Spectroscopic detection of Altair’s non-radial pulsations

Abstract

Context. Rapid rotation is a common feature of early-type stars but remains a challenge for models. Nevertheless, we need to understand its effect on stellar evolution in order to interpret the observed properties of numerous stars. Aims. We wish to provide more observational constraints on the properties of fast rotating stars, especially their oscillation modes. Methods. We focus on the nearby star Altair which is known to be a very rapidly rotating star with an equatorial velocity recently estimated at 313 km s−1. We observed this star with the high-resolution spectropolarimeter Neo-Narval over six nights, with one night of interruption, in September 2020. Results. We detect significant line profile variations on the mean line profile of the spectra. Their time-frequency analysis shows that these variations are induced by gravito-inertial waves propagating at Altair’s surface with azimuthal wavenumbers of the order m = 10 − 15. With a preliminary computation of the eigenspectrum using the most recent concordance model of Altair we carried out a first modelling of the observed waves. Conclusions. Altair was known as the brightest δ Scuti star. We now see that it is the brightest hybrid oscillating star with gravito-inertial waves and acoustic waves being excited. Clearly, more observations and more advanced models are needed to explain the observations in greater details.