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Most simulations of galaxies and massive giant molecular clouds (GMCs) cannotexplicitly resolve the formation (or predict the main-sequence masses) ofindividual stars. So they must use some prescription for the amount of feedbackfrom an assumed population of massive stars (e.g. sampling the initial massfunction [IMF]). We perform a methods study of simulations of a star-formingGMC with stellar feedback from UV radiation, varying only the prescription fordetermining the luminosity of each stellar mass element formed (according todifferent IMF sampling schemes). We show that different prescriptions can leadto widely varying (factor of ~3) star formation efficiencies (on GMC scales)even though the average mass-to-light ratios agree. Discreteness of sources isimportant: radiative feedback from fewer, more-luminous sources has a greatereffect 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 magneticfields). Moreover the differences in these methods are not purely numerical:some make different implicit assumptions about where and how massive starsform, and this remains deeply uncertain in star formation theory.

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