# Variable H$^{13}$CO$^+$ Emission in the IM Lup Disk: X-ray Driven Time-Dependent Chemistry?

## TL;DR

This study reports a significant brightening event in H$^{13}$CO$^+$ emission in the IM Lup protoplanetary disk, likely driven by X-ray flares from the host star, revealing dynamic chemistry influenced by stellar activity.

## Contribution

First detection of X-ray driven, time-dependent chemistry effects in a protoplanetary disk through observed H$^{13}$CO$^+$ emission variability.

## Key findings

- H$^{13}$CO$^+$ emission doubled during the third observation.
- X-ray flares can temporarily enhance HCO$^+$ abundance in disk atmospheres.
- Potential to spatially resolve and study chemical effects of stellar flares in disks.

## Abstract

We 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 H$^{13}$CO$^+$ $J=3-2$ rotational transition was observed three times toward IM Lup between July 2014 and May 2015 with the Atacama Large Millimeter 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 H$^{13}$CO$^+$ 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 a 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.