Pulsation properties of DA white dwarfs - Radial mode instabilities

2014

A&A

Context. Many low-mass white dwarfs with masses M * / M < ∼ 0.45, including the so-called extremely low-mass white dwarfs (M * / M < ∼ 0.20−0.25), have recently been discovered in the field of our Galaxy through dedicated photometric surveys. The subsequent discovery of pulsations in some of them has opened the unprecedented opportunity of probing the internal structure of these ancient stars.Aims. We present a detailed adiabatic pulsational study of these stars based on full evolutionary sequences derived from binary star evolution computations. The main aim of this study is to provide a detailed theoretical basis of reference for interpreting present and future observations of variable low-mass white dwarfs. Methods. Our pulsational analysis is based on a new set of He-core white-dwarf models with masses ranging from 0.1554 to 0.4352 M derived by computing the non-conservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star. We computed adiabatic radial ( = 0) and non-radial ( = 1, 2) p and g modes to assess the dependence of the pulsational properties of these objects on stellar parameters such as the stellar mass and the effective temperature, as well as the effects of element diffusion. Results. We found that for white dwarf models with masses below ∼0.18 M , g modes mainly probe the core regions and p modes the envelope, therefore pulsations offer the opportunity of constraining both the core and envelope chemical structure of these stars via asteroseismology. For models with M * > ∼ 0.18 M , on the other hand, g modes are very sensitive to the He/H compositional gradient and therefore can be used as a diagnostic tool for constraining the H envelope thickness. Because both types of objects have not only very distinct evolutionary histories (according to whether the progenitor stars have experienced CNO-flashes or not), but also have strongly different pulsation properties, we propose to define white dwarfs with masses below ∼0.18 M as ELM (extremely low-mass) white dwarfs, and white dwarfs with M * > ∼ 0.18 M as LM (low-mass) white dwarfs.

Section: Interpretation Of Observationsmentioning

confidence: 99%

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

2014

A&A

“…It is interesting to note that radial modes and p-modes with periods between 0.1 and 10 s have been found to be pulsationally unstable in a number of theoretical stability analysis (Saio et al 1983;Kawaler 1993) but, however, these short period modes have not been ever observed in any white dwarf star, although the interest in the topic has been recently renewed ). Finally, we mention that until now, only modes with ℓ = 1 and ℓ = 2 have been identified with high confidence in pulsating white dwarfs.…”

Section: Overview Of Non-radial Pulsationsmentioning

confidence: 99%

Evolutionary and pulsational properties of white dwarf stars

Isern

et al. 2010

Astron Astrophys Rev

368

372

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications to different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants.These potential applications of white dwarfs have led to a renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.

“…For the nonradial computations, the code solves the sixth-order complex system of linearized equations and boundary conditions as given by Unno et al (1989). For the case of radial modes, LP-PUL solves the fourth-order complex system of linearized equations and boundary conditions according to Saio et al (1983) with the simplifications of Kawaler (1993). Our nonadiabatic computations rely on the frozen-convection (FC) approximation, in which the perturbation of the convective flux is neglected.…”

Section: Pulsation Codementioning

confidence: 99%

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

2015

A&A

Context. Low-mass (M /M < ∼ 0.45) white dwarfs, including the so-called extremely low-mass white dwarfs (ELM, M /M < ∼ 0.18−0.20), are being currently discovered in the field of our Galaxy through dedicated photometric surveys. That some of them pulsate raises the unparalleled chance to investigate their interiors. Aims. We present a detailed nonadiabatic pulsational analysis of such stars, employing full evolutionary sequences of low-mass He-core white dwarf models derived from binary star evolution computations. The main aim of this study is to provide a detailed description of the pulsation stability properties of variable low-mass white dwarfs during the terminal cooling branch. Methods. Our nonadiabatic pulsation analysis is based on a new set of He-core white-dwarf models with masses ranging from 0.1554 to 0.4352 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. We computed nonadiabatic radial ( = 0) and nonradial ( = 1, 2) g and p modes to assess the dependence of the pulsational stability properties of these objects with stellar parameters such as the stellar mass, the effective temperature, and the convective efficiency. Results. We found that a dense spectrum of unstable radial modes and nonradial g and p modes are driven by the κ − γ mechanism due to the partial ionization of H in the stellar envelope, in addition to low-order unstable g modes characterized by short pulsation periods that are significantly excited by H burning via the ε mechanism of mode driving. In all the cases, the characteristic times required for the modes to reach amplitudes large enough to be observable (the e-folding times) are always shorter than cooling timescales. We explore the dependence of the ranges of unstable mode periods (the longest and shortest excited periods) with the effective temperature, the stellar mass, the convective efficiency, and the harmonic degree of the modes. We also compare our theoretical predictions with the excited modes observed in the seven known variable low-mass white dwarfs (ELMVs) and found excellent agreement.