The effects of X-ray photoionization and heating on the structure of circumstellar discs

Abstract

We present the results of a theoretical study investigating the effects of photoionization and heating by X-rays on discs around low-mass stars. In particular, we address the question of whether or not X-rays can drive a disc wind. First, we construct a one-dimensional 'quasi-hydrostatic' model, which solves for the vertical structure introduced by X-ray heating. We consider uniform X-ray illumination of the disc, but the X-ray fluxes required to heat the disc significantly are much greater than those seen by recent observations. When the model is extended to consider heating from a central X-ray source, we find that the one-dimensional model is only valid very close to the star. We extend our analysis to consider a simple two-dimensional model, treating the disc as a two-layered structure and solving for its density profile self-consistently. For T Tauri stars, we are able to set a crude upper limit on the mass-loss rate that can be driven by X-ray photoevaporation, with a value of ≃10-13 g cm-2 s-1. Our model is designed to maximize this value, and most likely overestimates it significantly. However, we still find a mass-loss rate which is less than that found in studies of ultraviolet photoevaporation. We conclude that in the presence of a significant ultraviolet field, X-ray driven disc winds are unlikely to play a significant role in the evolution of discs around low-mass stars.