The effect of X-ray and accretion variability on mid-infrared lines in the young disk-bearing binary DQ Tau

Abstract

A star's radiation field is the main energy source of a planet-forming disk. Stellar photons -- from X-rays to optical -- are reprocessed in the inner protoplanetary disk, and this process enables mid-infrared (mid-IR) diagnostics of the disk's physical conditions. Most pre-main-sequence stars accrete at moderate rates and emit X-rays efficiently. Such activity is expected to drive stochastic variability in the stellar spectrum. It is not clear to what extent such variability affects the thermochemical structure of protoplanetary disks. We examine the effect of routine accretion variability and X-ray flares on the thermochemical structure and mid-IR line emission spectra of protoplanetary disks. We combined multi-epoch contemporaneous X-ray and NUV/optical observations of the DQ Tau binary with JWST/MIRI spectra of its circumbinary disk. We interpreted the data and assessed the effect of a varying stellar spectrum on the disk using a forward thermochemical model illustrative of the system. Synthetic mid-IR fluxes of , , , and are systematically stronger at periastron than at apastron due to a fourfold increase in accretion luminosity. Variations in the integrated fluxes lie within a factor of two between epochs and are in reasonable agreement with the JWST/MIRI spectra of DQ Tau. Routine stellar variability induces only modest changes in the disk's thermochemical structure, yet it can still shift the snowline outward by 17% relative to its apastron position. Because the JWST observations did not coincide with X-ray flares, we relied on simulations to analyze these phenomena. We find that moderate-intensity flares leave most mid-IR molecular emission unchanged. In contrast, X-ray flares strongly enhance the abundances and mid-IR luminosities of , , and on the disk surface. CO CO2 HCN H2O H I Ar II Ne II Our results imply that the line variability observed in non-outbursting systems is consistent with moderately variable stellar spectra driven by accretion and magnetospheric activity.