2009, Radiation-hydrodynamic models of X-ray and EUV photoevaporating protoplanetary discs

We present the first radiation-hydrodynamic model of a protoplanetary disc irradiated with an X-ray extreme ultraviolet (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 1.4 × 10−8 M yr−1, 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 (∼1022 cm−2) and can thus effect heating in denser regions and at larger radius than the EUV. The radial extent of the launching region of the X-ray-heated wind is 1–70 au 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 when 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 per cent of the disc’s lifetime. This is consistent with the 105 yr transitional time-scale estimate