is little direct evidence that they apply to massive star forming clumps at present. Indeed, the Plume et al. (1997) study upon which McKee & Tan draw, states that they found no correlation between linewidth and size. Most of these cores also already have plenty of star formation activity in them, which will inevitably affect the observed linewidths.

The spherical accretion rates in the turbulent core model are sufficient to overcome radiation pressure. The time variable rates also make the star formation timescale rather independent of the mass which helps to produce apparently coeval clusters containing a range of masses. Their fiducial model also has a radial density distribution on large scales that is close to r$^{-1.5}$, similar to that seen in several studies of massive star forming regions (e.g. Hatchell & van der Tak 2003). This type of gradient has usually been interpreted as being consistent with infall in a rapid star formation scenario in which the whole region is collapsing. In the McKee & Tan model it is seen as a longer lived quasi-equilibrium structure supported by turbulence.

It has been fairly clear ever since the first examples of massive young stars were found that they form from 