accretion exerts a high radiation pressure on the dust grains in the infalling cloud. In a spherically symmetric treatment Kahn (1974) deduced that this effect would limit the mass of a star that could be formed by accretion to about 40 M$_{{\mbox{$_{\odot}$}}}$. The adoption of more appropriate dust parameters removes this strict limitation, but does require very high infall rates for the ram pressure to overcome the radiation pressure. Wolfire & Cassinelli (1987) updated this with a full radiative transfer solution and concluded that normal interstellar dust opacities would not allow inflow to occur. Even with depleted dust models they found that high accretion rates ($\sim$10$^{-3}$) would be necessary to form the most massive stars.

Such accretion rates were thought to be unreasonably high compared to the $\sim$10$^{-5}$ expected from the collapse of a cloud initially close to equilibrium and held up by thermal pressure alone (${\mbox{$\stackrel{.}{M}$}}_{acc} \sim 
c^{3}/G$); as is thought to be the case for low-mass star formation. However, several reasons have been put forward to argue for higher and time variable accretion rates, which can also overcome the radiation pressure problem. In any case, one might intuitively expect more massive objects to accrete faster.

One approach taken 