Evaluating the chromospheric structure model of AD Leo using RH1.5D and magnetic field data

TL;DR

This work tests whether Zeeman-Doppler imaging (ZDI) maps can anchor a multi-component chromospheric model for the active M dwarf AD Leo and link magnetic flux distribution to emission-line formation. Using RH1.5D non-LTE radiative transfer, the authors synthesize $H\alpha$ and Ca II IRT profiles for two active chromospheric components plus a quiet background, constrained by 46 CARMENES spectra and contemporaneous ZDI maps. The results show that a two-component model—with a dynamic low-latitude region and a relatively stable polar region—reproduces the observed spectra across epochs and aligns with the ZDI-derived magnetic topology; the polar component remains 12–17% of the visible surface while the low-latitude component varies 55–86%. The study demonstrates that integrating spectroscopic modeling with magnetic-field maps provides a powerful framework to map magneto-chromospheric structure in M dwarfs and offers insights into magnetic cycles and space-weather environments relevant to exoplanets around active low-mass stars.

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 validate the link between magnetic flux distribution and emission-line formation. Methods. We analyze high-resolution CARMENES spectra of H-alpha and the Ca II infrared triplet, together with ZDI maps. Synthetic profiles are computed using the RH1.5D non-LTE radiative transfer code 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 qualitative references for the large-scale magnetic topology but are not used as input to the optimization. Results. Our model reproduces the spectral line profiles across multiple epochs. The low-latitude active region shows notable variability, accounting for approximately 55-86% of the emission, while the polar region remains relatively constant in area (12-17%) but exhibits temperature variations over time, particularly during periods of increased activity. The spatial locations of the active regions derived from spectroscopy agree well with the radial magnetic field distribution from ZDI. Conclusions. 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.

Figures (7)

• Figure 1: Observations of AD Leo (top panels) and of the inactive template LP 776-46 (middle panels). All spectra have been continuum‐normalized consistently across epochs. The shaded yellow region marks the wavelength interval used for model fitting. Colored traces correspond to sequential observations, progressing from red to blue, with four exceptionally strong emission events in AD Leo highlighted in black. The bottom panels show the highest-SNR time-resolved spectra (SNR$\sim$98 for H $\alpha$ and 150 for CaII IRT) of the inactive template star LP 776-46, with overlaid Lorentzian profile fits (red curves).
• Figure 2: Temporal evolution of filling factors during two observational campaigns. Left panel (2018): the low-latitude filling factor varies between 60% and 86%, while the polar component ranges from 12% to 17%. Right panel (2020): the low-latitude filling factor remains stable between 55% and 72%, with the polar component consistently contributing 12--17%.
• Figure 3: ZDI maps of AD Leo show the magnetic field components (radial, azimuthal, and meridional) in a flattened polar view across the epochs: 2019a, 2019b, 2019 optical, 2020a, and 2020b. Each component is color-coded to indicate magnetic polarity (red for positive, blue for negative). Radial ticks indicate the rotational phases during observations, and concentric circles represent stellar latitudes at $-30^\circ$, $+30^\circ$, $+60^\circ$, and the equator. The color bar is scaled to the maximum magnetic field strength for each epoch. For a stellar inclination of $i \approx 20^\circ$, latitudes south of $-20^\circ$ are not visible; the $-30^\circ$ latitude line is shown as a plotting reference only. Credit:2023AA...676A..56B
• Figure 4: Temperature stratification across chromospheric models: the plot illustrates how temperature varies with column mass (log m) for the low-latitude active model (dashed blue line) and for the polar models in both their normal (dashed orange) and enhanced (dotted orange) states.
• Figure 5: Convolved synthetic profiles for H$\alpha$ (6561.50–6564.50 Å), CaII 8498 Å (8497.00–8499.00 Å), and CaII 8542 Å (8540.00–8544.00 Å) at two magnetic field strengths: $B_z$ = 0.0 kG (left) and 3.6 kG (right). Each panel shows normalized flux versus wavelength for three activity levels: low-latitude active (solid orange), normal polar active (solid blue), and strong polar active (dotted orange).