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

Abstract

Convective driving, the mechanism originally proposed by Brickhill (1991Brickhill ( , 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.

GD 358: The Case for Oblique Pulsation and Temperature Change

“…The oblique pulsation model makes aprediction for the relative amplitudes within each of the triplets in Table 1. These expressions are givenin, e.g., the appendix of Montgomery et al [4]. Finding: The oblique pulsation model provides an excellent fit to the observed amplitudes within each triplet.…”

“…It wast he first DBV to be discovered [1], it wasthe first DAVorDBV to have awell defined period spacing [2], and it wast he first DAVo rD BV to showc lear evidence of triplets for manyd ifferent radial overtones [3]. So it should perhaps come as no surprise that twomore "firsts" have been added to the list: 1) first single white dwarf to showevidence of alarge change in T eff ,a nd 2) first white dwarf to showe vidence of oblique pulsation [4]. We examine these claims below.…”

“…We have since extended this technique to multi-periodic stars [8] and have taken into account the nonlinear relationship between the bolometric and observed flux variations [9,4]. Our approach is well-summarized in Montgomery [5] and Montgomery [9].…”

A survey of pulsating DA and DB white dwarfs Observations with the Whole Earth Telescope

“…structure of the surface layers and the degenerate core ( [4]). A second diagnostic is the presence of multiplets.…”

Concluding remarks

“…The non-linear response of the convection zone results in the pulsating white dwarf having a non-sinusoidal light curve. Montgomery et al [63] extend the analysis in [62] to multiperiodic stars and develop a non-linear technique for fitting non-sinusoidal light curves. An empirical determination of convection parameters is then possible as Provencal et al [64] demonstrate and future applications to pulsating white dwarfs should enable changes to white dwarf convection zones to be mapped as cooling proceeds.…”