pushed sideways, across the surface of the disc forming an equatorial wind. This is the same mechanism that drives the stellar winds in field main sequence stars where the terminal velocity is a few thousand , i.e. of order the escape speed at the stellar surface (Prinja et al. 1990). In the massive YSO case, the surface gravity in the disc atmosphere a few stellar radii out from the star is lower and so the typical speed of the equatorial wind is a few hundred . Hence, this mechanism has the potential to explain both the morphology, speed and mass-loss rate of the observed equatorial wind systems. Initial attempts to simulate the line profiles arising from such winds have too high a rotational component and result in double-peaked line profiles whereas only single-peaks are usually seen (Sim et al. 2005).

Note that this radiatively pressure driven flow is very different from the photo-evaporation mechanism discussed by Franco et al. (1989) and Hollenbach et al. (1994). That drives a much slower, thermal flow from the outer regions of the disc and they did not consider the effect of radiation pressure on the inner disc. Both mechanisms could operate simultaneously, but if 