Chemical Processes in Protoplanetary Disks
  We have developed a high resolution combined physical and chemical model of a
protoplanetary disk surrounding a typical T Tauri star. Our aims were to use
our model to calculate the chemical structure of disks on small scales
(sub-milli-arcsecond in the inner disk for objects at the distance of Taurus, ~
140 pc) to investigate the various chemical processes thought to be important
in disks and to determine potential molecular tracers of each process. Our
gas-phase network was extracted from the UMIST Database for Astrochemistry to
which we added gas-grain interactions including freeze out and thermal and
non-thermal desorption (cosmic-ray induced desorption, photodesorption and
X-ray desorption) and a grain-surface network. We find that cosmic-ray induced
desorption has the least effect on our disk chemical structure while
photodesorption has a significant effect, enhancing the abundances of most
gas-phase molecules throughout the disk and affecting the abundances and
distribution of HCN, CN and CS, in particular. In the outer disk, we also see
enhancements in the abundances of H2O and CO2. X-ray desorption is a
potentially powerful mechanism in disks, acting to homogenise the fractional
abundances of gas-phase species across the depth and increasing the column
densities of most molecules although there remain significant uncertainties in
the rates adopted for this process. The addition of grain-surface chemistry
enhances the fractional abundances of several small complex organic molecules
including CH3OH, HCOOCH3 and CH3OCH3 to potentially observable values (i.e. a
fractional abundance of >~ 1.0E-11).
