High-degree modes in the southern pole star: The δ Scuti star σ Octantis

We report on a spectroscopic and photometric study of the δ Scuti variable star σ Octantis, which has been reported to be a hybrid δ Scuti /γ Doradus star previously. A large set of 350 high-resolution spectra were obtained with the 1-metre McLellan Telescope and HERCULES spectrograph at the University of Canterbury Mt John Observatory and these have been supplemented by an analysis of an extensive data set of photometric observations obtained during four Antarctic winters, as well as several TESS sectors of observations. From the spectra, a value of 100 km/s was obtained for the vsini. For the spectra, frequency analysis and mode identfication were carried out with SigSpec and FAMIAS. Many of the dominant frequencies were also seen in the photometry, and variable amplitudes of some frequencies was noted. Notably, the dominant photometric frequency (10.49 c/d) was detected in the spectroscopy, but was not the strongest frequency. Spectroscopic mode identification showed that most modes were prograde high-degree (l ≥ 9) modes. No evidence for a radial mode was found. In order to gain further insights into the pulsation modes in this star, we have started to model this star using iSpec (to refine the fundamental stellar parameters) and MESA/GYRE (to identify and confirm the pulsation frequencies and mode identifications).