Zeeman broadening in cool stars

We investigate detectability of magnetic fields by Zeeman broadening of well-isolated spectral lines in F, G and K type stars. Data of unprecedented quality were taken with CES ¹ mounted on the 3.6m ESO telescope at La Silla, Chile, in three campaigns in the optical range between 5770Å and 6280Å, each with a wavelength coverage of roughly 40Å. We use the SPINOR/STOPRO (cf. Frutiger et al. [1]) package developed by ETH ² and MPS ³ to perform spectral line inversion via χ ² minimization. Starting from constraints given by previous measurements of stellar parameters, we fit a number of extracted spectral lines. Eventually, our goal is to determine the product of the magnetic field strength B and the surface filling factor, B × f. Our work is in progress and thus no final measurements can be presented at this stage.