Source of the Fe Kα emission in T Tauri stars

Radiation induced by relativistic electrons during flares. An application to RY Tau

¹ AEGORA Research Group – Joint Center for Ultraviolet Astronomy, Universidad Complutense de Madrid, Plaza de Ciencias 3, 28040 Madrid, Spain
e-mail: aig@ucm.es
² Departamento de Fisica de la Tierra y Astrofisica, Fac. de CC. Matematicas, Plaza de Ciencias 3, 28040 Madrid, Spain
³ I.I.S. Enrico Fermi, Via Monte Nero, 15/A, 28041 Arona, Italy

Received: 6 March 2024
Accepted: 15 May 2024

Abstract

Context. T Tauri stars (TTSs) are magnetically active stars that accrete matter from the inner border of the surrounding accretion disk; plasma becomes trapped into the large-scale magnetic structures and falls onto the star, heating the surface through the so-called accretion shocks. The X-ray spectra of the TTSs show prominent Fe Kα fluorescence emission at 6.4 keV (hereafter, Fe Kα emission) that cannot be explained in a pure accretion scenario because its excitation requires significantly more energy than the maximum available through the well-constrained free-fall velocity. Neither can it be produced by the hot coronal plasma.

Aims. TTSs display all signs of magnetic activity, and magnetic reconnection events are expected to occur frequently. In these events, electrons may become accelerated to relativistic speeds, and their interaction with the environmental matter may result in Fe Kα emission. It is the aim of this work to evaluate the expected Fe Kα emission in the context of the TTS research and compare it with the actual Fe Kα measurements obtained during the flare detected while monitoring RY Tau with the XMM-Newton satellite.

Methods. The propagation of high-energy electrons in dense gas generates a cascade of secondary particles that results in an electron shower of random nature, whose evolution and radiative throughput was simulated in this work using the Monte Carlo code PENELOPE. A set of conditions representing the environment of the TTSs where these showers may impinge was taken into account to generate a grid of models that can aid the interpretation of the data.

Results. The simulations show that the electron beams produce a hot spot at the point of impact; strong Fe Kα emission and X-ray continuum radiation are produced by the spot. This emission is compatible with RY Tau observations.

Conclusions. The Fe Kα emission observed in TTSs could be produced by beams of relativistic electrons accelerated in magnetic reconnection events during flares.