Abstract
An attempt has been made to study the effects of the opacity mechanisms on pulsational instability of some theoretical models having different masses and in different evolutionary stages. Different combinations of the opacity exponentsn, s, and helium-hydrogen ratio,B, have been considered. Based on these combinations, Gamma-mechanism and Kappa-mechanism have been compared. Gamma-mechanism (n=0,s=0) is found to be totally ineffective forB=0.01 in producing a driving force. Even a high value ofB (=0.38) is also not suitable for instability. The combination ofn=0.7,s=2.1, andB=0.25 has been found to give the largest negative dissipation for a 15.6M⊙ star in hydrogen-burning phase although the same does not hold for helium-burning phase. The conditions in which these excitation mechanisms could be most effective have been discussed.
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