Brightness Fluctuation Spectra of Sun-Like Stars. I. The Mid-Frequency Continuum

We analyze space-based time series photometry of Sun-like stars, mostly in the Pleiades, but also field stars and the Sun itself. We focus on timescales between roughly 1 hour and 1 day. In the corresponding frequency band these stars display brightness fluctuations with a decreasing power-law continuous spectrum. K2 and Kepler observations show that the RMS flicker due to this Mid-Frequency Continuum (MFC) can reach almost 1%, approaching the modulation amplitude from active regions. The MFC amplitude varies by a factor up to 40 among Pleiades members with similar Teff, depending mainly on the stellar Rossby number Ro. For Ro<0.04, the mean amplitude is roughly constant at about 0.4%; at larger Ro the amplitude decreases rapidly, shrinking by about two orders of magnitude for Ro~1. Among stars, the MFC amplitude correlates poorly with that of modulation from rotating active regions. Among field stars observed for 3 years by Kepler, the quarterly average modulation amplitudes from active regions are much more time-variable than the quarterly MFC amplitudes. We argue that the process causing the MFC is largely magnetic in nature, and that its power-law spectrum comes from magnetic processes distinct from the star's global dynamo, with shorter timescales. By analogy with solar phenomena, we hypothesize that the MFC arises from a (sometimes energetic) variant of the solar magnetic network, perhaps combined with rotation-related changes in the morphology of supergranules.