Vibrational instability of Population III very massive main-sequence stars due to the -mechanism

ABSTRACT Very massive stars are thought to be formed in the early Universe because of a lack of cooling process by heavy elements, and might have been responsible for the later evolution of the Universe. We had an interest in the vibrational stability of their evolution and have carried out a linear non-adiabatic analysis of radial and non-radial oscillations for Population III very massive main-sequence stars with . We found that only the radial fundamental mode becomes unstable due to the -mechanism for these stars. The instability appears just after the CNO cycle is activated and the nuclear energy generation rate becomes large enough to stop the pre-main-sequence contraction, and continues during the early stage of core hydrogen burning. Also, we have roughly estimated the amount of mass loss due to the instability to evaluate its significance.

Surface and volumetric effects in a fluid subjected to high-frequency vibration

In this article, the results of experimental study of surface and volumetric effects arising in a fluid under the action of high-frequency vibration are considered. Some elements in the theory of the surface effects are analyzed with regard to their origin and interrelations with other similar phenomena. The height of splashing fountains on the fluid surface is assessed based on the analogy with particle bounce over a vibrating plane. It is found that the influence of vibration on the average velocity of fluid flow is more pronounced at such excitation parameters that enhance the surface effects under study, namely, the formation of cellular structures (Faraday ripple), the creation of turbulence in the fluid surface layer and splashing. The analytical study of motion of a bubble in an oscillating volume of a fluid, saturated with gas at a certain depth, is provided, with compressibility of both the bubble and surrounding gas-saturated fluid layer being taken into account. The condition of bubble sinking in such a compressible medium and the condition of vibrational instability of the separate state of the gas–fluid system are determined.