2004/03/08 by Isabel Aleman, I. Aleman, R. Gruenwald +1 · 3 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #H II region #Hydrogen #Ionization #Line (geometry) #Molecule #Nebula #Planetary nebula #Plasma #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/383562
published as Astrophys.J. 607 (2004) 865-872 · 13 pages, 4 figures. Accepted for publication in ApJ
arxiv created 2004/03/08 · openalex publication_date 2004/05/19 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
This paper presents an analysis of the concentration of hydrogen molecules inside the ionized region of planetary nebulae (PNs). The equations corresponding to the ionization and chemical equilibria of H, H + , H - , H 2 , H , and H are coupled with the equations of ionization and thermal balance for a photoionized atomic gas. A total of 40 different reactions related to the formation or destruction of these species are included. The presence of dust is taken into account, since grains act as catalysts for the production of H 2 as well as shield the molecules against the stellar ionizing radiation. We analyze the effect of the stellar ionizing continuum as well as of the gas and grain properties on the calculated H 2 mass. It is shown that a significant concentration of H 2 can survive inside the ionized region of planetary nebulae, mostly in the inner region of the recombination zone. The total H 2 to total hydrogen mass ratio inside the ionized region increases with the central star temperature, and, depending on the PN physical conditions, it may be of the order of ~10 -6 or even higher. The increase of the recombination zone with stellar temperature can account for such correlation. This may explain why H 2 emission is more frequently observed in bipolar PNs (Gatley's rule), since this kind of object typically has hotter stars. Applying our results to the planetary nebula NGC 6720, we obtain an H 2 to hydrogen mass ratio similar to the value obtained from the observed H 2 line emission.