Photoevaporation of protoplanetary discs with PLUTO+PRIZMO I. Lower X-ray-driven mass-loss rates due to enhanced cooling

TL;DR

This study uses hydrodynamic simulations with detailed thermochemistry to show that enhanced cooling mechanisms significantly lower X-ray-driven mass-loss rates in protoplanetary discs, impacting disc longevity and evolution.

Contribution

It introduces a new simulation approach combining PLUTO and PRIZMO, revealing the importance of specific cooling processes and spectral variations on mass-loss rates.

Findings

Cooling from neutral H excitation of O reduces mass-loss rates by an order of magnitude.

Less soft X-ray spectra lead to 2-3 times lower mass-loss rates.

H2 survives into the wind, acting as a dominant coolant and potential tracer.

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 for 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 conduct hydrodynamic simulations with pluto where the thermochemistry is calculated using prizmo. We explore the contribution of certain key microphysical processes and the impact of using different spectra used previously 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 Msun/yr. 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 2-3 times lower. The chemistry is significantly out of equilibrium, with the survival of H2 into the wind aided by advection. This leads to its role as the dominant coolant at 10s au - thus stabilising a larger radial temperature gradient across the wind - as well as providing a possible wind tracer.