and that previous studies were affected by Malmquist bias.

Another approach has been to use the fact that the cores that form massive stars are not supported by thermal pressure, but by a combination of turbulence and MHD waves (see the book edited by Franco & Carramiñana 1999). Bernasconi & Maeder (1996) used the empirically derived Larson (1981) relations, which show the non-thermal linewidth increasing with increasing size of cloud. This is thought to be a natural consequence of turbulent or magnetic support. In the simple picture in which ${\mbox{$\stackrel{.}{M}$}}_{acc} \propto c^{3}$, this gives a physical basis for the accretion rate increasing with time as the inside-out collapse proceeds. In this way Bernasconi & Maeder (1996) found accretion rates up to $\sim$10$^{-4}$  for the most massive stars.

McKee & Tan (2003) have developed a turbulent core model, which results in accretion rates about an order of magnitude higher than this ($\sim 
10^{-3}$  for the most massive stars). This they justify from the observed high pressures of massive star forming cores that require higher linewidths than given by the usual Larson relations. They still appeal to the form of the Larson relations to get accretion rates increasing with time, although there 