Molecular Hydrogen in Planetary Nebulae

Author(s)

Isabel Regina Guerra Aleman

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD)
• Defense date: 2002-06-21

Examining board members

Ruth Bomfim Gruenwald; Francois Christophe Cuisinier; Walter Junqueira Maciel

• Advisor: Ruth Bomfim Gruenwald

Abstract

The goal of this work is the study of the H2 molecule survival and the determination of its abundance in different typical planetary nebulae conditions inside the ionized region. In order to do these calculations, we developed Fortran subroutines for the Aangaba one-dimensional photoionization code that, until this work, only took into account the atomic species (H, He, C, N, O, Mg, Ne, Si, S, Ar, Cl, and Fe) and their ions. Ionization and chemical equilibria of H, H+, H-, H2, H2+, and H3+ are assumed. The H3 molecule is not included because it is unstable. Fortyone different reactions that could form and destroy these species are taken into account. Reaction on grain surfaces, the most important mechanism for the production of H2 molecules in the interstellar medium, is analyzed in detail in the conditions of planetary nebulae ionized regions. We make a careful analysis of the grain survival in these regions. We also study the influence of the central star properties and gas density, as well as the astrophysical grain properties in the obtained H2 concentration. It is shown that a significant concentration of H2 can exist inside the ionized region of planetary nebulae, mostly in the recombination zone. The H2 concentration relative to the total hydrogen concentration reaches values as high as 1E-4 and the H2 mass to total hydrogen mass ratio inside the ionized region reaches values as high as 4E-4. The ratio increases with increasing temperature. This fact can explain why the H2 emission is more often observed in bipolar planetary nebulae (Gatley?s rule), since this kind of object has typically hotter stars. In the literature a H2 mass to total hydrogen mass ratio equal to 6.9E-5 is estimated from observations for the planetary nebula NGC6720. With the same input parameters for the gas density and the stellar spectrum, we calculated a ratio equal to 3.3E-5, close to the observed value. (AU)