Evidence for Temperature Change and Oblique Pulsation from Light Curve Fits of the Pulsating White Dwarf GD 358

TL;DR

This study uses nonlinear light curve fitting to analyze pulsating white dwarf GD 358, revealing evidence of temperature changes, a thinner convection zone in 1996, and the first detection of oblique pulsation in such stars.

Contribution

It presents the first evidence of oblique pulsation in a white dwarf and links convection zone variations to temperature changes through nonlinear light curve modeling.

Findings

Thinner convection zone in 1996 linked to higher surface temperature.

First detection of oblique pulsation in a white dwarf.

Temperature change correlates with pulsation mode variations.

Abstract

Convective driving, the mechanism originally proposed by Brickhill (1991, 1983) for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the DBV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope (WET); this data set was multiperiodic, containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358's convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.