Abstract
Radiation pressure on dust grains may be an important mechanism in driving
winds in a wide variety of astrophysical systems. However, the efficiency of
the coupling between the radiation field and the dusty gas is poorly understood
in environments characterized by high optical depths like those in
ultra-luminous infrared galaxies (ULIRGs) and massive dense star clusters. We
present a series of idealized numerical experiments, performed with the
radiation-hydrodynamic code orion, in which we study the dynamics of
such winds and quantify their properties. We find that, after wind acceleration
begins, radiation Rayleigh-Taylor instability forces the gas into a
configuration that reduces the rate of momentum transfer from the radiation
field to the gas by a factor ~ 10 - 100 compared to an estimate based on the
optical depth at the base of the atmosphere; instead, the rate of momentum
transfer from a driving radiation field of luminosity L to the gas is roughly
L/c multiplied by half the optical depth at the dust photosphere, which is far
smaller than the optical depth in the deep interior. When we apply our results
to conditions appropriate to ULIRGs and star clusters, we find that the
asymptotic wind momentum flux from such objects should not significantly exceed
that carried by the direct radiation field, L/c. This result constrains the
expected mass loss rates from systems that exceed the Eddington limit to be of
order the so-called "single-scattering" limit, and not significantly higher. We
present an approximate fitting formula for the rate of momentum transfer from
radiation to dusty gas through which it passes, which is suitable for
implementation in sub-grid models of galaxy formation. Finally, we provide a
first map of the column density distribution of gas in a radiatively-driven
wind as a function of velocity, and velocity dispersion.
Description
[1302.4440] Numerical Simulations of Radiatively-Driven Dusty Winds
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