Molecular line emission from a protoplanetary disk irradiated externally
  by a nearby massive star
  Star formation often occurs within or nearby stellar clusters. Irradiation by
nearby massive stars can photoevaporate protoplanetary disks around young stars
(so-called proplyds) which raises questions regarding the ability of planet
formation to take place in these environments. We investigate the
two-dimensional physical and chemical structure of a protoplanetary disk
surrounding a low-mass (T Tauri) star which is irradiated by a nearby massive
O-type star to determine the survivability and observability of molecules in
proplyds. Compared with an isolated star-disk system, the gas temperature
ranges from a factor of a few (in the disk midplane) to around two orders of
magnitude (in the disk surface) higher in the irradiated disk. Although the UV
flux in the outer disk, in particular, is several orders of magnitude higher,
the surface density of the disk is sufficient for effective shielding of the
disk midplane so that the disk remains predominantly molecular in nature. We
also find that non-volatile molecules, such as HCN and H2O, are able to freeze
out onto dust grains in the disk midplane so that the formation of icy
planetesimals, e.g., comets, may also be possible in proplyds. We have
calculated the molecular line emission from the disk assuming LTE and
determined that multiple transitions of atomic carbon, CO (and isotopologues,
13CO and C18O), HCO+, CN, and HCN may be observable with ALMA, allowing
characterization of the gas column density, temperature, and optical depth in
proplyds at the distance of Orion (~400 pc).
