MAXIMUM RATES OF PERIOD CHANGE FOR DA-WHITE DWARF MODELS WITH CARBON AND OXYGEN CORES

Observational investigations of the rates of period change in several DAV (= ZZ Ceti) stars are fast approaching theoretically interesting limits. Although the known observational errors are still large, in three of these objects the best current observational values for rates of period change (= PI) are of order 10(-14) s s-1-substantially larger than published calculations for carbon core white dwarf models (1 to 4 x 10(-15) s s-1). In this Letter, we explore the maximum rates of period change expected from theoretical evolutionary models of 0.5 and 0.6 M. DAV stars with core compositions (carbon and oxygen) suggested by standard evolutionary theory. The largest rates of period change occur for nontrapped modes in models near the observed blue edge of the DAV instability strip (T(eff) approximately 13,000 K). As expected from simple scaling arguments, models with pure oxygen core composition have the largest rates of period change, up to 9 x 10(-15) s s-1. Trapped modes have PI values ranging from 1 to 4 x 10(-15) s s-1, about half the values we obtain for nontrapped modes. Observationally, we find that the hottest DAVs have power spectra dominated by short-period (approximately 100-300 s) low-amplitude modes-a result of short thermal time scales in the driving region-these short-period modes are not trapped in the models. Thus, observational rates of period change in hot DAVs may be more easily detected than previously suspected. Our results also suggest that measured rates of period change may provide a useful probe of mode trapping in the DAV instability strip.