Ionization fronts in interstellar gas and the expansion of HII regions

Abstract

The gas dynamical effects of an expanding nearly fully ionized hydrogen region (H II region), which is associated with the formation of O and B stars, are investigated. The radiation from the hot star is absorbed by the surrounding interstellar gas (mainly neutral hydrogen) and leads to its ionization. Previous analyses have disregarded the internal motions set up in expanding H II regions. Similarity solutions of the equations of motion are presented for spherical and cylindrical problems, thus enabling the effects of groups of stars as well as individual stars to be discussed. For similarity to be applicable the initial density variations of the undisturbed neutral gas have to be like 1/ r ® in the spherical case and like 1/ r in the cylindrical case. This does not, however, limit their use in describing the general picture of events for any other given density distribution. Recombination of the ions and electrons and subsequent re-ionization by radiation within the H II region is allowed for; cooling processes such as that due to the excitation of O<super>+</super> ions are also taken into account. It is shown that the temperature of the ionized gas in the H II region is approximately uniform even though the region as a whole is expanding. Rates of expansion are calculated and it is also determined whether a shock propagates ahead of the ionized gas. In particular for rates of expansion less than about 20 km/s a shock wave occurs ahead, but for speeds greater than about 20 km/s, which would occur in the initial motion, the rate of expansion of the ionized gas is too great and an ‘isothermal’ shock occurs within the H II region. The boundary between the ionized and neutral gases can be regarded as a discontinuity and is termed an ionization front. The present paper is concerned with the propagation of such fronts and accompanying shocks; a companion paper by W. I. Axford investigates the structures of ‘isothermal shock’ and ionization fronts. The lack of uniqueness, which occurs in the present paper, is removed when the results are combined with Axford’s work.