Molecular Hydrogen in the Ionized Region of Planetary Nebulae

This paper presents an analysis of the concentration of the hydrogen moleculeinside the ionized region of planetary nebulae. The equations corresponding tothe ionization and chemical equilibria of H, H+, H-, H2, H2+, and H3+ arecoupled with the equations of ionization and thermal balance for a photoionizedatomic gas. Forty different reactions related to the formation or thedestruction of these species are included. The presence of dust is taken intoaccount, since grains act as catalysts for the production of H2, as well asshield the molecules against the stellar ionizing radiation. We analyze theeffect of the stellar ionizing continuum, as well as of the gas and grainproperties on the calculated H2 mass. It is shown that a significantconcentration of H2 can survive inside the ionized region of planetary nebulae,mostly in the inner region of the recombination zone. The total H2 to totalhydrogen mass ratio inside the ionized region increases with the central startemperature, and, depending on the PN physical conditions, it can be of theorder of 10^-6 or even higher. The increase of the recombination zone with thestellar temperature can account for such correlation. This can explain why theH2 emission is more frequently observed in bipolar planetary nebulae (Gatley'srule), since this kind of object has typically hotter stars. Applying ourresults for the planetary nebula NGC 6720, we obtain an H2 to hydrogen massratio similar to the value obtained from the observed H2 line emission.Comment: 13 pages, 4 figures. Accepted for publication in Ap