Misc,
The Necessity of Feedback Physics in Setting the Peak of the Initial Mass Function
(Oct 16, 2015)
Abstract
A popular theory of star formation is gravito-turbulent fragmentation, in
which self-gravitating structures are created by turbulence-driven density
fluctuations. Simple theories of isothermal fragmentation successfully
reproduce the core mass function (CMF) which has a very similar shape to the
initial mass function (IMF) of stars. However, numerical simulations of
isothermal turbulent fragmentation thus far have not succeeded in identifying a
fragment mass scale that is independent of the simulation resolution. Moreover,
the fluid equations for magnetized, self-gravitating, isothermal turbulence are
scale-free, and do not predict any characteristic mass. In this paper we show
that, although an isothermal self-gravitating flow does produce a CMF with a
mass scale imposed by the initial conditions, this scale changes as the parent
cloud evolves. In addition, the cores that form undergo further fragmentation
and after sufficient time forget about their initial conditions, yielding a
scale-free pure power-law distribution \$d N/d M\propto
M^-2\$ for the stellar IMF. We show that this problem can be alleviated by
introducing a simple model for stellar radiation feedback. Radiative heating,
powered by accretion onto forming stars, arrests the fragmentation cascade and
imposes a characteristic mass scale that is nearly independent of the
time-evolution or initial conditions in the star-forming cloud, and that agrees
well with the peak of the observed IMF. In contrast, models that introduce a
stiff equation of state for denser clouds but that do not explicitly include
the effects of feedback do not yield an invariant IMF.
