Complexity of magnetic fields on red dwarfs

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.

arXiv comment15 pages, 6 figures

References in corpus (27)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• A terrestrial planet candidate in a temperate orbit around Proxima Centauri
• The Limiting Effects of Dust in Brown Dwarf Model Atmospheres
• The Theory of Brown Dwarfs and Extrasolar Giant Planets
• Magnetic fields of non-degenerate stars
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• Large-scale magnetic topologies of late M dwarfs
• Radiative MHD simulation of sunspot structure
• The Palomar/MSU Nearby Star Spectroscopic Survey III: Chromospheric Activity, M-dwarf Ages and the Local Star Formation History
• The First Direct Measurements of Magnetic Fields on Very Low-Mass Stars
• NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
• The Effective Temperature Scale of M dwarfs
• Line Intensities and Molecular Opacities of the FeH $F^4{\Delta }_i-X^4{\Delta }_i$ Transition
• Measuring Magnetic Fields in Ultracool Stars and Brown Dwarfs
• SPIRou Input Catalogue: Global properties of 440 M dwarfs observed with ESPaDOnS at CFHT
• SPIRou Input Catalog: Activity, Rotation and Magnetic Field of Cool Dwarfs
• The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
• Exploring the magnetic field complexity in M dwarfs at the boundary to full convection
• Modelling the molecular Zeeman effect in M-dwarfs: methods and first results
• Molecules as magnetic probes of starspots
• First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
• Surface magnetism of cool stars
• Rotation, magnetism, and metallicity of M dwarf systems
• An increased rate of large flares at intermediate rotation periods for mid-to-late M dwarfs
• First polarimetric observations and modeling of the FeH F^4 Delta-X^4 Delta system
• First spectropolarimetric measurement of a brown dwarf magnetic field in molecular bands
• First polarimetric measurements and modeling of the Paschen-Back effect in CaH transitions

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• Constraining atmospheric parameters and surface magnetic fields with $\text{ZeeTurbo}$: an application to SPIRou spectra
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• The CARMENES search for exoplanets around M dwarfs: Benchmarking the impact of activity in high-precision radial velocity measurements
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• Coronal X-Ray Emission from Nearby, Low-Mass, Exoplanet Host Stars Observed by the MUSCLES and Mega-MUSCLES HST Treasury Survey Projects
• Magnetic Inflation and Stellar Mass. V. Intensification and saturation of M dwarf absorption lines with Rossby number
• Correlating Changes in Spot Filling Factors with Stellar Rotation: The Case of LkCa 4
• Starspots as the origin of ultrafast drifting radio bursts from an active M dwarf
• The Mysterious Affair of the H${}_2$ in AU Mic
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