region phase is not to do with the star evolving to a stage where it is hot enough to ionize hydrogen, but more to do with the cessation of accretion. If accretion swelling the star is the cause, then the end of the high accretion rate phase will then allow the star to contract down onto the ZAMS. In so doing, the Lyman continuum output will increase and the H II region phase begin.

When ionization of the surrounding material does begin the ionization front will move faster in the low density directions. These are most likely to be the polar lobes due to the flattening effect of the centrifugal barrier during the infall phase and bipolar outflows during the YSO phase. In this scenario, one would expect the youngest, most compact H II regions to be bipolar. Within the class of so-called hyper-compact H II regions (HCHII, see Kurtz 2005) there are examples of bipolar objects. These also tend to display broad recombination lines (40 ), which has been used as another criterion to set some of the hyper-compact objects apart as a new class (Jaffe & Martin-Pintado 1999; Sewilo et al 2004).

Figure \[fig:ratio\] shows a plot of 