A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

While evolving toward their terminal white-dwarf cooling branches, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could shed some light on the internal structure of these stars through asteroseismology. In this work, we perform a pulsational stability analysis considering the effects of the ε mechanism in destabilizing gravity-mode periods for sequences of low-mass white dwarfs evolving through CNO flashes. We show that the ε mechanism due to stable hydrogen burning can excite low-order (ℓ=1,2) gravity modes with periods between ∼80 and 500 s, for stars with 0.2025≲M⋆/M⊙≲0.3630 located in an extended region of the logg−Teff diagram.