Sounding stellar cycles with Kepler - II. ground-based observations

Abstract

We have monitored 20 Sun-like stars in the Kepler field-of-view for excess flux with the Fibrefed Echelle Spectrograph on the Nordic Optical Telescope since the launch ofKepler spacecraft in 2009. These 20 stars were selected based on their asteroseismic properties to sample the parameter space (effective temperature, surface gravity, activity level, etc.) around the Sun. Though the ultimate goal is to improve stellar dynamo models, we focus the present paper on the combination of space-based and ground-based observations that can be used to test the age- rotation-activity relations. In this paper we describe the considerations behind the selection of these 20 Sun-like stars and present an initial asteroseismic analysis, which includes stellar age estimates. We also describe the observations from the Nordic Optical Telescope and present mean values of measured excess fluxes. These measurements are combined with estimates of the rotation periods obtained from a simple analysis of the modulation in photometric observations from Kepler caused by starspots, and asteroseismic determinations of stellar ages, to test relations between age, rotation and activity. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.