Photoevaporated flows from H II regions

Abstract

We model the dynamics of a fast, isothermal ionized stellar wind loaded with mass injected from photoevaporated globules surrounding the massive star. The effect of the mass injection is to produce a density profile such that the ionization front can be trapped for 10(5) yr, depending on the physical characteristics of the neutral globules inside the H II region. We find that for neutral globules with sizes R(g) similar to 0.01 pc, masses of M(g) similar to 1 M., and number densities N-g similar to 2 x 10(4) pc(-3), thought to be representative of globules in regions of massive star formation, the implied mean density and size of the mass-loaded regions of ionized gas are about 10(3)-10(4) cm(-3) and about 0.1 pc, respectively, similar to those of compact H II regions. Dust absorption of ionizing photons is important and decreases the densities of the mass-loaded winds with respect to their dust-free counterparts. Also, mass-loaded winds with dust evolve more slowly, since the dusty globules survive for longer times than the dust-free ones. Our models predict ionized hows with mass flow rates of M similar to 10(-5) to 10(-4) M. yr(-1). These ionized flows could be studied in radio recombination lines. Assuming N-g does not decline sharply with distance to the central star, the ionized flow will recombine after the characteristic ''Stromgren'' radius r(S) at which the ionizing photon rate goes to zero. Therefore, after this radius a neutral flow will accelerate adiabatically to a terminal velocity of v(HI) similar to 40 km s(-1). Neutral flows of this type could be searched for in the neutral hydrogen line at 21 cm in absorption against the continuum of the compact H II regions.