X-ray radiative transfer in protoplanetary disks - The role of dust and X-ray background fields

TL;DR

This study develops a new X-ray radiative transfer model for protoplanetary disks, examining the effects of stellar and background X-ray irradiation on disk chemistry, especially considering dust influence and cluster environment effects.

Contribution

It introduces a novel X-ray radiative transfer module for ProDiMo that accounts for scattering and absorption by gas and dust, and evaluates the impact of background X-ray fields on disk chemistry.

Findings

X-ray opacities are dominated by gas except at high energies.

Background X-ray fields can dominate local ionization beyond 20 au.

Observable effects are significant only under strong background X-ray conditions.

Abstract

The X-ray luminosities of T Tauri stars are about two to four orders of magnitude higher than the luminosity of the contemporary Sun. As these stars are born in clusters, their disks are not only irradiated by their parent star but also by an X-ray background field produced by the cluster members. We aim to quantify the impact of X-ray background fields produced by young embedded clusters on the chemical structure of disks. Further, we want to investigate the importance of the dust for X-ray radiative transfer in disks. We present a new X-ray radiative transfer module for the radiation thermo-chemical disk code ProDiMo, which includes X-ray scattering and absorption by both the gas and dust component. For the X-ray radiative transfer, we consider irradiation by the star and by X-ray background fields. To study the impact of X-rays on the chemical structure of disks we use the well-established disk ionization tracers N2H+ and HCO+. For evolved dust populations, X-ray opacities are mostly dominated by the gas; only for photon energies $E\gtrsim5-10\,$keV, dust opacities become relevant. Consequently, the local disk X-ray radiation field is only affected in dense regions close to the disk midplane. X-ray background fields can dominate the local X-ray disk ionization rate for disk radii $r\gtrsim20\,$au. However, the N2H+ and HCO+ column densities are only significantly affected in case of low cosmic-ray ionization rates, or if the background flux is at least a factor of ten higher than the flux level expected for clusters typical for the solar vicinity. Observable signatures of X-ray background fields in low-mass star-formation regions, like Taurus, are only expected for cluster members experiencing a strong X-ray background field. For the majority of the cluster members, the X-ray background field has only little impact on the disk chemical structure.