On radiation pressure in static, dusty H II regions

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Radiation pressure acting on gas and dust causes HII regions to have central densities that are lower than the density near the ionized boundary. HII regions in static equilibrium comprise a family of similarity solutions with three parameters: β, γ, and the product Q 0 n rms; β characterizes the stellar spectrum, γ characterizes the dust/gas ratio, Q 0 is the stellar ionizing output (photons/s), and n rms is the rms density within the ionized region. Adopting standard values for β and γ, varying Q 0 n rms generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q 0 n rms) to shell-like (large Q 0 n rms). When Q 0 n rms ≳ 1052 cm-3 s-1, dusty HII regions have conspicuous central cavities, even if no stellar wind is present. For given β, γ, and Q 0 n rms, a fourth quantity, which can be Q 0, determines the overall size and density of the HII region. Examples of density and emissivity profiles are given. We show how quantities of interest - such as the peak-to-central emission measure ratio, the rms-to-mean density ratio, the edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by the gas - depend on β, γ, and Q 0 n rms. For dusty HII regions, compression of the gas and dust into an ionized shell results in a substantial increase in the fraction of the stellar photons that actually ionize H (relative to a uniform-density HII region with the same dust/gas ratio and density n = n rms). We discuss the extent to which radial drift of dust grains in HII regions can alter the dust-to-gas ratio. The applicability of these solutions to real HII regions is discussed.

Original languageEnglish (US)
Article number100
JournalAstrophysical Journal
Issue number2
StatePublished - May 10 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • H ii regions
  • ISM: bubbles
  • ISM: structure
  • dust, extinction
  • infrared: ISM
  • radio continuum: ISM


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