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

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 1-dimensional “quasihydrostatic” model, which solves for the vertical structur e 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 1-dimensional model is only valid very close to the star. We extend our analysis to consider a simple 2dimensional 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 se t a crude upper limit on the massloss 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 maximise this value, and most likely over-estimates it significantly. However we still find a mass-loss rate which is less than that f ound in studies of ultraviolet photoevaporation. We conclude that in the presence of a significant UV field, X-ray driven disc winds are unlikely to play a significant role in the evolu tion of discs around low-mass stars.