partially ionized, and is difficult to predict at this moment how the ionization fraction of MHD driven jets would change for higher mass stars. If they are significantly ionized then their ability to confine the H II regions would be dramatically reduced.

Again a difficulty is that as soon as the central star becomes at all luminous it will start to drive strong stellar winds due to radiation pressure. These will open up cavities further exposing more material to the ionizing radiation. The equatorial winds discussed above would also not be able to confine the H II region since they do not cover the polar regions. Indeed, in the models by Drew et al. (1998) the polar regions are occupied by a normal O star stellar wind flowing at a few thousand .

One possible solution is to tackle the problem at source by examining whether the star really is emitting large amounts of Lyman continuum radiation during the massive YSO phase. Although the core is very likely to be already on the main sequence it is unclear that the outer layers of the star have contracted fully into a main sequence configuration. It is well known that accreting stars 