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On the formation and expansion of H II regions
Franco, Jose; Tenorio-Tagle, Guillermo; Bodenheimer, Peter
AA(Universidad Nacional Autonoma de Mexico, Mexico City, Mexico; Max-Planck-Institut für Physik und Astrophysik, Garching bei München, Germany), AB(Max-Planck-Institut für Physik und Astrophysik, Garching bei München, Germany), AC(Lick Observatory, Santa Cruz, CA)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 349, Jan. 20, 1990, p. 126-140. Research supported by NASA and CONACYT. (ApJ Homepage)
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NASA/STI Keywords:
Computational Astrophysics, H Ii Regions, Stellar Evolution, Density Distribution, Gas Ionization, Gravitational Effects, Radiation Distribution, Shock Fronts
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The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

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