K2 photometry on oscillation mode variability: the new pulsating hot B subdwarf star EPIC 220422705

TL;DR

This study analyzes oscillation mode variability in the pulsating hot B subdwarf star EPIC 220422705 using K2 photometry, revealing complex frequency patterns, rotation periods, and amplitude modulations linked to nonlinear mode interactions.

Contribution

First detailed analysis of mode variability and nonlinear interactions in a hot B subdwarf star using K2 data, linking observations to stellar oscillation theory.

Findings

Detected 66 significant frequencies including multiplets.

Derived core and surface rotation periods of approximately 36 and 29 days.

Observed intrinsic and periodic amplitude and frequency modulations.

Abstract

We present an analysis of oscillation mode variability in the hot B subdwarf star EPIC~220422705, a new pulsator discovered from $\sim78$~days of {\em K}2 photometry. The high-quality light curves provide a detection of 66 significant independent frequencies, from which we identified 9 incomplete potential triplets and 3 quintuplets. Those {\sl g-} and {\sl p-}multiplets give rotation periods of $\sim$ 36 and 29 days in the core and at the surface, respectively, potentially suggesting a slightly differential rotation. We derived a period spacing of 268.5\,s and 159.4\,s for the sequence of dipole and quadruple modes, respectively. We characterized the precise patterns of amplitude and frequency modulations (AM and FM) of 22 frequencies with high enough amplitude for our science. Many of them exhibit intrinsic and periodic patterns of AM and FM, with periods on a timescale of months as derived by the best fitting and \texttt{MCMC} test. The nonlinear resonant mode interactions could be a natural interpretation for such AMs and FMs after other mechanisms are ruled out. Our results are the first step to build a bridge between mode variability from {\em K}2 photometry and nonlinear perturbation theory of stellar oscillation.