On the possible existence of short-period g-mode instabilities powered by nuclear burning shells in post-AGB H-deficient (PG1159-type) stars

TL;DR

This study investigates the pulsational stability of post-AGB H-deficient stars, revealing a new instability strip driven by nuclear burning shells that could explain short-period g-modes, with VV 47 as a potential example of dual-mode excitation.

Contribution

It identifies a new instability strip in PG1159 stars caused by the $ ext{ε}$-mechanism, expanding understanding of pulsation driving mechanisms in these stars.

Findings

Confirmed existence of a separate PG1159 instability strip for short-period g-modes.

Found that VV 47's long periods are driven by the $ ext{κ}$-mechanism, short periods by the $ ext{ε}$-mechanism.

Proposed VV 47 could be the first star with simultaneous $ ext{κ}$- and $ ext{ε}$-mechanism pulsations.

Abstract

We present a pulsational stability analysis of hot post-AGB H-deficient pre-white dwarf stars with active He-burning shells. The stellar models employed are state-of-the-art equilibrium structures representative of PG1159 stars derived from the complete evolution of the progenitor stars. On the basis of fully nonadiabatic pulsation computations, we confirmed theoretical evidence for the existence of a separate PG1159 instability strip in the $\log T_{\rm eff} - \log g$ diagram characterized by short-period $g$-modes excited by the $\epsilon$-mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long period $g$-modes destabilized by the classical $\kappa$-mechanism acting on the partial ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159 stars characterized by thick He-rich envelopes and located inside this overlapping region could exhibit both short and intermediate/long periods simultaneously. we study the particular case of VV 47, a pulsating planetary nebula nucleus that has been reported to exhibit a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical $\kappa$-mechanism, while the observed short-period branch below $\approx 300$ s could correspond to modes triggered by the He-burning shell through the $\epsilon$-mechanism, although more observational work is needed to confirm the reality of these short-period modes. Were the existence of short-period $g$-modes in this star convincingly confirmed by future observations, VV 47 could be the first known pulsating star in which both the $\kappa$-mechanism and the $\epsilon$-mechanism of mode driving are simultaneously operating.