The flow of ionized gas from a globule in interstellar space

Abstract

The flow of ionized gas from a globule in interstellar space illustrates rather well the various physical processes that often occur in or near ionization fronts. Thus the non-ionized gas is largely shielded from the incident radiation by the HII region that forms around it. The low frequency photons are absorbed more easily in this region so that the typical photon reaching the front has more than average energy, and the usual processes govern the thermal balance of the gas. In the case of a globule the front is strong D type, so that it advances subsonically into the gas ahead of it, but the ionized gas streams away supersonically in the rear. The strong D condition provides one of the boundary conditions needed to define the flow; this conclusion is similar to one reached by Axford 6) when he was studying the expansion of a spherically symmetrical HII region around a star. Finally some estimates are made for the mass of gas likely to be contained in the globule, and of the condition for it to be stable against gravitational collapse.