Initial Ionization of Compressible Turbulence

We study the effects of the initial conditions of turbulent molecular clouds on the ionization structure in newly formed H ii regions, using three-dimensional, photon-conserving radiative transfer in a precomputed density field from three-dimensional compressible turbulence. Our results show that the initial density structure of the gas cloud can play an important role in the resulting structure of the H ii region. The propagation of the ionization fronts, the shape of the resulting H ii region, and the total mass ionized depend on the properties of the turbulent density field. Cuts through the ionized regions generally show “butterfly” shapes rather than spherical ones, while emission measure maps are more spherical if the turbulence is driven on scales small compared to the size of the H ii region. The ionization structure can be described by an effective clumping factor ζ = 〈n〉 · 〈n〉/〈n〉, where n is number density of the gas. The larger the value of ζ, the less mass is ionized, and the more irregular the H ii region shapes. Because we do not follow dynamics, our results apply only to the early stage of ionization when the speed of the ionization fronts remains much larger than the sound speed of the ionized gas, or Alfvén speed in magnetized clouds if it is larger, so that the dynamical effects can be negligible. Subject headings: H ii regions: ionization structure – H ii regions: morphology – radiative transfer – ISM: clouds – ISM: hydrodynamics – magneto-hydrodynamics (MHD) – turbulence