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#256 new enhancement

update N_2 photodissociation rate

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: Cc:


arXiv:1305.1781v1 [astro-ph.GA] 8 May 2013

Photodissociation of interstellar N2
Xiaohu Li1, Alan N. Heays1;2;3, Ruud Visser4, Wim Ubachs3, Brenton R. Lewis5, Stephen T. Gibson5, and Ewine F.
van Dishoeck1;6

Context. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation
under interstellar conditions has never been properly studied. The partitioning of nitrogen between N and N2 controls
the formation of more complex prebiotic nitrogen-containing species.
Aims. The aim of this work is to gain a better understanding of the interstellar N2 photodissociation processes based on recent detailed
theoretical and experimental work and to provide accurate rates for use in chemical models.
Methods. We used an approach similar to that adopted for CO in which we simulated the full high-resolution line-by-line absorption
+ dissociation spectrum of N2 over the relevant 912–1000 Å wavelength range, by using a quantum-mechanical model which solves
the coupled-channels Schr¨odinger equation. The simulated N2 spectra were compared with the absorption spectra of H2, H, CO, and
dust to compute photodissociation rates in various radiation fields and shielding functions. The e ects of the new rates in interstellar
cloud models were illustrated for di use and translucent clouds, a dense photon dominated region and a protoplanetary disk.
Results. The unattenuated photodissociation rate in the Draine (1978, ApJS, 36, 595) radiation field assuming an N2 excitation temperature
of 50K is 1:65  10????10 s????1, with an uncertainty of only 10%. Most of the photodissociation occurs through bands in the
957–980 Å range. The N2 rate depends slightly on the temperature through the variation of predissociation probabilities with rotational
quantum number for some bands. Shielding functions are provided for a range of H2 and H column densities, with H2 being
much more e ective than H in reducing the N2 rate inside a cloud. Shielding by CO is not e ective. The new rates are 28% lower than
the previously recommended values. Nevertheless, di use cloud models still fail to reproduce the possible detection of interstellar N2
except for unusually high densities and/or low incident UV radiation fields. The transition of N!N2 occurs at nearly the same depth
into a cloud as that of C+ ! C ! CO. The orders-of-magnitude lower N2 photodissociation rates in clouds exposed to black-body
radiation fields of only 4000 K can qualitatively explain the lack of active nitrogen chemistry observed in the inner disks around cool
Conclusions. Accurate photodissociation rates for N2 as a function of depth into a cloud are now available that can be applied to a
wide variety of astrophysical environments.

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