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Soft X-ray emission as diagnostics for Maunder minimum stars

Abstract The identification of stars in a Maunder minimum state purely from their chromospheric emission (for example in Ca II lines) has proven to be difficult. Photospheric contributions, metallicities and possible deviations from the main sequence stage may lead to very low values of the traditional chromospheric activity indicators, while no Maunder minimum state may be present. X-ray observations can be a key tool for identifying possible Maunder minimum stars: We have detected very soft X-ray emission from low-temperature coronal plasma, similar to emission from solar coronal holes, in several stars with very low chromospheric activity indicators. The coronal properties inferred from X-ray observations can therefore yield a crucial piece of information to verify Maunder minimum states in stars.