Multi-campaign asteroseismic analysis of eight solar-like pulsating stars observed by the K2 mission

Abstract

The NASA K2 mission that succeeded the nominal Kepler mission observed several hundred thousand stars during its operations. While most of the stars were observed in single campaigns of ∼80 days, some of them were targeted for more than one campaign. We perform an asteroseismic study of a sample of eight solar-like stars observed during K2 Campaigns 6 and 17, allowing us access to up to 160 days of data. With these two observing campaigns, we determine not only the stellar parameters but also study the rotation and magnetic activity of these stars. We first extract the light curves for the two campaigns using two different pipelines, EVEREST and Lightkurve. The seismic analysis is done on the combined light curve of C6 and C17, where the gap between them was removed and the two campaigns were ‘stitched’ together. We determine the global seismic parameters of the solar-like oscillations using two different methods: one using the A2Z pipeline and the other the Bayesian apollinaire code. With the latter, we also perform the peak-bagging of the modes to characterize their individual frequencies. By combining the frequencies with the Gaia DR2 effective temperature and luminosity, and metallicity for five of the targets, we determine the fundamental parameters of the targets using the IACgrids based on the MESA (Modules for Experiments in Stellar Astrophysics) code. We find that four of the stars are on the main sequence, two stars are about to leave it, and two stars are more evolved (a subgiant and an early red giant). While the masses and radii of our targets probe a similar parameter space compared to the Kepler solar-like stars, with detailed modeling, we find that for a given mass our more evolved stars seem to be older than previous seismic stellar ensembles. We calculate the stellar parameters using two different grids of models, one incorporating and one excluding the treatment of diffusion, and find that the results agree generally within the uncertainties, except for the ages. The ages obtained using the models that exclude diffusion are older, with differences of greater than 10% for most stars. The seismic radii and the Gaia DR2 radii present an average difference of 4% with a dispersion of 5%. Although the agreement is relatively good, the seismic radii are slightly underestimated compared to Gaia DR2 for our stars, the disagreement being greater for the more evolved ones. Our rotation analysis provides two candidates for potential rotation periods but longer observations are required to confirm them.