Ionisation of inner T Tauri star discs: effects of in-situ energetic particles produced by strong magnetic reconnection events

TL;DR

This study investigates how energetic particles produced by magnetic reconnection in T Tauri star discs can significantly enhance local ionisation rates, surpassing other known sources and affecting disc evolution.

Contribution

It provides the first detailed modeling of energetic particle propagation and ionisation effects in T Tauri star circumstellar discs based on observational and theoretical data.

Findings

Energetic particles can cause ionisation rates several orders of magnitude higher than X-rays.

The ionisation effect depends on magnetic configuration and particle spectra.

Local ionisation by energetic particles may influence disc chemistry and dynamics.

Abstract

Magnetic reconnection is one of the major particle acceleration processes in space and astrophysical plasmas. Low-energy supra-thermal particles emitted by magnetic reconnection are a source of ionisation for circumstellar discs, influencing their chemical, thermal and dynamical evolution. The aim of this work is to propose a first investigation to evaluate how energetic particles can propagate in the circumstellar disc of a T Tauri star and how they affect the ionisation rate of the disc plasma. To that end, we have collected experimental and theoretical cross sections for the production of H$^+$, H$_2^+$ and He$^+$ by electrons and protons. Starting from theoretical injection spectra of protons and electrons emitted during magnetic reconnection events, we have calculated the propagated spectra in the circumstellar disc considering the relevant energy loss processes. We have considered fluxes of energetic particles with different spectral indices and different disc magnetic configurations, generated at different positions from the star considering the physical properties of the flares as deduced from the observations obtained by the Chandra Orion Ultra Deep point source catalogue. We have then computed the ionisation rates for a disc whose structure has been calculated with the radiation thermo-chemical code {\tt ProDiMo}. We find that energetic particles are potentially a very strong source of local ionisation with ionisation rates exceeding by several orders of magnitude the contribution due to X-rays, stellar energetic particles and radioactivity in the inner disc.