  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.
