A theoretical study of population II variables withperiods in the range 10 - 20 days (W Virginis variables) ispresented. A modified hydrodynamic Christy code is used inconjunction with the Carson opacities, in preference to the LosAlamos tables, following the work of Carson, Stothers andVemury on the shorter period BL Herculis variables.Twenty-five survey models are presented, along with nine othercomparison models of varying masses and opacities.A study of the observations shows that the division ofthese variables into two types by observers might be explainedby a slightly different mass for each type, thus making thedivision dependent on the star's previous evolution.The non-linear results obtained by this study show that amass of 0.6 M☉ is a good one to use, and that M = 0.5 M☉ makeslittle difference (although M = 0.8 M☉ seems to be too high).The results in general compare well with the observations, asboth also show the split into two types of light curve. Three good models of individual stars are presented, on a par with the models of BL Herculis published by Carson, Stothers andVemury. The bumps in the light and velocity curves of many ofthe models seem to be real, caused by the Christy "echo".A few of the models show some RV Tauri behaviour. One inparticular shows very strange behaviour, involving a violent81ternation of light curve shapes. Models constructed usingthe Los Alamos opacities do not produce results as consistentwith observations as those of the main survey.The study shows that these stars can be represented byhydrodynamic models of mass 0.6 M☉ using the Carson opacities, but also that convection may be important in the cooler stars in order to model them accurately. This indicates thedirection in which further theoretical work may lie.
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A theoretical study of population II Cepheids with periods in the range 10-20 days
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