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

TL;DR

This study investigates the uncorrelated amplitude and frequency variations of harmonics in the SX Phoenicis star XX Cygni, revealing complex behaviors that challenge traditional assumptions in asteroseismology.

Contribution

It presents the first detailed analysis of harmonic amplitude and frequency variations in XX Cygni, highlighting their uncorrelated nature and potential implications for asteroseismic studies.

Findings

Harmonics dominate the periodogram of XX Cygni.

Amplitude and frequency of harmonics vary over time.

Harmonics show uncorrelated amplitude and frequency variations.

Abstract

Harmonics are quite common in pulsating stars, and they are always considered to mimic the behaviors of their independent parent pulsation modes and not taken as the key information for asteroseismology. Here, we report an SX Phoenicis star XX Cygni, whose periodogram is dominated by the fundamental frequency $f_{0} = 7.41481 \pm 0.00004\ \mathrm{cd}^{-1}$ and its 19 harmonics. According to the analysis of the archival data from \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 by neighbouring 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 the further research of this hidden corner in current asteroseismology.