Astron. Astrophys. 356, 1149-1156 (2000)

5. Metal-free H II regions

An ionization-bounded H II region with primordial abundancies of hydrogen and helium, but without heavier elements, must be much hotter than present day H II regions as shown in the previous section. Even then, not all ionizing photons will be absorbed and converted to soft photons, but reach the actual boundary of the H II region and drive an ionization front into the surrounding neutral gas. Some fraction of the stellar Lyc luminosity is consumed at the front for ionization and heating of the gas. We approximate the ionizing spectrum of the star and the radiation at the front by a Wien-law of temperature . The gas heating at the front is then

which also suffers from losses inside the ionized region by an opacity factor . If we set up the energy balance for the ionizing photons injected by the star, we find a minimal temperature such that is real, but can balance the energy budget for any temperature above that. The number of ionizing photons is reduced in the H II region only by free-bound transitions, which equal the number of recombinations determined by Eq. 34 in ionization equilibrium. The total energy balance is then

with the recombination rate given by Eq. 34 and the cooling from Eqs. 39 and 38. The free-free luminosity

includes the spectral averaged Gaunt factor . The balance Eq. 42 has two solutions: One at high temperatures of several K, where the cooling is dominated by free-free emission, and another one between and a few K, where the cooling is due to free-bound and bound-bound transitions. In both cases the equilibrium temperature is very sensitive to density and radius or the product of the H II region. The high temperature solution faces two severe problems in our context. The expansion time for the ionization front driven by a massive star is of the order of years while the recombination time scale for reaching ionization balance in a region of mean density cm^-3 and temperatures of a few K is itself years. So we cannot expect an ionization equilibrium to hold in this case and the determination of an equilibrium temperature is time dependent. The second conjecture comes from the thermal balance of the electrons alone. The main cooling of free electrons in our situation is provided by free-free emission while the heating takes place at the ionization front (Eq. 41) and by reionization inside the H II region after recombination. The energy gain in the ionized region is proportional to the recombination rate in equilibrium and the mean energy of a new electron created by ionization. We neglect the energy of recombining electrons here. The mean energy of a new electron is determined by the ionizing spectrum, taken to follow a Wien law and the absorption cross section, which declines steeply as close to the ionization edge. From that we get the mean energy as

with

the energy of the ionization edge from Eq. 31 and the exponential integral (e.g., Abramowitz & Stegun 1972). The approximation in Eq. 45 holds only for low effective stellar temperatures . The inside heating of H II regions is

and we get an approximate electron energy balance

which is only satisfied to within a factor of two for present in Eq. 41 in case of the lower temperature solution. The electron energy balance Eq. 48 cannot hold for the high energy solution. In that case the free-free cooling rises with temperature while the recombination rate declines and the gas heating at the ionization front is more or less independent of in the H II region. The opacity due to conversion of photons inside the ionized region becomes less important with increasing temperature.

For illustration we have performed a simplified 1D radiative transfer and ionization balance calculation including hydrogen and helium. The local calculation finds only the lower temperature solution. We have taken a density distribution with a central cusp

with cm^-3 and cm^-3 and a cusp radius of for central stars with effective temperatures of K with , K with , K with and K with . The stellar spectrum is approximated by a black body. The density reaches a constant value beyond pc. The derived temperature profiles are shown in Fig. 4.

Fig. 4. of H II regions of primordial composition. The central ionizing source is a black body of K (dashed), K (solid), (long-short dashed) and K (dash-dotted curve). r is the radial distance from the ionizing star. The density structure of the H II and the parameters of the central star are given in the text.

© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000
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