Magnetic fields from low mass stars to brown dwarfs

Abstract

Magnetic fields have been detected on stars across the H-R diagram and substellar objects either directly by their effect on the formation of spectral lines, or through the activity phenomena they power which can be observed across a large part of the electromagnetic spectrum. Stars show a very wide variety of magnetic properties in terms of strength, geometry or variability. Cool stars generate their magnetic fields by dynamo effect, and their properties appear to correlate - to some extent - with stellar parameters such as mass, rotation and age. With the improvements of instrumentation and data analysis techniques, magnetic fields can now be detected and studied down to the domain of very-low-mass stars and brown dwarfs, triggering new theoretical works aimed, in particular, at modelling dynamo action in these objects. After a brief discussion on the importance of magnetic field in stellar physics, the basics of dynamo theory and magnetic field measurements are presented. The main results stemming from observational and theoretical studies of magnetism are then detailed in two parts: the fully-convective transition, and the very-low mass stars and brown dwarfs domain.

30 pages, 9 figures. Notes for lectures presented at the Evry Schatzman school on "Low-mass stars and the transition from stars to brown dwarfs", September 2011, Roscoff, France. To appear in the EAS Conference Series, edited by C. Charbonnel, C. Reyle, M. Schultheis

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• M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets
• A hot Jupiter orbiting a 2-Myr-old solar-mass T Tauri star
• The global and small-scale magnetic fields of fully convective, rapidly spinning M dwarf pair GJ65A and B
• Monitoring the large-scale magnetic field of AD~Leo with SPIRou, ESPaDOnS and Narval. Toward a magnetic polarity reversal?
• Surface magnetism of cool stars
• Timing Analysis of the Periodic Radio and Optical Brightness Variations of the Ultracool Dwarf, TVLM 513-46546
• PSR J1306--40: An X-ray Luminous Redback with an Evolved Companion
• Size and strength of self-excited dynamos in Jupiter-like extrasolar planets
• The energetic particle environment of a GJ 436 b-like planet
• Misalignment of the outer disk of DK Tau and a first look at its magnetic field using spectropolarimetry
• Galactic cosmic ray propagation through M dwarf planetary systems
• The long-term evolution of stellar activity
• Reconstruction of spider system's observables from orbital period modulations via the Applegate mechanism