The Complicated Case of δ Scuti Pulsations and Rotation in KIC 6951642; a long-orbit Single-lined Spectroscopic Binary Star

Abstract: More than four years of HERMES observations have confirmed KIC 6951642 is a very long orbit (≈1770 d) single-lined spectroscopic binary (F0-type) with a fast-rotating companion (vsin i = 123±3 Km/s). The Fourier spectrum of its four-year photometric observations includes plenty of significant frequencies (594) in low- and high-frequency regions. The high-frequency modes appear with various time-delay patterns. We detected several rotationally split 𝛿 Scuti pulsations centered at 13.96 per day (and average frequency spacing of Δ𝑓= 0.723±0.006 per day) for KIC 6951642. The detailed study of all significant low frequencies, extended from 0.72 to 3.60 per day, revealed that the two most dominant frequencies (with the same amplitude and larger than of p-modes) are a combina3on the lowest-frequency modes (𝑓₃ = 𝑓< 0.17 per day), i.e. 𝑓_orhrm + 𝑚𝑓_orhrm (𝑚 = 12,14). We suggest the lowest-frequency modes are very large harmonics (orders of 10) of orbital frequency (≈0.0006 per day). We verified the other most dominant low-frequencies as harmonics of rotation frequency 0.721 per day and its combinations. Finally, we reject the probability of hybrid pulsations in the fast-rotating companion of KIC 6951642. We introduce it as a 𝛿 Scuti pulsator with a candidate rotation frequency of 0.721 per day.