2019ApJ...883..197W

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 H₂O 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 midplane 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 H₂O abundance relative to H by more than a factor of ∼3-5 along the disk surface (Z/R >= 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 H₃O⁺, H₂O adsorption and desorption onto dust grains, and ultraviolet photolysis of H₂O and related species are found to be the three dominant processes regulating the gas-phase H₂O 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.