Evaluating the chromospheric structure model of AD Leo using RH 1.5D and magnetic field data

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100011, PR China
² Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁴ School of Physics and Technology, Nantong University, Nantong 226019, China
⁵ Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, PR China

^★ Corresponding authors: sjr@nao.cas.cn; whg@nao.cas.cn

Received: 14 May 2025
Accepted: 20 October 2025

Abstract

Context. The interplay between surface magnetic topology and chromospheric heating in active M dwarfs remains poorly constrained, limiting our understanding of their magnetic cycles and high-energy environments.

Aims. We aim to test whether detailed Zeeman–Doppler imaging (ZDI) maps of AD Leo can be used to spatially anchor a multi-component chromospheric model and thus validate the link between magnetic flux distribution and emission-line formation.

Methods. We analyze high-resolution CARMENES spectra of H_α and the Ca II infrared triplet, together with detailed ZDI maps. The RH1.5D non-local thermodynamic equilibrium (non-LTE) radiative transfer code is employed to synthesize spectral lines with two active atmospheric components (low-latitude near the equator and polar near the pole) and a quiet background. Their relative filling factors and temperature structures are optimized per epoch. The ZDI maps serve as a qualitative reference for the large-scale magnetic topology but are not used as input to the optimization procedure.

Results. Our model reproduces the spectral line profiles over multiple epochs. The low-latitude active region shows notable variability – accounting for approximately 55−86% of the emission, while the polar active region remains relatively constant in area (12−17%) but exhibits temperature variations over time, particularly during the periods of increased activity. The spatial locations of the active regions derived from spectroscopy are in good agreement with the radial magnetic field distribution obtained from ZDI.

Conclusions. Our results indicate that combining spectroscopic modeling with magnetic field maps is an effective approach for mapping magneto-chromospheric structures in M dwarfs. This framework deepens our understanding of stellar magnetic cycles and chromospheric dynamics, paving the way for detailed time-resolved studies in active low-mass stars.