A theoretical study of population II Cepheids with periods in the range 10-20 days
Abstract
A theoretical study of population II variables with
periods in the range 10 - 20 days (W Virginis variables) is
presented. A modified hydrodynamic Christy code is used in
conjunction with the Carson opacities, in preference to the Los
Alamos tables, following the work of Carson, Stothers and
Vemury on the shorter period BL Herculis variables.
Twenty-five survey models are presented, along with nine other
comparison models of varying masses and opacities.
A study of the observations shows that the division of
these variables into two types by observers might be explained
by a slightly different mass for each type, thus making the
division dependent on the star's previous evolution.
The non-linear results obtained by this study show that a
mass of 0.6 M☉ is a good one to use, and that M = 0.5 M☉ makes
little difference (although M = 0.8 M☉ seems to be too high).
The results in general compare well with the observations, as
both 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 and
Vemury. The bumps in the light and velocity curves of many of
the models seem to be real, caused by the Christy "echo".
A few of the models show some RV Tauri behaviour. One in
particular shows very strange behaviour, involving a violent
81ternation of light curve shapes. Models constructed using
the Los Alamos opacities do not produce results as consistent
with observations as those of the main survey.
The study shows that these stars can be represented by
hydrodynamic 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 the
direction in which further theoretical work may lie.
Type
Thesis, PhD Doctor of Philosophy
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