Cluster formation in molecular clouds – I. Stellar populations, star formation rates and ionizing radiation

We present a model for the radiative output of star clusters in the process of star formation suitable for use in hydrodynamical simulations of radiative feedback. Gas in a clump, defined as a region whose density exceeds 104 cm−3, is converted to stars via the random sampling of the Chabrier initial mass function. A star formation efficiency controls the rate of star formation. We have completed a suite of simulations which follow the evolution of accretion-fed clumps with initial masses ranging from 0 to 105 M⊙ and accretion rates ranging from 10−5 to 10−1 M⊙ yr−1. The stellar content is tracked over time which allows the aggregate luminosity, ionizing photon rate, number of stars and star formation rate (SFR) to be determined. For a fiducial clump of 104 M⊙, the luminosity is ∼4 × 106 L⊙ with an SFR of roughly 3 × 10−3 M⊙ yr−1. We identify two regimes in our model. The accretion-dominated regime obtains the majority of its gas through accretion and is characterized by an increasing SFR, while the reservoir-dominated regime has the majority of its mass present in the initial clump with a decreasing SFR. We show that our model can reproduce the expected number of O stars, which dominate the radiative output of the cluster. We find a nearly linear relationship between SFR and mass as seen in observations. We conclude that our model is an accurate and straightforward way to represent the output of clusters in hydrodynamical simulations with radiative feedback.