Classification of X-ray Flare Driven Chemical Variability in Protoplanetary Disks

TL;DR

This study models how stochastic X-ray flares from young stars influence the chemical composition of protoplanetary disks over short and long timescales, revealing significant effects on small ions and some molecules.

Contribution

Introduces a 2D chemical model with a stochastic flaring module to assess the impact of X-ray flares on disk chemistry, highlighting effects on ion abundances and chemical steady states.

Findings

Flares cause temporary increases in small gas-phase cations like H3+ and HCO+.

Long-term flaring alters the abundance of species such as O and O2 by a few percent.

The specific history of flaring events does not significantly impact chemical evolution.

Abstract

Young stars are highly variable in the X-ray regime. In particular, bright X-ray flares can substantially enhance ionization in the surrounding protoplanetary disk. Since disk chemical evolution is impacted by ionization, X-ray flares have the potential to fundamentally alter the chemistry of planet forming regions. We present 2D disk chemical models that incorporate a stochastic X-ray flaring module, named \xgen, and examine the flares' overall chemical impact compared to models that assume a constant X-ray flux. We examine the impact of 500 years of flaring events and find global chemical changes on both short time scales (days) in response to discrete flaring events and long time-scales (centuries) in response to the cumulative impact of many flares. Individual X-ray flares most strongly affect small gas-phase cations, where a single flare can temporarily enhance the abundance of species such as H$_3^+$, HCO$^+$, CH$_3^+$, and C$^+$. We find that flares can also drive chemistry out of "steady state" over longer time periods, where the disk-integrated abundance of some species, such as O and O$_2$, changes by a few percent over the 500 year model. We also explore whether the specific history of X-ray flaring events (randomly drawn but from the same energy distribution) impacts the chemical evolution and find that it does not. Finally, we examine the impact of X-ray flares on the electron fraction. While most molecules modeled are not highly sensitive to flares, certain species, including observable molecules, are very reactive to the dynamic environment of a young star.