Abstract
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 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 characterizing 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.
Highlights
The brightest member of the rapidly oscillating Ap class of variable stars is α Circini (HD 128898; HR 5463)
Since our aim is to benchmark High Resolution Spectrograph (HRS) against other high resolution spectrographs used for asteroseismology of rapidly oscillating Ap (roAp) stars, we have extracted data from the European Southern Observatory (ESO) archive of α Cir taken with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph (R ∼ 115 000)
We have clearly demonstrated that Southern African Large Telescope (SALT) HRS is wellsuited for the detection and characterisation of pulsations in Ap stars
Summary
The brightest member of the rapidly oscillating Ap (roAp) class of variable stars is α Circini (HD 128898; HR 5463). New insights from radial velocity studies of spectral lines in the roAp stars were provided by Savanov et al (1999) through the analysis of 14 nights of high-resolution (R ∼ 35 000) observations of γ Equ, a bright wellstudied roAp star They found that pulsation amplitudes were highest in lines of doubly ionised Pr and Nd, while lines of Ba ii and Fe ii were essentially stable. The TESS photometric data are not as sensitive to low-amplitude modes as spectroscopy is, so ground-based time-resolved spectra are still needed to fully exploit these new class members. Frequency analysis of photometric data by Kurtz et al (1994) showed α Cir to pulsate with a dominant frequency of 211 d−1 (2442 μHz, P = 6.8 min), with several other low amplitude frequencies present Their multisite campaign enabled them to deduce a rotation period of 4.48 d through identification of rotationally split side lobes to the principal peak. The analysis of the HARPS data used ‘integral’ RV measurements which, providing higher precision, remove information about individual lines, and atmospheric height (Hatzes & Mkrtichian 2004)
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