Search for Activity Variatons in Kepler Red Giants

The frequencies of Solar acoustic oscillation modes vary in phase with the magnetic activity, shifting to lower frequencies during activity minima and to higher frequencies during activity maxima. On the other hand, the mode powers vary in anti-phase with the solar cycle. Similar analyses have been extended to other main-sequence stars exhibiting similar results. Thus, the frequency shifts and variations in mode power provide valuable diagnostics of magnetic cycles in stars. In this study, we aim to investigate frequency shifts in a sample of over 16,000 Kepler red giants using the cross-correlation function (CCF) method. The CCF method tracks global shifts in the frequency spectra over time by cross-correlating power spectra from different time segments. We also discuss strategies for improving the uncertainties and error estimation in measuring frequency shifts of red giant stars, as they tend to oscillate more slowly compared to main-sequence stars; hence, they are more prone to noise in their frequency spectrum. Using our method, we reproduced the literature results for main-sequence stars, including the Sun. Finally, we illustrate the measured frequency shifts for a few red giant stars that display significant variations.