The Dynamical Evolution of HII Regions in Non-Uniform Environments

  • G. Tenorio-Tagle
Part of the Astrophysics and Space Science Library book series (ASSL, volume 93)


The perfect balance established between the ionizing photon output from an early type star and the recombinations in a volume of gas surrounding it, was first envisaged by Strömgren (1939). This principle allowed him to deduce, under the assumption of constant density, values for the size of the ionized region — termed the Strömgren sphere — and to infer the existence of a sharp discontinuity between ionized and neutral matter — termed the ionization front (I-front). Strömgren’s idea gave rise to the “classical” theory for the evolution of the HII regions, that which predicts the expansion of the hot ionized gas into the surrounding cold neutral matter, due to the pressure difference generated by ionization,. The “classical” theory was developed by many authors. Kahn (1954), Axford (1961) and Goldsworthy (1961) classified the various possible types of I fronts expected to occur during the formation and evolution of a Strömgren sphere. They also predicted the formation of a shock front, moving into the neutral matter, ahead of D type I fronts. Analytical treatments of the problem (Savedoff and Greene 1955; Newman and Axford 1968) demonstrated that a small separation exists between the two fronts, and that a fair amount (1%) of the stellar radiative energy could be converted, through the shock, into kinetic energy of the compressed neutral gas (Spitzer, 1968).


Star Formation Shock Front Rarefaction Wave Molecular Cloud Ionization Front 
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Copyright information

© D. Reidel Publishing Company 1982

Authors and Affiliations

  • G. Tenorio-Tagle
    • 1
  1. 1.Max-Planck-Institut für AstrophysikGarching bei MünchenGermany

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