ON THE PULSATION OF VARIABLE STARS

Abstract

The present thesis is an attempt to study some aspects of trie pulsation theory of the variable stars. Keeping in view the theoretical developments on the structure and evolution of stars, a stellar model of 15.6 solar masses in the helium burning phase of its evolution has been selected to see if it could be a representative model of the pulsating cepheids. After calculating the inner structure of the model in detail, it has been studied for its pulsation properties on the basis of the Eddington's hypothesis of small radial oscillations. The periods of pulsation of the model in the fundamental and the next three higher modes of pulsation have be -n calculated. Skewness and shape of the radial velocity curve of the model are det ermined on the basis of the theory of anharmonic pulsations The effect of the radiation pressure, which is quite significant and a variable quantity in the model, on the pulsation periods and the shape and skewness of the radial velocity curve has been studied. An attempt has also been made to study the effects of the inclusion of the different number of higher modes, the inclusion of second and third order terms, and the variation of the amplitude of pulsat ion on the shape and skewness of the radial velocity curve. -ii- The capacity of the nuclear energy sources in the interior of the star in sustaining the pulsations is investigated. Inclusion of non-adiabatic effects of the outer layers and the contribution of the region of second helium ionization towards the pulsational stability of the model and the observed phase lag of the model has also been explored. An attempt has also been made to see which mode out of the various modes of pulsation considered will be most favoured for pulsations. A detailed introduction to the present work and a chapter-wise summary of the contents of the thesis is given in section 1.7 of Chapter I.