The Structure and Evolution of Early Cosmological HII Regions

Abstract

We study the formation and evolution of HII regions around the first stars formed at z=10-30. We use a one-dimensional Lagrangian hydrodynamics code which self-consistently incorporates radiative transfer and non-equilibrium primordial gas chemistry. The star-forming region is defined as a spherical gas cloud with a Population III star embedded at the center. We explore a large parameter space of host halo mass, gas density profile, and stellar luminosity. The formation of the HII region is characterized by initial slow expansion of a weak D-type ionization front near the center, followed by rapid propagation of an R-type front throughout the outer gas envelope. We find that the transition between the two front types is indeed a critical condition for the complete ionization of halos of cosmological interest. In small mass (< 10^6 M_sun) halos, the transition takes place within a few 10^5 yr, yielding high escape fractions (>80%) of both ionizing and photodissociating photons. In larger mass (> 10^7 M_sun) halos, the ionization front remains to be of D-type over the lifetime of the massive star, the HII region is confined well inside the virial radius, and the escape fractions are essentially zero. We derive an analytic formula that reproduces well the results of our simulations. We discuss immediate implications of the present results for the star formation history and early reionization of the Universe.