X-ray irradiated protoplanetary disk atmospheres I: Predicted emission line spectrum and photoevaporation

TL;DR

This study models X-ray irradiated protoplanetary disks around T Tauri stars, predicting emission lines, gas temperatures, and mass loss rates due to photoevaporation, providing insights into disk dispersal timescales.

Contribution

The paper introduces detailed 2D radiative transfer models of X-ray irradiated disks, predicting emission spectra and mass loss rates, advancing understanding of disk photoevaporation processes.

Findings

Hot gas layer with temperatures up to 10^6 K at small radii

Predicted observable emission lines from heavy elements and hydrogen/helium

Estimated mass loss rate of ~10^-8 M_sun yr^-1 due to X-ray photoevaporation

Abstract

We present MOCASSIN 2D photoionisation and dust radiative transfer models of a prototypical T Tauri disk irradiated by X-rays from the young pre-main sequence star. The calculations demonstrate a layer of hot gas reaching temperatures of ~10^6 K at small radii and ~10^4 K at a distance of 1 AU. The gas temperatures decrease sharply with depth, but appear to be completely decoupled from dust temperatures down to a column depth of ~5*10^21 cm^-2. We predict that several fine-structure and forbidden lines of heavy elements, as well as recombination lines of hydrogen and helium, should be observable with current and future instrumentation, although optical lines may be smothered by the stellar spectrum. Predicted line luminosities are given for the the brightest collisionally excited lines (down to ~10^-8L_sun, and for recombination transitions from several levels of HI and HeI. The mass loss rate due to X-ray photoevaporation estimated from our models is of the order of 10^-8 M_sun yr^-1, implying a dispersal timescale of a few Myr for a disk of mass 0.027 M_sun, which is the mass of the disk structure model we employed. We discuss the limitations of our model and highlight the need for further calculations that should include the simultaneous solution of the 2D radiative transfer problem and the 1D hydrostatic equilibrium in the polar direction.