The Impact of Feedback During Massive Star Formation by Core Accretion
  We study feedback during massive star formation using semi-analytic methods,
considering the effects of disk winds, radiation pressure, photoevaporation and
stellar winds, while following protostellar evolution in collapsing massive gas
cores. We find that disk winds are the dominant feedback mechanism setting star
formation efficiencies (SFEs) from initial cores of ~0.3-0.5. However,
radiation pressure is also significant to widen the outflow cavity causing
reductions of SFE compared to the disk-wind only case, especially for >100Msun
star formation at clump mass surface densities Sigma<0.3g/cm2. Photoevaporation
is of relatively minor importance due to dust attenuation of ionizing photons.
Stellar winds have even smaller effects during the accretion stage. For core
masses Mc~10-1000Msun and Sigma~0.1-3g/cm2, we find the overall SFE to be
0.31(Rc/0.1pc)^{-0.39}, potentially a useful sub-grid star-formation model in
simulations that can resolve pre-stellar core radii,
Rc=0.057(Mc/60Msun)^{1/2}(Sigma/g/cm2)^{-1/2}pc. The decline of SFE with Mc is
gradual with no evidence for a maximum stellar-mass set by feedback processes
up to stellar masses of ~300Msun. We thus conclude that the observed truncation
of the high-mass end of the IMF is shaped mostly by the pre-stellar core mass
function or internal stellar processes. To form massive stars with the observed
maximum masses of ~150-300Msun, initial core masses need to be >500-1000Msun.
We also apply our feedback model to zero-metallicity primordial star formation,
showing that, in the absence of dust, photoevaporation staunches accretion at
~50Msun. Our model implies radiative feedback is most significant at
metallicities ~10^{-2}Zsun, since both radiation pressure and photoevaporation
are effective in this regime.
