Magnetism, rotation, and non-thermal emission in cool stars - New results from almost 300 M dwarf magnetic field measurements

by Ansgar Reiners (Institut für Astrophysik, Georg-August-Universität, Göttingen)

Tuesday 15 Mar 2022, 15:00 → 16:00 Europe/Stockholm

https://stockholmuniversity.zoom.us/j/940229961

We report measurements of surface-average magnetic fields in 292 M dwarfs from comparison to radiative transfer calculations; for 260 of them, this is the first measurement of this kind. Our data were obtained from more than 15,000 high-resolution spectra taken during the CARMENES project. Our results reveal a comprehensive picture of magnetic fields in cool stars. We find a clear relation between average field strength, <B>, and Rossby number, Ro, resembling the well studied rotation-activity relation. Among the slowly rotating stars, we find that magnetic flux, Phi_B, is proportional to rotation period, P, and among the rapidly rotating stars that average surface fields do not grow significantly beyond the level set by the available kinetic energy. Furthermore, we find close relations between non-thermal coronal X-ray emission, chromospheric Halpha and Ca H&K emission, and magnetic flux. Taken together, these relations demonstrate empirically that the rotation-activity relation can be traced back to a dependence of the magnetic dynamo on rotation, and we provide relations estimating average field strengths and non-thermal emission for cool stars.

Reiners_2022-Mar-15.mp4