White dwarf pulsators-a study with the whole earth telescope

Abstract

We have presented in this thesis the first observational results on the location of the driving region and the processes involved in white dwarf pulsations. We selected one object from each of the three main instability strips of white dwarfs. The objects chosen were: G 2938 (a DAV), GD 358 (a DBV), and PG 1159 (a DOV). We have analysed longterm photometric data collected collaboratively, and the results of our analysis are summarised below: 1) Period limits of the pulsation spectrum For each star, the lower and upper limits of the observed periods are:

G 2938: Pw = 57 seconds Pg = 333 seconds

GD 358: Pw = 400 seconds Pg = 900 seconds

PG 1159035: Pw = 457 seconds Pg = 1000 seconds

Based on these period limits, we obtain the following locations of the driving regions:

StarPosition of outer layer of the driving region (in stellar mass units)Position of inner layer of the driving region (in stellar mass units)G 29387 × 10¹4 × 10¹²GD 3586 × 10¹²2 × 10¹PG 11590354 × 101 × 10 These results indicate that the driving regions are fairly thin and lie near the surface (> 0.98 R*). Their locations favour partial ionization of the predominant surface element as the driving mechanism-instead of convective blocking (DAV/DBV) or nuclear burning (DOV).

2) Total energy in pulsations We calculated and compared the total pulsation energy for the three stars:

GD 358 has about 1.6 times the pulsation energy of G 2938, consistent with helium partial ionization (DBV) vs. hydrogen partial ionization (DAV). PG 1159035 has about 5.7 times the energy of G 2938 and 3.6 times that of GD 358. This supports C/O partial ionization as the driving mechanism, and rules out nuclear burning as the cause of pulsations in PG 1159035.

6.1 The Onward Journey We outline the immediate and longterm goals that could improve and extend this research. Immediate directions i) Apply alternative reduction techniques to the same datasets (e.g., extinction fits, channel2 division) to reduce atmospheric effects. ii) Fit the predominant peaks in the power spectrum with a suitable distribution to study thermal timescale distributions. For PG 1159035, where modes are identified, compare distributions for = 1 and = 2 modes. iii) Investigate the effect of removing parts of the dataset (since no new data can be added) to understand alias patterns and changes in estimated total pulsation energy.

Longterm goals i) Mode coupling: Study the causes of mode coupling and whether total pulsation energy remains conserved. Understanding how convection and other physical processes affect coupling may provide insight into the red and blue edges of instability strips. G 2938, whose light curve has changed over the last two years, is an ideal future WET candidate. ii) New targets: Select additional objects from each instability strip for future WET observations to determine whether the current results are representative of each class. iii) Asteroseismology: Detailed asteroseismology will help determine whether observed differences arise from differences in mode indices. iv) Theoretical modelling: Test whether theoretical models reproduce the observed period limits. Successful reproduction will yield more accurate stellar parameters. v) Application to broader stellar physics: Extending these asteroseismological techniques to white dwarfs will eventually help in understanding more complex pulsators and stellar evolutionary processes.