ON TURBULENT PRESSURE CONFINEMENT OF ULTRACOMPACT H-II REGIONS

It has been proposed recently that the small size and long lifetime of ultracompact (UC) H II regions could be due to pressure confinement i f the thermal pressure of the ambient gas is higher than previous esti mates. We point out that confinement by thermal pressure alone implies emission measures in excess of observed values. We show that turbulen t pressure, inferred from observed nonthermal velocities, is sufficien t to confine UC H II regions and explain their longevity. We predict a n anticorrelation between the size of UC H Ir regions and the velocity dispersion of the ambient neutral gas and show that it is consistent with existing observations.