SALT HRS capabilities for time resolved pulsation analysis: a test with the roAp star $α$ Circini

Spectroscopy is a powerful tool for detecting variability in the rapidly oscillating Ap (roAp) stars. The technique requires short integrations times and high resolution, and so is limited to only a few telescopes and instruments. To test the capabilities of the High Resolution Spectrograph (HRS) at the Southern African Large Telescope (SALT) for the study of pulsations in roAp stars, we collected 2.45 hr of high-resolution data of the well studied roAp star $α$ Cir in a previously unused instrument configuration. We extracted radial velocity measurements using different rare earth elements, and the core of H$_α$, via the cross correlation method. We performed the same analysis with a set of $α$ Cir data collected with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph to provide a benchmark for our SALT HRS test. We measured significant radial velocity variations in the HRS data and show that our results are in excellent agreement between the two data sets, with similar signal-to-noise ratio detections of the principal pulsation mode. With the HRS data, we report the detection of a second mode, showing the instrument is capable of detecting multiple and low-amplitude signals in a short observing window. We concluded that SALT HRS is well-suited for characterising pulsations in Ap stars, opening a new science window for the telescope. Although our analysis focused on roAp stars, the fundamental results are applicable to other areas of astrophysics where high temporal and spectral resolution observations are required.

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