Magnetic field are ubiquitous to young and mature low-mass stars, and can potentially impact their formation, their evolution and their internal structure; yet the physical processes that succeed at amplifying and sustaining these fields (called dynamo processes), like those capable of changing the fate of the host stars (and in particular their rotation rates), are still somewhat enigmatic. Although theoretical modelling and numerical simulations (e.g., of stellar dynamo action and magnetospheric accretion processes) showed breathtaking progress in the last decade, they are not yet in the state of accurately predicting the various magnetic topologies that different low-mass stars can generate nor the impact such fields can have on stellar formation.
Thanks to the advent of new-generation instruments, spectropolarimetric observations coupled to tomographic techniques can now reveal the large-scale magnetic topologies of both young and mature low-mass stars, and in particular their poloidal and toroidal components. More specifically, one can now investigate magnetic topologies of cool dwarfs, all the way from the brown dwarf threshold (spectral type M8) where stars are fully convective up to the limit beyond which outer convective zones get vanishingly small (spectral type F5); one can also explore the magnetic topologies of young low-mass stars that are still accreting mass from their circumstellar disc (i.e., the classical T Tauri stars) and study how such fields can impact mass accretion processes from the surrounding discs.
We review herein the latest observational advances in this field, showing in particular that large-scale magnetic topologies of low-mass stars and protostars can drastically vary with mass and rotation rate, and discuss their implications for our understanding of dynamo processes, stellar formation and stellar evolution.