The Elephant in the Room: The Importance of Where and When Massive Stars Form in Molecular Clouds

TL;DR

This study investigates how different prescriptions for assigning luminosity to stellar mass elements in GMC simulations significantly affect star formation efficiencies, highlighting the importance of source discreteness and underlying assumptions in feedback modeling.

Contribution

It demonstrates that varying IMF sampling schemes in simulations lead to substantial differences in star formation outcomes, emphasizing the impact of source discreteness and assumptions about massive star formation.

Findings

Star formation efficiency varies by a factor of ~3 depending on feedback prescription.

Fewer, more luminous sources exert a greater feedback effect than many dimmer sources.

Methodological differences can overshadow other factors like initial conditions or magnetic fields.

Abstract

Most simulations of galaxies and massive giant molecular clouds (GMCs) cannot explicitly resolve the formation (or predict the main-sequence masses) of individual stars. So they must use some prescription for the amount of feedback from an assumed population of massive stars (e.g. sampling the initial mass function [IMF]). We perform a methods study of simulations of a star-forming GMC with stellar feedback from UV radiation, varying only the prescription for determining the luminosity of each stellar mass element formed (according to different IMF sampling schemes). We show that different prescriptions can lead to widely varying (factor of ~3) star formation efficiencies (on GMC scales) even though the average mass-to-light ratios agree. Discreteness of sources is important: radiative feedback from fewer, more-luminous sources has a greater effect for a given total luminosity. These differences can dominate over other, more widely-recognized differences between similar literature GMC-scale studies (e.g. numerical methods, cloud initial conditions, presence of magnetic fields). Moreover the differences in these methods are not purely numerical: some make different implicit assumptions about where and how massive stars form, and this remains deeply uncertain in star formation theory.