The periods of ZZ Ceti variables
White dwarf pulsators (ZZ Ceti variables) occur in the extension of the radial pulsation envelope ionization instability strip to the observed luminosities of 3 × 10−3 L⊙ according to van Horn (1979). Investigations have been 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 δ Scuti variables. Masses used in this study are 0.60 and 0.75 M⊙ and the temperature Te used ranged between 10,000 K and 14,000 K, the observed range in Te. Helium rich surface compositions like Y = 0.78, Z = 0.02 as well as Y = 0.28, Z = 0.02 have been 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 i s due to so much convection in the pulsation driving region that no radiative luminosity is available for modulation by they and κ effects. It is speculated that the very long observed periods (100–1000 sec) of ZZ Ceti variables are not due to nonradial pulsations, but are possibly aliases due to data undersampling.
KeywordsMixed Convection White Dwarf Radial Pulsation Helium Ionization Blue Edge
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