# Complexity of magnetic fields on red dwarfs

## TL;DR

This study investigates the complexity of magnetic fields in M-dwarfs by analyzing Zeeman-sensitive molecular and atomic lines, revealing large, height-dependent magnetic structures with strengths up to 6 kG, and highlights the importance of multi-line spectropolarimetric analysis.

## Contribution

The paper introduces a multi-line spectropolarimetric method to analyze magnetic field complexity and height dependence in M-dwarfs, expanding understanding beyond previous single-line studies.

## Key findings

- Magnetic fields in M-dwarfs are large, up to 6 kG.
- Magnetic features occupy up to 100% of the stellar atmosphere.
- Magnetic structures depend on formation height and species used.

## Abstract

Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature -- and Zeeman -- sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how "surface" inhomogeneities are distributed in height -- the dimension that is usually neglected in stellar magnetic studies. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1-M7). Our chi^2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3 kG to 6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma beta is less than one, implying highly magnetized stars. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.