A search for long-term variations in asteroseismic parameters of Kepler red giants

Despite decades of observations of the solar cycle, the underlying mechanism is not entirely understood. Observation of activity cycles in stars that evolved further from the main sequence, such as red giants, may help put additional constraints on the dynamo models. The surface magnetic fields sustained by the dynamo are known to affect the oscillations inside the stars. Observations of the Sun have shown that the oscillation frequencies change in phase while the mode amplitudes change out of phase with the solar cycle. Similar observations have been made on other solar-type stars using asteroseismology to detect activity cycles. Using the long-cadence photometric data from Kepler, we analysed the long-term trends in the global asteroseismic parameters of 16,094 red giants with solar-like oscillations. We identified stars displaying significant variation in these parameters by studying the long-term trends and computed their mean frequency for each Kepler quarter using a cross-correlation method. We estimated the error bars of these shifts using Monte Carlo simulations. The expected frequency shifts for red giants were predicted to be a few μHz, but our results show the shifts to be around 1/100-1/10th of a μHz. Additionally, we found that the estimated error bars are significantly larger than the quarterly variations in frequency shifts, making it challenging to detect activity cycles in red giants.