On the Impact of 22 Ne on the Pulsation Periods of Carbon–Oxygen White Dwarfs with Helium-dominated Atmospheres

Abstract

We explore changes in the adiabatic low-order g-mode pulsation periods of 0.526, 0.560, and 0.729 M ⊙ carbon–oxygen white dwarf models with helium-dominated envelopes due to the presence, absence, and enhancement of 22 Ne in the interior. The observed g-mode pulsation periods of such white dwarfs are typically given to 6−7 significant figures of precision. Usually white dwarf models without 22 Ne are fit to the observed periods and other properties. The rms residuals to the ≃150−400 s low-order g-mode periods are typically in the range of σ rms ≲ 0.3 s, for a fit precision of σ rms / P ≲ 0.3%. We find average relative period shifts of Δ P / P ≃ ±0.5% for the low-order dipole and quadrupole g-mode pulsations within the observed effective temperature window, with the range of Δ P / P depending on the specific g-mode, abundance of 22 Ne, effective temperature, and the mass of the white dwarf model. This finding suggests a systematic offset may be present in the fitting process of specific white dwarfs when 22 Ne is absent. As part of the fitting processes involves adjusting the composition profiles of a white dwarf model, our study on the impact of 22 Ne can provide new inferences on the derived interior mass fraction profiles. We encourage routinely including 22 Ne mass fraction profiles, informed by stellar evolution models, to future generations of white dwarf model-fitting processes.