Lifetime of Surface Features and Stellar Rotation: A Wavelet Time-Frequency Approach

We explore subtle variations in disk-integrated measurements spanning ≲18 yr of stellar surface magnetism by using a newly developed time-frequency gapped wavelet algorithm. We present results based on analysis of the Mount Wilson Ca II H and K emission fluxes in four, magnetically active stars (HD 1835 [G2 V], HD 82885 [G8 IV-V], HD 149661 [K0 V], and HD 190007 [K4 V]) and sensitivity tests using artificial data. When the wavelet basis is appropriately modified (i.e., when the time-frequency resolution is optimized), the results are consistent with the existence of spatially localized and long-lived Ca II features (assumed here as activity regions that tend to recur in narrowly confined latitude bands), especially in HD 1835 and HD 82885. This interpretation is based on the observed persistence of relatively localized Ca II wavelet power at a narrow range of rotational timescales, enduring as long as ≳10 yr.