Photoevaporation of protoplanetary discs with PLUTO+PRIZMO

I. Lower X-ray–driven mass-loss rates due to enhanced cooling

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
⁵ Exzellenzcluster “Origins,” Boltzmannstr. 2, 85748 Garching, Germany

^★ Corresponding author; sellek@strw.leidenuniv.nl

Received: 28 March 2024
Accepted: 31 July 2024

Abstract

Context. Photoevaporation is an important process for protoplanetary disc dispersal, but there has so far been a lack of consensus from simulations over the mass-loss rates and the most important part of the high-energy spectrum involved in driving the wind.

Aims. We aim to isolate the origins of these discrepancies through carefully benchmarked hydrodynamic simulations of X-ray photoevaporation with time-dependent thermochemistry calculated on the fly.

Methods. We conducted hydrodynamic simulations with PLUTO where the thermochemistry is calculated using PRIZMO. We explored the contribution of certain key microphysical processes and the impact of employing different spectra previously used in literature studies.

Results. We find that additional cooling results from the excitation of O by neutral H, which leads to dramatically reduced mass-loss across the disc compared to previous X-ray photoevaporation models, with an integrated rate of ~10⁻⁹ M_⊙ yr⁻¹. Such rates would allow for longer-lived discs than previously expected from population synthesis. An alternative spectrum with less soft X-ray produces mass-loss rates around a factor of two to three times lower. The chemistry is significantly out of equilibrium, with the survival of H₂ into the wind being aided by advection. This leads to H₂ becoming the dominant coolant at 10s au, thus stabilising a larger radial temperature gradient across the wind as well as providing a possible wind tracer.