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

TL;DR

This study re-analyzed Kepler light curves of 38 SPB stars to detect nonlinear resonant mode coupling among gravity modes, revealing that most stars exhibit such resonances, which are crucial for understanding their internal physics.

Contribution

It introduces a new method combining multiple iterative prewhitening strategies to detect nonlinear mode resonances in SPB stars, highlighting their prevalence.

Findings

Most SPB stars show at least one nonlinear resonance involving large-amplitude modes.

Resonances are detected consistently across different regression models.

Results suggest mode coupling is common and significant in these stars.

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.