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

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 ionization 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 ionization 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 ionization rates for a disc whose structure has been calculated with the radiation thermo-chemical code prodimo. We find that energetic particles are potentially a very strong source of local ionization with ionization rates exceeding by several orders of magnitude the contribution due to X-rays, stellar energetic particles, and radioactivity in the inner disc.