Time series photometry of the helium atmosphere pulsating white dwarf EC 04207-474

TL;DR

This study analyzes 71 hours of photometric data of the pulsating white dwarf EC 04207-4748, identifying multiple eigenmodes and modeling nonlinear flux variations to estimate the star's convective response time.

Contribution

It provides the first detailed Fourier analysis and nonlinear modeling of EC 04207-4748, estimating its convective response time and linking it to stellar surface temperature.

Findings

Four independent pulsation modes identified.

Dominant mode period of 447 seconds.

Convective response time estimated at ~150 seconds.

Abstract

We present the analysis of 71 hours of high quality time-series CCD photometry of the helium atmosphere pulsating white dwarf (DBV) EC 04207-4748 obtained using the facilities at Mt John University Observatory in New Zealand. The photometric data set consists of four week-long observing sessions covering the period March to November 2011. A Fourier analysis of the lightcurves yielded clear evidence of four independent eigenmodes in the star with the dominant mode having a period of 447 s. The lightcurve variations exhibit distinct nonsinusoidal shapes, which results in significant harmonics of the dominant frequency appearing in the Fourier transforms. These observed variations are interpreted in terms of nonlinear contributions from the energy flux transmission through the subsurface convection zone in the star. Our modelling of this mechanism, using the methods first introduced by Montgomery (2005), yields a time-averaged convective response time of tau_0 ~ 150 s for the star, and this is shown to be broadly consistent with a MLT/alpha parameter value between 0.8 and 1.2. It is argued that for the DBV pulsators the measured value of tau_0 is a better estimate of the relative stellar surface temperatures than those obtained via spectroscopic techniques.