Modeling Time Dependent Water Chemistry Due to Powerful X-ray Flares from T-Tauri Stars

TL;DR

This study models how powerful X-ray flares from young stars temporarily alter water chemistry in protoplanetary disks, revealing significant short-term increases in gas-phase water abundance that could be observable with future infrared observations.

Contribution

It introduces a time-dependent chemical model of water in disks during stellar X-ray flares, highlighting the impact of rare, strong flares on water abundance.

Findings

Strong flares can increase gas-phase water by 3-5 times temporarily.

Typical flares do not cause significant observable changes.

Water abundance changes are short-lived, lasting days.

Abstract

Young stars emit strong flares of X-ray radiation that penetrate the surface layers of their associated protoplanetary disks. It is still an open question as to whether flares create significant changes in disk chemical composition. We present models of the time-evolving chemistry of gas-phase water during X-ray flaring events. The chemistry is modeled at point locations in the disk between 1 and 50 au at vertical heights ranging from the mid-plane to the surface. We find that strong, rare flares, i.e., those that increase the unattenuated X-ray ionization rate by a factor of 100 every few years, can temporarily increase the gas-phase water abundance relative to H can by more than a factor of $\sim3-5$ along the disk surface (Z/R $\ge$ 0.3). We report that a "typical" flare, i.e., those that increase the unattenuated X-ray ionization rate by a factor of a few every few weeks, will not lead to significant, observable changes. Dissociative recombination of H$_3$O$^+$, water adsorption and desorption onto dust grains, and ultraviolet photolysis of water and related species are found to be the three dominant processes regulating the gas-phase water abundance. While the changes are found to be significant, we find that the effect on gas phase water abundances throughout the disk is short-lived (days). Even though we do not see a substantial increase in long term water (gas and ice) production, the flares' large effects may be detectable as time varying inner disk water 'bursts' at radii between 5 and 30 au with future far infrared observations.