Recent advances point out the crucial role of magnetic fields in many aspects of stellar physics, throughout stellar evolution. From the very formation of stars and their planetary systems, stellar spin evolution, activity, to the latest stages of stellar evolution and the shaping of planetary nebulae, we now have evidence that magnetic fields are a key ingredient. We therefore think that characterizing how magnetic properties depend on stellar parameters, and ultimately unveiling the interplay between internal structure, rotation and magnetic fields along stellar evolution is of prime interest to make new advances in stellar physics.

Main-sequence stars with spectral types cooler than about F5 possess an extended convective envelope. Their rotation and internal shear, when interacting with convection, is able to trigger large-scale dynamo processes which are at the origin of their widespread magnetism. The efficiency of global stellar dynamos is primarily controlled by the stellar rotation rate and depth of the convective zone. Both parameters display strong variation among cool stars, generating a wide variety of dynamo signatures, so that dynamo modes marginally detected, or simply inactive in the Sun, become accessible by observing solar-type stars. If the activity level alone is an indicator of the dynamo efficiency, the temporal evolution of the magnetic field also carries a large fraction of the information about the physical parameters of the dynamo.

Fig. 1: Numerical simulation of the solar dynamo. From Augustson et al. (2015)

A direct characterization of surface magnetic fields of cool stars was until recently limited to a few peculiar objects such as bright RS CVn systems. With the advent of efficient high-resolution spectropolarimeters and associated data analysis techniques it is now possible to detect and characterize magnetic fields on stars throughout the Hertzsprung-Russell diagram. Within the Bcool project we aim to provide theorists with stringent observational constraints on the properties of surface magnetic fields, and activity in the time-domain for a large sample of stars spanning a wide range of stellar parameters. We also aim to build on these observational results to derive empirical trends and performs more elaborate modelling that allow us to analyse the effect of magnetic fields on the physics of stars and of their planetary systems.

Fig. 2: Plot of Log(maximum measured |Bl|) against and chromospheric age. From Marsden et al. (2014) Fig. 3: Wind simulation of the M dwarf star DT Vir. From Vidotto et al. (2014)