Chemistry in Disks. V: CN and HCN in proto-planetary disks
  The chemistry of proto-planetary disks is thought to be dominated by two
major processes: photodissociation near the disk surface, and depletion on dust
grains in the disk mid-plane, resulting in a layered structure with molecules
located in a warm layer above the disk mid-plane. We attempt here to confront
this warm molecular layer model prediction with the distribution of two key
molecules for dissociation processes: CN and HCN. Using the IRAM Plateau de
Bure interferometer, we obtained high spatial and spectral resolution images of
the CN J=2-1 and HCN J=1-0 lines in the disks surrounding the two T-Tauri DM
Tau and LkCa 15 and the Herbig Ae MWC 480. Disk properties are derived assuming
power law distributions. The hyperfine structure of the observed transitions
allows us to constrain the line opacities and excitation temperatures. We
compare the observational results with predictions from existing chemical
models, and use a simple PDR model (without freeze-out of molecules on grains
and surface chemistry) to illustrate dependencies on UV field strength, grain
size and gas-to-dust ratio. We also evaluate the impact of Lyman alpha
radiation. The temperature ordering follows the trend found from CO lines, with
DM Tau being the coldest object and MWC 480 the warmest. Although CN indicates
somewhat higher excitation temperatures than HCN, the derived values in the
T-Tauri disks are very low (8-10 K). They agree with results obtained from CCH,
and are in contradiction with thermal and chemical model predictions. These
very low temperatures, as well as geometrical constraints, suggest that
substantial amounts of CN and HCN remain in the gas phase close to the disk
mid-plane, and that this mid-plane is quite cold. The observed CN/HCN ratio
(5-10) is in better agreement with the existence of large grains, and possibly
also a substantial contribution of Lyman alpha radiation.
