2021ApJ...912..136S

Protoplanetary disk evolution is strongly impacted by ionization from the central star and local environment, which collectively have been shown to drive chemical complexity and are expected to impact the transport of disk material. Nonetheless, ionization remains a poorly constrained input to many detailed modeling efforts. We use new and archival ALMA observations of N₂H⁺ 3-2 and H¹³CO⁺ 3-2 to derive the first observationally motivated ionization model for the IM Lup protoplanetary disk. Incorporating ionization from multiple internal and external sources, we model N₂H⁺ and H¹³CO⁺ abundances under varying ionization environments and compare these directly to the imaged ALMA observations by performing non-LTE radiative transfer, visibility sampling, and imaging. We find that the observations are best reproduced using a radially increasing cosmic-ray (CR) gradient, with low CR ionization in the inner disk, high CR ionization in the outer disk, and a transition at ∼80-100 au. This location is approximately coincident with the edge of spiral structure identified in millimeter emission. We also find that IM Lup shows evidence for enhanced UV-driven formation of HCO⁺, which we attribute to the disk's high flaring angle. In summary, IM Lup represents the first protoplanetary disk with observational evidence for a CR gradient, which may have important implications for IM Lup's ongoing evolution, especially given the disk's young age and large size.