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KIC 2857323: A Double-mode High-amplitude δ Scuti Star with Amplitude Decline of the First Overtone Mode

We investigate the pulsating behavior of KIC 2857323 using high-precision observations from the Kepler mission. Fourier analysis of 4 yr time-series data reveals five independent frequencies for the light variation. Among them, two strong frequencies f 1 and f 3 with a period ratio of 0.774 identify this star as a double-mode (i.e., the fundamental mode F0 and first overtone mode F1) high-amplitude δ Scuti star (HADS). Seismic modeling using the two radial modes F0 and F1 indicates that KIC 2857323 is a main-sequence star with mass M = 1.78 ± 0.02 M ⊙ and metallicity Z from 0.009 to 0.012. We analyze the phase and amplitude variations of F0 and F1 using the phase modulation method and find that the first overtone mode F1 shows a slow decline in amplitude. We discuss several possible causes for the amplitude variation and speculate that the amplitude decline in this star may be due to pulsation energy loss. We note that KIC 2857323 is the first double-mode HADS to show amplitude decline and warrants further study to ascertain its nature.