The photodissociation and chemistry of CO isotopologues: applications to
  interstellar clouds and circumstellar disks
  Aims. Photodissociation by UV light is an important destruction mechanism for
CO in many astrophysical environments, ranging from interstellar clouds to
protoplanetary disks. The aim of this work is to gain a better understanding of
the depth dependence and isotope-selective nature of this process.
  Methods. We present a photodissociation model based on recent spectroscopic
data from the literature, which allows us to compute depth-dependent and
isotope-selective photodissociation rates at higher accuracy than in previous
work. The model includes self-shielding, mutual shielding and shielding by
atomic and molecular hydrogen, and it is the first such model to include the
rare isotopologues C17O and 13C17O. We couple it to a simple chemical network
to analyse CO abundances in diffuse and translucent clouds, photon-dominated
regions, and circumstellar disks.
  Results. The photodissociation rate in the unattenuated interstellar
radiation field is 2.6e-10 s^-1, 30% higher than currently adopted values.
Increasing the excitation temperature or the Doppler width can reduce the
photodissociation rates and the isotopic selectivity by as much as a factor of
three for temperatures above 100 K. The model reproduces column densities
observed towards diffuse clouds and PDRs, and it offers an explanation for both
the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The
photodissociation of C17O and 13C17O shows almost exactly the same depth
dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally
fractionated with respect to 16O. This supports the recent hypothesis that CO
photodissociation in the solar nebula is responsible for the anomalous 17O and
18O abundances in meteorites.
