The classical Dtype expansion of spherical H II regions
Abstract
Recent numerical and analytic work has highlighted some shortcomings in our understanding of the dynamics of H II region expansion, especially at late times, when the H II region approaches pressure equilibrium with the ambient medium. Here we reconsider the idealized case of a constant radiation source in a uniform and spherically symmetric ambient medium, with an isothermal equation of state. A thickshell solution is developed that captures the stalling of the ionization front and the decay of the leading shock to a weak compression wave as it escapes to large radii. An acoustic approximation is introduced to capture the latetime damped oscillations of the H II region about the stagnation radius. Putting these together, a matched asymptotic equation is derived for the radius of the ionization front which accounts for both the inertia of the expanding shell and the finite temperature of the ambient medium. The solution to this equation is shown to agree very well with the numerical solution at all times, and is superior to all previously published solutions. The matched asymptotic solution can also accurately model the variation of H II region radius for a timevarying radiation source.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 September 2018
 DOI:
 10.1093/mnras/sty1484
 arXiv:
 arXiv:1805.09273
 Bibcode:
 2018MNRAS.479.2016W
 Keywords:

 ISM: bubbles;
 HII regions;
 ISM: kinematics and dynamics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 8 pages, 5 figures. Authors' original version, revised version with minor changes accepted for publication in MNRAS