Evidence for a Cosmic Ray Gradient in the IM Lup Protoplanetary Disk

TL;DR

This study provides the first observational evidence of a cosmic ray ionization gradient in the IM Lup protoplanetary disk, revealing a radially increasing cosmic ray influence that impacts disk chemistry and evolution.

Contribution

It introduces a new observationally-motivated ionization model for IM Lup, incorporating multiple ionization sources and revealing a cosmic ray gradient in the disk.

Findings

Cosmic ray ionization increases with radius, with a transition at 80-100 au.

Enhanced UV-driven HCO+ formation linked to disk's high flaring angle.

IM Lup is the first disk with observational evidence for a cosmic ray gradient.

Abstract

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 N$_2$H$^+$ 3--2 and H$^{13}$CO$^+$ 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 N$_2$H$^+$ and H$^{13}$CO$^+$ 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 $\sim 80 - 100$ 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 on-going evolution, especially given the disk's young age and large size.