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 circumstellar 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 1.4MJ self-gravitating proto-GGP. Here we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCOþ. Our main results are as follows: (1) Very high lying HCOþ transitions (e.g., HCOþ J 1⁄4 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) HCOþ 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 HCOþ 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 ALMAmay be able to differentiate between competing theories of GGP formation. Subject headinggs: circumstellar matter — line: formation — line: profiles — planetary systems: formation — planetary systems: protoplanetary disks — radiative transfer