The Chemical Effects of Mutual Shielding in Photon Dominated Regions

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 1102Å. 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.

Accepted to MNRAS, 9 Pages, 2 Figures

References in corpus (3)

• Nitrogen hydrides in interstellar gas: Herschel/HIFI observations towards G10.6-0.4 (W31C)
• The Dense Molecular Gas in the Circumnuclear Disk of NGC 1068
• Dense molecular gas toward W49A: A template for extragalactic starbursts?

Cited by in corpus (14)

• Photodissociation and photoionisation of atoms and molecules of astrophysical interest
• Photodissociation and X-Ray Dominated Regions
• Exocometary gas structure, origin and physical properties around $\beta$ Pictoris through ALMA CO multi-transition observations
• Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel
• ALMA observations of the $\eta$ Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3-30 M${}_{\oplus }$ planets?
• Photodissociation of interstellar N2
• Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs
• Population synthesis of exocometary gas around A stars
• ALMA resolves CI emission from the $\beta$ Pictoris debris disk
• Photodissociation of interstellar ArH+
• The surprisingly low carbon mass in the debris disk around HD 32297
• Lack of other molecules in CO-rich debris discs: is it primordial or secondary gas?
• Vertical evolution of exocometary gas: I. How vertical diffusion shortens the CO lifetime
• Is acetylene essential for carbon dust formation?