Amplitude and frequency variations in PG~0101+039 from K2 photometry -- A pulsating hot B subdwarf star in an unsynchronized binary system

TL;DR

This study analyzes K2 photometry of the pulsating hot B subdwarf PG~0101+039, revealing mode variability, rotation, and binary properties, with implications for stellar nonlinear theory and mode coupling mechanisms.

Contribution

It provides the first detailed analysis of amplitude and frequency variations in PG~0101+039, including mode extraction, rotation measurement, and nonlinear modulation characterization.

Findings

137 independent frequencies extracted

Rotation period estimated at ~8.8 days

Amplitude and frequency modulations linked to nonlinear couplings

Abstract

K2 photometry is suitable for the exploitation of mode variability on short timescales in hot B subdwarf stars, which is important to constrain nonlinear quantities addressed by the stellar theory of high-order perturbation in the future. We analyze the $\sim80$~d high-quality K2 data collected on PG~0101+039 and extract the frequency content of oscillation. We then determine its rotational and orbital properties, as well as characterize the dynamics of amplitude and frequency. The frequencies are extracted from light curves via a standard prewhitening technique. The binary information is obtained from variations both in brightness and radial velocities. Amplitude and frequency modulation of oscillation modes are measured by piece-wise light curves and characterized by EMCMC method. We have extracted 137 independent frequencies in PG~0101+039 and derived period spacing of ~252s and 144s for the dipole and quadruple modes, respectively. We derive a rotation rate of 8.81+-0.06d and ~8.60+-0.16d based on g- and p-mode multiplets, implying a marginally differential rotation with a probability of ~ 60%. We find that the rotation period is much shorter than the orbital period of ~0.57d, indicating that this system is not synchronized. Amplitude and frequency modulation are measurable for 44 frequencies with high enough amplitude, including 12 rotational components. We characterize their modulating patterns and find a clear correlation between amplitude and frequency variation, which is linked to nonlinear resonant couplings. In general, the modulating scale and timescale are on an order of a few dozen of nano hertz and a few tens of days, respectively, whose values are important constraints to future calculations of nonlinear amplitude equations.