The Chemical Effects of Mutual Shielding in Photon Dominated Regions
  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.
