The Discovery of a Photoevaporation-Driven Molecular Outflow from the T Tauri Transitional Disk GM Aur

Circumstellar disks are not only a byproduct of star formation, but are also the place where planets form and migrate. The dominant gas-phase constituent of disks early in their evolution is H2, and its lifetime in the disk limits the time available for gas giant planet formation and migration. A number of mechanisms have been proposed to remove gas, including photoevaporation in the presence of the stellar X-ray, EUV, and FUV radiation field, but the relative importance of these different components and the point in disk evolution where they become significant remain uncertain. Some models predict enhanced evaporation of gas in the outer disk once the inner portions of the disk have begun to clear. One such system is the T Tauri star GM Aur which hosts a large disk with an r=20 AU central cavity. We have carried out the first high-contrast FUV imaging of this star+disk using HST ACS/SBC and report the detection of the inner 1" (140 AU) of the disk in the FUV and the discovery of a roughly cylindrical structure 90 AU in radius and extending 200 AU orthogonal to the disk, aligned with the previously reported red, polar lobes. The structure is brightest at wavelengths where there are numerous fluorescent molecular hydrogen transitions, both in our imagery and in an archival HST/STIS long-slit spectrum. The cylinder is marginally detected in the ACS/SBC F165LP band indicating that there is some sub-0.2 micron-sized dust entrained in it, but is not detected in ACS/SBC F122M imagery. The radial scale of the footprint of the cylinder on the disk and the absence of atomic emission lines associated with the structure exclude a conventional jet, but are consistent with a photoevaporation-driven outflow. We compare the properties of this outflow with predictions of X-ray, EUV, and FUV-driven disk winds.