Vol. 635
7. Stellar structure and evolution

Hidden magnetic fields of young suns

Spectropolarimetric observations analyzed with the Zeeman Doppler imaging (ZDI) technique routinely measure the large-scale magnetic fields of active solar-like stars. Due to the cancelation of opposite field polarities, polarimetry is however blind to any small-scale component of the field. Zeeman broadening in high-resolution intensity spectra, by contrast, probes the total magnetic field and can therefore reveal these hidden small-scale components. Kochukhov et al. developed a new magnetic field diagnostic method based on the relative Zeeman intensification of optical atomic lines with different magnetic sensitivity, which bypasses the traditional difficulty of disentangling the Zeeman effect from the other broadening mechanisms which affect spectral lines. By using this new technique and comparing the same stars with spectropolarimetry, they find that ZDI recovers about 1% of the total energy of the magnetic field in the most active solar-like stars and just 0.01% for their least active targets. Their results also suggest that magnetic regions have roughly the same local field strength in all solar-like stars, with the filling factor of such regions systematically increasing with stellar activity.