The Chemical Effects of Mutual Shielding in Photon Dominated Regions

TL;DR

This paper explores how mutual shielding effects, especially by molecular hydrogen and carbon continuum, significantly influence chemical abundances in photon dominated regions, revealing their importance in astrochemical modeling.

Contribution

It introduces the first detailed modeling of mutual shielding effects by H2 and carbon continuum in PDRs, highlighting their impact on chemical abundances.

Findings

N2, N2H+, NH3, and CN abundances increase by factors of 3-100 due to shielding effects.

Shielding effects significantly alter photorates and chemical compositions in PDRs.

Mutual shielding effects have been previously overlooked in recent models.

Abstract

We investigate the importance of the shielding of chemical photorates by molecular hydrogen photodissociation lines and the carbon photoionization continuum deep within models of photon dominated regions. In particular, the photodissociation of N2 and CN are significantly shielded by the H2 photodissociation line spectrum. We model this by switching off the photodissociation channels for these species behind the HI to H2 transition. We also model the shielding effect of the carbon photoionization continuum as an attenuation of the incident radiation field shortwards of 1102\AA. Using recent line and continuum cross section data, we present calculations of the direct and cosmic ray induced photorates for a range of species, as well as optically thick shielding factors for the carbon continuum. Applying these to a time dependent PDR model we see enrichments in the abundances of N2, N2H+, NH3 and CN by factors of roughly 3-100 in the extinction band Av=2.0 to Av=4.0 for a range of environments. While the precise quantitative results of this study are limited by the simplicity of our model, they highlight the importance of these mutual shielding effects, neither of which has been discussed in recent models.