Detection of nonlinear resonances among gravity modes of slowly pulsating B stars: results from five iterative prewhitening strategies

Abstract

Context. Slowly pulsating B (SPB) stars are main-sequence multi-periodic oscillators that display non-radial gravity modes. For a fraction of these pulsators, 4-year photometric light curves obtained with the Kepler space telescope reveal period spacing patterns from which their internal rotation and mixing can be inferred. In this inference, any direct resonant mode coupling has usually been ignored so far. Aims. We re-analysed the light curves of a sample of 38 known Kepler SPB stars. For 26 of those, the internal structure, including rotation and mixing, was recently inferred from their dipole prograde oscillation modes. Our aim is to detect direct nonlinear resonant mode coupling among the largest-amplitude gravity modes. Methods. We extract up to 200 periodic signals per star with five different iterative prewhitening strategies based on linear and nonlinear regression applied to the light curves. We then identify candidate coupled gravity modes by verifying whether they fulfil resonant phase relations. Results. For 32 of 38 SPB stars we find at least 1 candidate resonance that is detected in both the linear and the best nonlinear regression model fit to the light curve and involves at least one of the two largest-amplitude modes. Conclusions. The majority of the Kepler SPB stars reveal direct nonlinear resonances based on the largest-amplitude modes. These stars are thus prime targets for nonlinear asteroseismic modelling of intermediate-mass dwarfs to assess the importance of mode couplings in probing their internal physics.

Towards a systematic treatment of observational uncertainties in forward asteroseismic modelling of gravity-mode pulsators

“…Whereas other codes prefer (a variant of) the Lomb-Scargle algorithm (Scargle 1982;Press & Rybicki 1989) for calculating the periodogram of a light curve (see e.g. Degroote et al 2009;Antoci et al 2019;Van Beeck et al 2021). In this work we utilize the former method, such that our results are fully consistent and verifiable with the Period04 software package.…”

“…In reality, however, the majority of coherent pulsators are not observed to have perfectly periodic pulsation modes (see e.g. Bowman et al 2016;Van Beeck et al 2021), which introduces additional uncertainties in the practical analysis of a pulsating star's light curve.…”

Rossby numbers and stiffness values inferred from gravity-mode asteroseismology of rotating F- and B-type dwarfs: consequences for mixing, transport, magnetism, and convective penetration

“…For the SPB stars, however, Van Beeck et al ( 2021) found 4 of the 26 SPB stars to have light curves with outbursts, similar to those observed in pulsating Be stars and interpreted in terms of GIWs (e.g., Neiner et al 2012Neiner et al , 2020. Following Van Beeck et al (2021) we divide the 26 SPB pulsators into 3 groups:…”

“…We rely on the residual light curves of the 37 γ Dor stars computed by Van Reeth et al (2015). For the 26 SPB stars, we take the residual light curve after prewhitening from the method that gave the highest scaled fractional variance among the five methods exploited by Van Beeck et al (2021). We compute the amplitude spectra of these 63 residual light curves.…”

“…Stochastically-excited GIWs have recently been detected in rapidly rotating Be stars (Neiner et al 2012(Neiner et al , 2020. Finally, nonlinear resonant mode coupling may also be an efficient excitation and energy exchange mechanism in dwarfs (e.g., Goupil & Buchler 1994;Buchler et al 1997), with promising recent observational detections for SPB stars by Van Beeck et al (2021).…”