Uncorrelated Amplitude and Frequency Variations of the Harmonics in SX Phoenicis Star XX Cygni

Abstract

Harmonics are quite common in pulsating stars. They are always considered to mimic the behaviors of their independent parent pulsation modes, and are not taken for key information in asteroseismology. Here, we report an SX Phoenicis star XX Cygni, whose periodogram is dominated by the fundamental frequency f 0 = 7.41481 ± 0.00004 c day−1 and its 19 harmonics. According to the analysis of the archival data from the Transiting Exoplanet Survey Satellite (TESS), we find that both the amplitudes and frequencies of the fundamental mode and the harmonics vary within TESS Sectors 14–17 and 54–57, which might be caused by the contamination of neighboring stars. What is more interesting is that the harmonics show significantly uncorrelated amplitude and frequency variations over time. Some possible origins and interesting issues are proposed to scheme further research of this hidden corner in current asteroseismology.