Bondi-Hoyle-Littleton accretion and the upper mass stellar IMF
  We report on a series of numerical simulations of gas clouds with
self-gravity forming sink particles, adopting an isothermal equation of state
to isolate the effects of gravity from thermal physics on the resulting sink
mass distributions. Simulations starting with supersonic velocity fluctuations
develop sink mass functions with a high-mass power-law tail $dN/d\log M \propto
M^{\Gamma}$, $\Gamma = -1 \pm 0.1$, independent of the initial Mach number of
the velocity field. Similar results but with weaker statistical significance
hold for a simulation starting with initial density fluctuations. This mass
function power-law dependence agrees with the asymptotic limit found by
Zinnecker assuming Bondi-Hoyle-Littleton (BHL) accretion, even though the mass
accretion rates of individual sinks show significant departures from the
predicted $\mdot \propto M^2$ behavior. While BHL accretion is not strictly
applicable due to the complexity of the environment, we argue that the final
mass functions are the result of a {\em relative} $M^2$ dependence resulting
from gravitationally-focused accretion. Our simulations may show the power-law
mass function particularly clearly compared with others because our adoption of
an isothermal equation of state limits the effects of thermal physics in
producing a broad initial fragmentation spectrum; $\Gamma \rightarrow -1$ is an
asymptotic limit found only when sink masses grow well beyond their initial
values. While we have purposely eliminated many additional physical processes
(radiative transfer, feedback) which can affect the stellar mass function, our
results emphasize the importance of gravitational focusing for massive star
formation.
