Institute for Advanced Study Informal Astrophysics Seminar

Planets form from the coldest and densest parts of circumstellar disks around young stars. During this phase, the active nature of the star subjects the disk to relatively high fluxes of UV and X-ray photons and energetic particles. Simultaneously, the local star-forming environment may provide additional external UV and/or radioactive pollutants from recent massive stellar populations. These energetic agents play a vital role in 1) setting protoplanetary disks' turbulent and thermal physics, and 2) regulating the important gas and grain surface chemical reactions, impacting the overall disk molecular composition. Consequently, chemistry can used to "map out" the important ionizing processes in disks using molecular emission. Using data from the Submillimeter Array and Atacama Large Millimeter/Submillimeter Array, we have put strong constraints on the ionization levels in the deep disk of TW Hya, constraining the cosmic ray rate to a value two orders of magnitude below the dense interstellar value. This result has major consequences for the active disk chemistry and, in particular, the chemistry of water. Finally, I will discuss future directions aiming to resolve ionization structure and time variability.