Letter to the Editor

Model of X-ray and extreme-UV emission from magnetically heated atmospheres in classical T Tauri stars: Case study of TW Hya

¹ Department of Earth and Planetary Science, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
² Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria, 16, I-20133 Milano, Italy
³ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

^⋆ Corresponding author; shoda.m.astroph@gmail.com

Received: 30 January 2025
Accepted: 5 March 2025

Abstract

Photoevaporation caused by X-ray and UV radiation from the central star has attracted attention as a key process driving the dispersal of protoplanetary discs. Although numerous models have been used to investigate the photoevaporation process, their conclusions vary, which is partly due to differences in the adopted radiation spectra of the host star, in particular, in the extreme-UV (EUV) and soft X-ray bands. This study aims to construct the EUV and (soft) X-ray emission spectrum from pre-main-sequence stars using a physics-based model that focuses on the radiation from magnetically heated coronae. We applied a magnetohydrodynamics model capable of reproducing the coronal emission of main-sequence stars to the pre-main-sequence star TW Hya, and we assessed its capability by comparing the predicted and observed emission line intensities. The emission lines that formed at coronal temperatures (T = 4 − 13 × 10⁶ K) are reproduced in intensity within a factor of three. Emission lines from lower-temperature (T < 4 × 10⁶ K) plasmas are systematically underestimated, with typical intensities at 10–30% of the observed values. This is consistent with previous findings that these emissions predominantly originate from accretion shocks. Emission lines emitted from extremely high temperatures (T > 13 × 10⁶ K) account for only about 1–10% of the observed values, probably because transient heating associated with flares was neglected. These results indicate that the quiescent coronal emission of pre-main-sequence stars can be adequately modelled using a physics-based approach.