Fully Nonadiabatic Analysis of Vibrational Instability of Population III Stars due to the ɛ-Mechanism

Abstract

A linear nonadiabatic analysis of the vibrational stability of population III main-sequence stars was carried out. It was demonstrated that, in the case of massive stars with M > ∼ 5 M ⊙ , helium burning (triple alpha reaction) starts during the main-sequence stage and produces 12 C, leading to the activation of a part of the CNO-cycle. It was found that, despite of that, those stars with M < ∼ 13M ⊙ become unstable against the dipole g 1 -and g 2 -modes due to the ε-mechanism, during the early evolutionary phase at which the pp-chain is still the dominant nuclear energy source. The instability due to the ε-mechanism occurs against g-modes having a large amplitude in the off-centered 3 He accumulation shell in the deep interior, and the growth time is much shorter than the evolutionary timescale. This instability is therefore likely to induce mixing in the stellar interior, and have a significant influence on the evolution of these stars.

The stability of massive main-sequence stars as a function of metallicity

“…However, when the time‐scales are comparable, one must account for phase shifts between the thermodynamic and abundance perturbations (cf. Kawaler 1988; Sonoi & Shibahashi 2012).…”

“…When the timescales associated with the individual reactions are sufficiently short or long compared to the mode period, simple approximations can be made to give appropriate, modified ǫ T,ρ values (as in the treatment of the p-p chain in Unno et al 1989). However, when the timescales are comparable, one must account for phase shifts between the thermodynamic and abundance perturbations (cf., Kawaler 1988;Sonoi & Shibahashi 2012).…”

Nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection

“…Therefore, the convective effects on pulsation are not definitively negligible. In this study, we implement the time-dependent convection (TDC) formula by [13] to the nonadiabatic code by [14], and analyze the pulsational stability of radial modes in hot massive stars. Fig.…”

Dipole low-order g-mode instability of metal-poor low-mass main-sequence stars due to the mechanism

“…As the metallicity and CNO abundance decrease, however, the CNO cycle becomes replaced by the pp chain for more massive stars. In the Population III case, the nuclear energy source is only the pp chain at the zero‐age main sequence (ZAMS) stage for stars with (Sonoi & Shibahashi 2012, hereafter Paper II).…”