Periods of ZZ Ceti variables

White dwarf pulsators (ZZ Ceti variables) osub solar acccur in the extension of the radial pulsation envelope ionization instability strip to the observed luminosities of 3 x 10-3L sub solar according to van Horn. Investigations were underway to see if the driving mechanisms of hydrogen and helium ionization can cause radial pulsations as they do for the Cepheids, the RR Lyrae variables, and the delta Scuti variables. Masses used in this study are 0.60 and 0.75 M sub solar for T/sub e/ between 10,000 K and 14,000 K, the observed range in T/sub e/. Helium rich surface compositions like Y = 0.78,, Z = 0.02 as well as Y = 0.28, Z = 0.02 were used in spite of observations showing only hydrogen lines in the spectrum. The deep layers are pure carbon, and several transition compositions are included. The models show radial pulsation instabilities for many overtone modes at periods between about 0.3 and 3 seconds. The driving mechanism is mostly helium ionization at 40,000 and 150,000 K. The blue edge at about 14,000 K is probably due to the driving region becoming too shallow, and the red edge at 10,000 K is due to so much convection in the pulsation deriving region that no radiative luminosity is available for modulation by the γ and kappa effects. It is speculated that the very long observed periods (100 to 1000 sec) of ZZ Ceti variables are not due to nonradial pulsations, but are possibly aliases due to data undersampling. 4 references