Recent measurements of stellar magnetic fields

Abstract

High resolution, high S/N spectra have been successfully modeled to yield information on the magnetic properties of cool stars for some 15 years now. These analyses yield estimates of the intensity weighted surface filling factor of active regions, f , and the mean unsigned field strength in these regions), B. The measurements are difficult, though, hampered by the small magnitude of the Zeeman effect, small f values for most stars, and the unknown spatial and thermodynamic properties of the active regions. Recent data, mostly in the infrared (IR) where the Zeeman effect (∞ λ 2 ) is larger, are yielding better measurements than ever before. I summarize the new observations here, and show how they are expanding and modifying our understanding of magnetic fields on cool stars.