Radiation-Hydrodynamic Models of X-Ray & EUV Photoevaporating Protoplanetary Discs

TL;DR

This study introduces a radiation-hydrodynamic model showing X-ray irradiation dominates photoevaporation in protoplanetary discs, significantly affecting their evolution and dispersal timescales.

Contribution

First radiation-hydrodynamic model of a protoplanetary disc irradiated with combined X-ray and EUV spectrum, demonstrating X-ray dominance in photoevaporative mass loss.

Findings

X-rays cause a higher photoevaporation rate than EUV alone.

The wind launching region extends from 1 to 70 AU with X-ray irradiation.

Inner disc clearing occurs at about 1 AU when accretion drops below wind loss rate.

Abstract

(Abridged) We present the first radiation-hydrodynamic model of a protoplanetary disc irradiated with an X-EUV spectrum. In a model where the total ionizing luminosity is divided equally between X-ray and EUV luminosity, we find a photoevaporation rate of 1.4e-8 M_sun/yr, which is two orders of magnitude greater than the case of EUV photoevaporation alone. Thus it is clear that the X-rays are the dominant driving mechanism for photoevaporation. This can be understood inasmuch as X-rays are capable of penetrating much larger columns (~1e22 cm^-2) and can thus effect heating in denser regions and at larger radius than the EUV can. The radial extent of the launching region of the X-ray heated wind is 1-70AU compared with the pure EUV case where the launch region is concentrated around a few AU. When we couple our wind mass-loss rates with models for the disc's viscous evolution, we find that, as in the pure EUV case, there is a photoevaporative switch, such that an inner hole develops at ~ 1 AU at the point that the accretion rate in the disc drops below the wind mass loss rate. At this point, the remaining disc material is quickly removed in the final 15-20% of the disc's lifetime. This is consistent with the 1e5 yr transitional timescale estimated from observations of T-Tauri stars. We also caution that although our mass-loss rates are high compared to some accretion rates observed in young stars, our model has a rather large X-ray luminosity of 2e30 erg/s; further modeling is required in order to investigate the evolutionary implications of the large observed spread of X-ray luminosities in T-Tauri stars.