Investigating magnetic activity cycles in solar-like oscillators using asteroseismic data from the K2 mission

Abstract

We present the results of an investigation into the possible presence of magnetic activity cycles in stars observed in two observational campaigns by the K2 mission. This study was based on the KEYSTONE asteroseismic sample of solar-like oscillators, which contained 20 stars for which we were able to determine whether the asteroseismic p-mode frequencies varied in time. These frequency shifts ($\delta\nu$) were determined using a cross-correlation method and using the individual mode frequencies, obtained by fitting power spectra. Three stars were found to exhibit $\delta\nu$ larger than their associated errors ($\sigma_{\delta\nu}$) using both methods, while two more stars exhibited $\delta\nu>\sigma_{\delta\nu}$ when the cross correlation was used and a further two stars exhibited $\delta\nu>\sigma_{\delta\nu}$ when the fitted frequencies were used. When considering the whole sample of 20 stars, the amplitude of $\delta\nu$ showed no dependence on the large frequency separation and metallicity. However, $\delta\nu$ was observed to increase with rotation rate and effective temperature. Our sample contained a number of evolved subgiant stars, allowing us to expand the parameter space usually considered when comparing $\delta\nu$ with stellar parameters. While $\delta\nu$ was small for all of the evolved stars, one was found to have $\delta\nu>\sigma_{\delta\nu}$, raising the possibility that these evolved stars may still exhibit variable magnetic activity.