On the Formation and Expansion of H II Regions
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1990
- DOI:
- 10.1086/168300
- Bibcode:
- 1990ApJ...349..126F
- Keywords:
-
- Computational Astrophysics;
- H Ii Regions;
- Stellar Evolution;
- Density Distribution;
- Gas Ionization;
- Gravitational Effects;
- Radiation Distribution;
- Shock Fronts;
- Astrophysics;
- HYDRODYNAMICS;
- NEBULAE: H II REGIONS;
- NEBULAE: INTERNAL MOTIONS;
- SHOCK WAVES