Comparative high-resolution spectroscopy of M dwarfs - exploring non-LTE effects

In the search for exoplanets M dwarfs are attractive targets. To be able to assess the habitability of planets around M dwarfs, the atmospheric parameters of the host star need to be accurately determined as well as abundances of individual chemical elements. Recent advances in high-resolution spectroscopy in the near-infrared have opened up the possibility to greatly improve the analysis of M dwarfs. The method of fitting the observed high-resolution spectra with synthetic spectra has recently been applied by Lindgren et al. (2016, 2017) and Passegger et al. (2018, 2019) using different instruments and theoretical approaches. We present a comparison of stellar parameters derived by Lindgren et al. and Passegger et al. for 11 different stars in common between the two studies. The derived effective temperature values agree within uncertainties for most of the stars while the surface gravity and metallicity show a larger spread and for half of the stars the differences are larger than the given uncertainties. We investigate the reasons for the diverging results by comparing synthetic spectra generated for the derived parameters with observed spectra. We present an assessment of non-LTE effects in the parameter range of M dwarfs for K and Fe. We demonstrate that non-LTE effects are negligible for Fe I in M-dwarf atmospheres but are important for K I, which has a number of strong lines in near-infrared spectra of these stars. These effects, leading to potassium abundance and metallicity corrections on the order of 0.2 dex, may be responsible for some of the discrepancies in the published analyses. The aim is to validate and improve the method of fitting synthetic spectra to observed spectra for M dwarfs.