Interiors Of White Dwarf Stars Research Articles

Exsolution process in white dwarf stars

Context. White dwarf stars are considered to be suitable cosmic laboratories for studying the physics of dense plasma. Furthermore, the use of white dwarf stars as cosmic clocks to date stellar populations and main sequence companions demands an appropriate understanding of the physics of white dwarfs in order to provide precise ages for these stars. Aims. We aim to study exsolution in the interior of white dwarf stars, a process in which a crystallized ionic binary mixture separates into two solid solutions with different fractions of the constituents. Depending on the composition of the parent solid mixture, this process can release or absorb heat, thus leading to a delay or a speed-up of white dwarf cooling. Methods. Relying on accurate phase diagrams for exsolution, we modeled this process in hydrogen(H)-rich white dwarfs with both carbon–oxygen (CO) and oxygen–neon (ONe) core composition, with masses ranging from 0.53 to 1.29 M⊙ and from 1.10 to 1.29 M⊙, respectively. Results. Exsolution is a slow process that takes place at low luminosities (log(L/L⊙)≲ − 2.75) and effective temperatures (Teff ≲ 18 000 K) in white dwarfs. We find that exsolution begins at brighter luminosities in CO than in ONe white dwarfs of the same mass. Massive white dwarfs undergo exsolution at brighter luminosities than their lower-mass counterparts. The net effect of exsolution on white dwarf cooling times depends on the stellar mass and the exact chemical profile. For standard core chemical profiles and preferred assumptions regarding miscibility gap microphysics, the cooling delay can be as large as ∼0.35 Gyr at log(L/L⊙)∼ − 5. We neglect any chemical redistribution possibly associated with this process, which could lead to a further cooling delay. Although the chemical redistribution is known to accompany exsolution in binary solid mixtures on Earth, given the solid state of the matter, it is hard to model in a reliable way, and its effect may be postponed until very low luminosities. Conclusions. Exsolution has a marginal effect on white dwarf cooling times and, accordingly, we find no white dwarf branches associated with it on the Gaia color–magnitude diagram. However, exsolution in massive white dwarfs can alter the faint end of the white dwarf luminosity function, thus impacting white dwarf cosmochronology.

Exploring the internal rotation of the extremely low-mass He-core white dwarf GD 278 with TESS asteroseismology

Context. The advent of high-quality space-based photometry, brought about by missions such as Kepler/K2 and TESS, makes it possible to unveil the fundamental parameters and properties of the interiors of white dwarf stars, particularly extremely low-mass white dwarfs, using the tools of asteroseismology. Aims. We present an exploration of the internal rotation of GD 278, the first known pulsating extremely low-mass white dwarf to show rotational splittings within its periodogram. Methods. We assessed the theoretical frequency splittings expected for different rotation profiles and compared them to the observed frequency splittings of GD 278. To this aim, we employed an asteroseismological model representative of the pulsations of this star, obtained by using the LPCODE stellar evolution code and the LP-PUL non-radial pulsation code. We also derived a rotation profile that results from detailed evolutionary calculations carried out with the MESA stellar evolution code and used it to infer the expected theoretical frequency splittings. Results. We find that the best-fitting solution when assuming linear profiles for the rotation of GD 278 leads to angular velocity values at the surface and center that are only slightly differential, and still compatible with rigid rotation. Additionally, the values of the angular velocity at the surface and the center for the simple linear rotation profiles and for the rotation profile derived from evolutionary calculations are in very good agreement. Also, the resulting theoretical frequency splittings are compatible with the observed frequency splittings, in general, for both cases. Conclusions. The results obtained from the different approaches followed in this work to derive the internal rotation of GD 278 agree. The fact that they were obtained by employing two independent stellar evolution codes gives our results robustness. Our results suggest only a marginally differential behavior for the internal rotation in GD 278 and, considering the uncertainties involved, this is very compatible with the rigid case, as has been observed previously for white dwarfs and pre-white dwarfs. The rotation periods derived for this star are also in line with the values determined asteroseismologically for white dwarfs and pre-white dwarfs in general.

Multi-dimensional Instability of Dust Acoustic Waves in Magnetized Quantum Plasmas with Positive or Negative Dust

A survey of dust acoustic waves (DAWs) in magnetized dusty plasmas composed of negatively or positively charged mobile dust and intertialess quantum electrons as well as ions is presented. The Zakharov-Kuznetsov (ZK) equation is derived via reductive perturbation technique. The solitary wave solution of ZK equation is given and the numerical analysis of the solution with parameters in dust crystals is performed to study the properties of the DAWs with positive or negative dust. The multi-dimensional instability of these solitary waves is investigated via small-k perturbation method. The instability criterion and growth rate relying on obliqueness, the density ratio between ions and dust, the dust cyclotron frequency, the Fermi temperature ratio between electrons and ions, and quantum diffraction parameter are discussed for both cases. The implications of these results to the interior of white dwarf stars and magnetars have been briefly discussed.

ION SOLITARY PULSES IN WARM PLASMAS WITH ULTRARELATIVISTIC DEGENERATE ELECTRONS AND POSITRONS

The nonlinear propagation of ion solitary pulses in a warm collisionless electron-positron-ion plasma with ultrarelativistic degenerate electrons and positrons has been investigated. Arbitrary and small- (but finite-) amplitude ion solitary pulses are investigated by deriving the Korteweg-de Vries equation and an energy-balance-like expression involving a Sagdeev-like pseudopotential. The existence regions for ion solitary pulses have been precisely defined and numerically investigated. The ion solitary pulse profiles are also displayed. Applications to the interior of white dwarf stars and the corona of magnetars are discussed.

Alfvén solitons in a Fermionic quantum plasma

The propagation of Alfvén envelope solitons through a Fermionic quantum plasma is considered. Starting from the governing equations for Hall magnetohydrodynamics including quantum corrections, coupled Zakharov-type equations are derived for circularly polarized Alfvén waves. The equations are numerically solved for time-independent and time-dependent cases. The time-independent case shows that variations in density take the form of dressed density solitons in which an approximately Gaussian peak is surrounded by smaller sinusoidal variations in the density envelope. The mathematical basis for this behavior is explained. A limited time-dependent case is obtained which uses the numerical time-independent soliton solutions as the initial conditions. This confirms that the soliton solutions retain the same profile as they propagate. The relevance of this work to dense astrophysical plasmas like the interiors of white dwarf stars is discussed.

THE PHYSICS OF CRYSTALLIZATION FROM GLOBULAR CLUSTER WHITE DWARF STARS IN NGC 6397

We explore the physics of crystallization in the deep interiors of white dwarf (WD) stars using the color-magnitude diagram and luminosity function constructed from proper-motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of WD stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn. We outline how these data can be used to observationally constrain the value of Γ ≡ E Coulomb/E thermal near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation.

Cold Dense Matter

FOR his presidential address to the Section of Physics at the thirty-first Indian Science Congress, held at Delhi in January, Dr. D. S. Kothari took as his subject "Cold Dense Matter". Here the word 'cold' is used in a technical sense as meaning that any free electrons present constitute a degenerate gas—the actual temperatures can range from planetary temperatures to those of the interiors of white dwarf stars. Dr. Kothari's address has now been published, and it gives us a welcome summary of the main features of temperature- and pressure-ionization of matter at high and low temperatures and at high and low densities.