Understanding the Cool DA White Dwarf Pulsator, G29−38

The white dwarfs are promising laboratories for the study of cosmochronologyand stellar evolution. Through observations of the pulsating white dwarfs, wecan measure their internal structures and compositions, critical tounderstanding post main sequence evolution, along with their cooling rates,allowing us to calibrate their ages directly. The most important set of whitedwarf variables to measure are the oldest of the pulsators, the cool DAVs,which have not previously been explored through asteroseismology due to theircomplexity and instability. Through a time-series photometry data set spanningten years, we explore the pulsation spectrum of the cool DAV, G29-38 and findan underlying structure of 19 (not including multiplet components) normal-mode,probably l=1 pulsations amidst an abundance of time variability and linearcombination modes. Modelling results are incomplete, but we suggest possiblestarting directions and discuss probable values for the stellar mass andhydrogen layer size. For the first time, we have made sense out of thecomplicated power spectra of a large-amplitude DA pulsator. We have shown itsseemingly erratic set of observed frequencies can be understood in terms of arecurring set of normal-mode pulsations and their linear combinations. Withthis result, we have opened the interior secrets of the DAVs to futureasteroseismological modelling, thereby joining the rest of the known whitedwarf pulsators.Comment: 29 pages including 5 figures To appear in ApJ 1 Mar 9