above. The stellar densities required for it to work are many orders of magnitude below those seen or inferred; and it seems inconceivable that energetic events such as stellar collisions are common without any observational sign of them.

Outflows
========

As mentioned above, bipolar molecular outflows have long indicated that accretion discs are present at the heart of massive star formation. It has been thought that outflows from massive YSOs are not as well collimated as those from their low-mass counterparts. Much of this impression may have arisen from low spatial resolution single-dish molecular line observations of high-mass systems that are on average at least ten times further away (Beuther et al. 2002a). The strong clustering in massive star forming regions can also cause multiple outflows to merge at low resolution. Again, the application of interferometric observations has revealed that some regions are made up of multiple, well-collimated outflows (Beuther et al. 2002b), although these are not particularly high luminosity systems.

However, other systems still appear to show little evidence for high degrees of collimation when observed at high resolution. Fig \[fig:s140co\] shows OVRO observations of the outflow from S140 IRS 1. The blueshifted lobe does display an approximate bow 