar amount of cloud-to-cloud scatter to random sampling. Therefore stochastic sampling does not generally model the stellar demographics of a star cluster as it is forming, but may describe the end result fairly well, if the correct IMF – and its environment-dependent upper cutoff – are known.