High Mass Star Formation by Gravitational Collapse of Massive Cores
  The current generation of millimeter interferometers have revealed a population of compact (r <~ 0.1 pc), massive (M ~ 100 Msun) gas cores that are the likely progenitors of massive stars. I review models for the evolution of these objects from the observed massive core phase through collapse and into massive star formation, with particular attention to the least well-understood aspects of the problem: fragmentation during collapse, interactions of newborn stars with the gas outside their parent core, and the effects of radiation pressure feedback. Through a combination of observation, analytic argument, and numerical simulation, I develop a model for massive star formation by gravitational collapse in which massive cores collapse to produce single stars or (more commonly) small-multiple systems, and these stars do not gain significant mass from outside their parent core by accretion of either gas or other stars. Collapse is only very slightly inhibited by feedback from the massive star, thanks to beaming of the radiation by a combination of protostellar outflows and radiation-hydrodynamic instabilities. Based on these findings, I argue that many of the observed properties of young star clusters can be understood as direct translations of the properties of their gas phase progenitors. Finally, I discuss unsolved problems in the theory of massive star formation, and directions for future work on them. 
Mark R. Krumholz (Princeton University)