H₂ infrared line emission from the ionized region of planetary nebulae

¹ Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG-USP), Universidade de São Paulo, Cidade Universitária, Rua do Matão 1226, São Paulo, SP 05508-090 Brazil
e-mail isabel@astro.iag.usp.br
² Jodrell Bank Centre for Astrophysics, The Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK

Received: 11 May 2010
Accepted: 20 December 2010

Abstract

Context. The analysis and interpretation of the H₂ line emission from planetary nebulae have been done in the literature by assuming that the molecule survives only in regions where the hydrogen is neutral, as in photodissociation, neutral clumps, or shocked regions. However, there is strong observational and theoretical evidence that at least part of the H₂ emission is produced inside the ionized region of these objects.

Aims. The aim of the present work is to calculate and analyze the infrared line emission of H₂ produced inside the ionized region of planetary nebulae using a one-dimensional photoionization code.

Methods. The photoionization code Aangaba was improved in order to calculate the statistical population of the H₂ energy levels, as well as the intensity of the H₂ infrared emission lines in the physical conditions typical of planetary nebulae. A grid of models was obtained and the results then analyzed and compared with the observational data.

Results. We show that the contribution of the ionized region to the H₂ line emission can be important, particularly in the case of nebulae with high-temperature central stars. This result explains why H₂ emission is more frequently observed in bipolar planetary nebulae (Gatley’s rule), since this kind of object typically has hotter stars. Collisional excitation plays an important role in populating the rovibrational levels of the electronic ground state of H₂ molecules. Radiative mechanisms are also important, particularly for the upper vibrational levels. Formation pumping can have minor effects on the line intensities produced by de-excitation from very high rotational levels, especially in dense and dusty environments. We included the effect of the H₂ molecule on the thermal equilibrium of the gas, concluding that, in the ionized region, H₂ only contributes to the thermal equilibrium in the case of a very high temperature of the central star or a high dust-to-gas ratio, mainly through collisional de-excitation.