Nonequilibrium photodissociation regions: lonization-dissociation fronts

Frank Bertoldi, B. T. Draine

Research output: Contribution to journalArticlepeer-review

118 Scopus citations


We discuss the theory of coupled ionization-dissociation fronts produced when molecular clouds are exposed to λ < 1110 Å radiation from hot stars. A steady, composite structure is developed, which generally includes an ionized outflow away from the cloud, an ionization front, a layer of photodissociated gas, a photodissociation front, and a shock wave preceding the photodissociation front. We show that the properties of the structure are determined by two dimensionless parameters, Ψ and δ, and by the Alfvén speed in the preshock gas. For a broad range of parameters of interest, the ionization front and the hydrogen photodissociation front do not separate, the H2 photodissociation and photoionization take place together, and a classical hydrogen "photodissociation region" (PDR) does not exist. We also show that even when a distinct PDR exists, in many cases the dissociation front propagates too rapidly for the usual stationary models of PDRs to be applicable. We discuss several famous PDRs, e.g., in M17 and Orion, and conclude that they cannot be described by equilibrium PDR models.

Original languageEnglish (US)
Pages (from-to)222-232
Number of pages11
JournalAstrophysical Journal
Issue number1 PART I
StatePublished - 1996
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • H ii regions
  • Ism : Kinematics and dynamics
  • Ism: Clouds
  • Ism: Individual (orion nebula)
  • Molecular processes
  • Shock waves


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