Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star

TL;DR

This study models the physical and chemical structure of protoplanetary disks irradiated by nearby massive stars, showing that molecules can survive and be observed, informing planet formation prospects in such environments.

Contribution

It provides the first detailed two-dimensional model of irradiated protoplanetary disks, analyzing molecular survivability and observability with ALMA.

Findings

Disk temperature increases significantly due to irradiation.

Molecular gas remains predominantly intact in the disk midplane.

Multiple molecular transitions are detectable with ALMA.

Abstract

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).