Molecules as magnetic probes of starspots

Abstract

Context. Stellar dynamo processes can be explored by measuring the magnetic field. This is usually obtained using the atomic and molecular Zeeman effect in spectral lines. While the atomic Zeeman effect can only access warmer regions, the use of molecular lines is of advantage for studying cool objects. The molecules MgH, TiO, CaH, and FeH are suited to probe stellar magnetic fields, each one for a different range of spectral types, by considering the signal that is obtained from modeling various spectral types. Aims. We have analyzed the usefulness of different molecules (MgH, TiO, CaH, and FeH) as diagnostic tools for studying stellar magnetism on active G-K-M dwarfs. We investigate the temperature range in which the selected molecules can serve as indicators for magnetic fields on highly active cool stars and present synthetic Stokes profiles for the modeled spectral type. Methods. We modeled a star with a spot size of 10% of the stellar disk and a spot comprising either only longitudinal or only transverse magnetic fields and estimated the strengths of the polarization Stokes V and Q signals for the molecules MgH, TiO, CaH, and FeH. We combined various photosphere and spot models according to realistic scenarios. Results. In G dwarfs, the molecules MgH and FeH show overall the strongest Stokes V and Q signals from the starspot, whereas FeH has a stronger Stokes V signal in all G dwarfs with a spot temperature of 3800 K. In K dwarfs, CaH signals are generally stronger, and the TiO signature is most prominent in M dwarfs. Conclusions. Modeling synthetic polarization signals from starspots for a range of G-K-M dwarfs leads to differences in the prominence of various molecular signatures in different wavelength regions, which helps to efficiently select targets and exposure times for observations.