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S.J. Kleinman (Iowa St. Univ.)
The size of the surface hydrogen layer mass in the DA white dwarfs is a critical issue: it is a key ingredient in determining the evolution to the white dwarf stage, it directly affects the ages we get for the coolest white dwarfs, and it is crucial to our ability to explain the observed distribution of white dwarf spectral types along the white dwarf cooling sequence. We provide three distinct lines of asteroseismological evidence that favor thick (\approx 10-4 -- 10-7M\sun) H layers in the DAs: 1) A similarity in mode structures of all the DAVs which requires a similarity in their structure and hence, their H layers. The easiest way to get such a uniform H layer is to have one determined by the remains of nuclear shell-burning, leaving a layer \approx 10-4M\sun for an average mass white dwarf. 2) Tentative model fitting of the similar structure unveiled through point 1) requires thick layers for a reasonable match. And 3) the best asteroseismological fits to individual DAVs all require thick layers. We present new data on G191--16, G38--29, and HL~Tau~76 and compare them to published data on other DAVs as well as with recent work on G29--38.