Radiation Feedback, Fragmentation, and the Environmental Dependence of
  the Initial Mass Function
  The fragmentation of star-forming interstellar clouds, and the resulting
stellar initial mass function (IMF), is strongly affected by the temperature
structure of the collapsing gas. Since radiation feedback from embedded stars
can modify this as collapse proceeds, feedback plays an important role in
determining the IMF. However, the effects and importance of radiative heating
are likely to depend strongly on the surface density of the collapsing clouds,
which determines both their effectiveness at trapping radiation and the
accretion luminosities of the stars forming within them. In this paper we
report a suite of adaptive mesh refinement radiation-hydrodynamic simulations
using the ORION code in which we isolate the effect of column density on
fragmentation by following the collapse of clouds of varying column density
while holding the mass, initial density and velocity structure, and initial
virial ratio fixed. We find that radiation does not significantly modify the
overall star formation rate or efficiency, but that it suppresses fragmentation
more and more as cloud surface densities increase from those typical of low
mass star-forming regions like Taurus, through the typical surface density of
massive star-forming clouds in the Galaxy, up to conditions found only in super
star clusters. In regions of low surface density, fragmentation during collapse
leads to the formation of small clusters rather than individual massive star
systems, greatly reducing the fraction of the stellar population with masses >~
10 Msun. Our simulations have important implications for the formation of
massive stars and the universality of the IMF.
