Cleeves, LaurenBergin, EdwinOberg, KarinAndrews, SeanWilner, DavidLoomis, Ryan2019-07-312017-06-23Cleeves, L. Ilsedore, Edwin A. Bergin, Karin I. Öberg, Sean Andrews, David Wilner, and Ryan Loomis. 2017. Variable H 13 CO Emission in the IM Lup Disk: X-Ray Driven Time-dependent Chemistry? The Astrophysical Journal Letters 843, no. 1: 7.2041-8213http://nrs.harvard.edu/urn-3:HUL.InstRepos:41024545We report the first detection of a substantial brightening event in an isotopologue of a key molecular ion, HCO+, within a protoplanetary disk of a T Tauri star. The H13CO+ $J=3-2$ rotational transition was observed three times toward IM Lup between 2014 July and 2015 May with the Atacama Large Millimeter/submillimeter Array. The first two observations show similar spectrally integrated line and continuum fluxes, while the third observation shows a doubling in the disk-integrated $J=3-2$ line flux compared to the continuum, which does not change between the three epochs. We explore models of an X-ray active star irradiating the disk via stellar flares, and find that the optically thin H13CO+ emission variation can potentially be explained via X-ray-driven chemistry temporarily enhancing the HCO+ abundance in the upper layers of the disk atmosphere during large or prolonged flaring events. If the HCO+ enhancement is indeed caused by an X-ray flare, future observations should be able to spatially resolve these events and potentially enable us to watch the chemical aftermath of the high-energy stellar radiation propagating across the face of protoplanetary disks, providing a new pathway to explore ionization physics and chemistry, including electron density, in disks.en-USJournal Article2019-07-3110.3847/2041-8213/aa76e2