Abstract

RAGA, A. G. et al. HII Regions in hydrostatic balance between gas pressure, radiation pressure and gravity. Rev. mex. astron. astrofis [online]. 2015, vol.51, n.1, pp.25-32. ISSN 0185-1101.

We study the solutions of a modified version of the isothermal Lane-Emden equation, which incorporates the effect of the (owtwards directed) radiation pressure resulting from photoionizations. These solutions are relevant for HII regions around a cluster with over ≈500 O stars, which can photoionize gas out to ≈ 200 √10 cm^-3n₀ pc (where no is the central gas density), where the effects of the self-gravity and the radiation pressure become important. We find that the solutions have a transition from a "gravity dominated" regime (in which the solutions converge at large radii to the non-singular, isothermal sphere solution) to a "radiation pressure dominated" regime (in which the density diverges at a finite radius) for central HII region densities above n_crit = 100 cm^-3. We argue that the high central density, radiation pressure dominated solutions will not occur in most astrophysically relevant situations, because of the absence of a possible confining environment with a high enough pressure.

Keywords : hydrodynamics; ISM [evolution]; ISM [kinematics and dynamics]; ISM [HII regions]; stars [formation].

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