Molecular line emission from gravitationally unstable protoplanetary disks

In the era of high-resolution submillimeter interferometers, it will soon be possible to observe the neutral circum-stellar medium directly involved in gas giant planet (GGP) formation at physical scales previously unattainable. In order to explore possible signatures of GGP formation via disk instabilities, we have combined a three-dimensional (3D), nonlocal thermodynamic equilibrium (LTE) radiative transfer code with a 3D, finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4M_J self-gravitating proto-GGP. Here we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCO⁺ . Our main results are as follows: (1) Very high lying HCO⁺ transitions (e.g., HCO⁺ J = 7-6) can trace dense clumps around circumstellar disks. Depending on the molecular abundance, the proto-GGP may be directly imageable by the Atacama Large Millimeter Array (ALMA). (2) HCO⁺ emission lines are heavily self-absorbed through the proto-GGP's dense molecular core. This signature is nearly ubiquitous and only weakly dependent on assumed HCO⁺ abundances. The self-absorption features are most pronounced at higher angular resolutions. Dense clumps that are not self-gravitating only show minor self-absorption features. (3) Line temperatures are highest through the proto-GGP at all assumed abundances and inclination angles. Conversely, due to self-absorption in the line, the velocity-integrated intensity may not be. High angular resolution interferometers such as the Submillimeter Array (SMA) and ALMA may be able to differentiate between competing theories of GGP formation.