2024MNRAS.528.2428K

The statistical technique 'Complex Demodulation' (CDM) is used to track the amplitude and phase changes of periodicities in five naked T Tauri stars. The periodicities are most likely caused by dark spots on the stellar surfaces, which are rotated into and out of view. Two of the stars (CD-56 1438, CD-72 248) show two independent periodicities, probably due to being binary weak-lined T Tauri stars. Two different low-pass filters, operating, respectively, in the frequency and time domains, are used as part of the CDM methodology. Statistical aspects of the estimated amplitudes and phases are investigated in some detail: in particular, expressions are derived for standard errors and for possible biases. A large variety of different types of amplitude and phase changes are found, including approximately linear or quadratic, abrupt level shifts, pulses, and oscillatory. Long term changes in amplitudes are aperiodic, but mimic long-term cycles.