Energy Transport in the Hydrogen Ionization Zone of Giant Stars

The relation of the hydrogen ionization zone to stellar pulsation is discussed. A detailed analysis of the behavior of this zone in SU Draconis, shows that it is an important energy source for driving pulsation. An examination of conditions in other RR Lyrae stars, cepheids, and W Virginis stars suggests that the hydrogen ionization zone is important in determining the phase lag and in driving their oscillation. It is proposed that non-spherical pulsationtype motions, driven by the hydrogen ionization instability, are present in all the late-type giants and supergiants and provide the principnl means of energy transport in the hydrogen ionization zone of such stars. These motions could account for indications of variability and supersonic velocities in their photospheres. Numerical calculations have been initiated to develop these ideas further. It is also shown that the dissociation transition of molecular hydrogen may provide the heat sink that is the key to the existence of the class of cool variables that include the Mira and semiregular red variables. (auth)