Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Serenelli

et al. 2016

A&A

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Context. Many low-mass (M /M < ∼ 0.45) and extremely low-mass (ELM, M /M < ∼ 0.18−0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars. Aims. Motivated by this finding, we present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Methods. Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M , which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Results. We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the T eff − log g diagram, where low-mass pre-white dwarfs are currently found. The destabilized modes are radial and nonradial p and g modes excited by the κ − γ mechanism acting mainly at the zone of the second partial ionization of He, with non-negligible contributions from the region of the first partial ionization of He and the partial ionization of H. The computations with element diffusion are unable to explain the pulsations observed in the two known pulsating pre-white dwarfs, suggesting that element diffusion might be inhibited at these stages of the pre-white dwarf evolution. Our nonadiabatic models without diffusion, on the other hand, naturally explain the existence and range of periods of the pulsating pre-white dwarf star WASP J1628+10B, although they fail to explain the pulsations of WASP J0247−25B, the other known member of the class, indicating that the He abundance in the driving region of this star might be substantially higher than predicted by our models. Conclusions. Discoveries of additional members of this new class of pulsating stars and their analysis in the context of the theoretical background presented in this paper will shed new light on the evolutionary history of their progenitor stars.

Section: Discussionmentioning

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Section: Numerical Codesmentioning

confidence: 99%

Section: Computations Neglecting Element Diffusionmentioning

confidence: 99%

Section: Computations Neglecting Element Diffusionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Serenelli

et al. 2016

A&A

Self Cite

Pulsating white dwarfs: new insights

Bertolami

et al. 2019

Astron Astrophys Rev

194

224

Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components -including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from "extra dimensions", and even crystallization.The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has

Pulsations powered by hydrogen shell burning in white dwarfs

Camisassa

et al. 2016

A&A

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Context. In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarf models evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. Aims. We investigate the pulsational stability of white dwarf models with thick envelopes to see whether nonradial g-mode pulsations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up during the asymptotic giant branch evolution of their progenitor stars. Methods. We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally pulsing and the post-asymptotic giant branch phases, and analyze their pulsation stability by solving the linear, nonadiabatic, nonradial pulsation equations for the models in the range of effective temperatures T eff ∼ 15 000 − 8 000 K. Results. We demonstrate that, for white dwarf models with masses M ⋆ 0.71 M ⊙ and effective temperatures 8 500 T eff 11 600 K that evolved from low-metallicity progenitors (Z = 0.0001, 0.0005, and 0.001), the dipole (ℓ = 1) and quadrupole (ℓ = 2) g 1 modes are excited mostly due to the hydrogen-burning shell through the ε-mechanism, in addition to other g modes driven by either the κ − γ or the convective driving mechanism. However, the ε mechanism is insufficient to drive these modes in white dwarfs evolved from solar-metallicity progenitors. Conclusions. We suggest that efforts should be made to observe the dipole g 1 mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant branch phase.