The Ionization Structure of Planetary Nebulae: II. Collisional Cooling of Pure Hydrogen Nebulae

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Abstract
Collisional excitation and ionization of hydrogen are mechanisms which lead to the removal of energy from planetary nebulae. The ionization and thermal balance equations including these effects are solved for a pure hydrogen nebula to determine the electron temperature in the nebula for a wide range of star temperatures. For reasonable stellar fluxes the electron temperature considered as a function of star temperature assumes a maximum value of 2 × 10 4 °K. Collisional excitation greatly enhances the line emission from low-lying states of hydrogen; in particular, it may be important to allow for collisional excitation of Ly α when one obtains star temperatures from estimates of the total energy radiated by the nebula. The role of hydrogen collisional cooling in real nebulae is discussed briefly. Accurate and convenient expressions are obtained for the collisional excitation and ionization rates for hydrogen.