White Dwarf Interiors Research Articles

Gravitational Settling of22Ne and White Dwarf Evolution

We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies, which adopted a simplified treatment of the effects of 22Ne sedimentation, this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior the delay introduced by the ensuing chemical differentation is minor for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd about 1.0 Msun, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of about 2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize as well on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.

Ion-temperature-gradient driven modes in very dense magnetoplasmas

By employing the quantum magnetohydrodynamic-Poisson model, a general dispersion relation for low-frequency electrostatic ion-temperature-gradient (ITG) modes in a very dense Fermi plasma is derived. The growth rate is found to be higher in the presence of ion-temperature gradients and electron corrections due to quantum fluctuations. Two new ITG driven modes in the Fermi plasma are found. These ITG modes are associated with an electron density response that differs from the Boltzmann law. It is expected that newly found ITG modes can play an important role in anomalous cross-field ion energy transport in the next-generation laser-solid density plasma experiments as well as in dense astrophysical bodies (e.g., neutron stars and the interior of white dwarfs).

Probing White Dwarf Interiors with LISA: Periastron Precession in Eccentric Double White Dwarfs

In globular clusters, dynamical interactions give rise to a population of eccentric double white dwarfs detectable by the Laser Interferometer Space Antenna (LISA) up to the Large Magellanic Cloud. In this Letter, we explore the detectability of periastron precession in these systems with LISA. Unlike previous investigations, we consider contributions due to tidal and rotational distortions of the binary components in addition to general relativistic contributions to the periastron precession. At orbital frequencies above a few mHz, we find that tides and stellar rotation dominate, opening up a possibly unique window to the study of the interior and structure of white dwarfs.

The ignition of thermonuclear flames in type Ia supernovae

In the framework of the Chandrasekar-mass deflagration model for Type Ia supernovae, a persisting free parameter is the initial location of the flame front, which is linked to the ignition process in the progenitor white dwarf. Previous analytical models indicate that the thermal runaway is driven by temperature perturbations (bubbles'') developing in the white dwarf's convective core. In this work, numerical 2D simulations of buoyant bubbles in the white dwarf interior are presented and discussed, in order to probe the bubble features at ignition (diameters, temperatures and evolutionary time scales). The results support the multi-point scenario as the most favored ignition model for the SN Ia explosion.

A deeper understanding of white dwarf interiors

A detailed record of the physical processes that operate during post-main-sequence evolution is contained in the internal chemical structure of white dwarfs. Global pulsations allow us to probe the stellar interior through asteroseismology, revealing the signatures of prior nuclear burning, mixing and diffusion in these stars. I review the rapid evolution of structural models for helium atmosphere variable (DBV) white dwarfs over the past five years, and I present a new series of model fits using recent observations to illustrate the relative importance of various interior structures. By incorporating physically motivated C/O profiles into double-layered envelope models for the first time, I identify an optimal asteroseismic model that agrees with both diffusion theory and the expected nuclear burning history of the progenitor. I discuss the implications of this fundamental result, and I evaluate the prospects for continued progress in the future.

Gravitational Radiation from Pulsating Magnetic White Dwarfs

Rotating white dwarfs undergoing quasi-radial oscillations can emit gravitational radiation in a frequency range from 0.1-0.3 Hz. Assuming that the energy source for the gravitational radiation comes from the oblateness of the white dwarf induced by the rotation, the strain amplitude is found to be 10-25 for a white dwarf at ∼50 pc. We had calculated thermal energy losses through a magneto-hydrodynamic mechanism and found it smaller than estimated before. The galactic population of these sources is estimated to be 107 and may produce a confusion-limited foreground for proposed advanced detectors in the frequency band between space-based and ground-based interferometers. Nearby oscillating white dwarfs may provide a clear enough signal to investigate white dwarf interiors through gravitational wave astroseismology.

Gravitational Radiation from Pulsating White Dwarfs

Rotating white dwarfs undergoing quasi-radial oscillations can emit gravitational radiation in a frequency range from 0.1 to 0.3 Hz. Assuming that the energy source for the gravitational radiation comes from the oblateness of the white dwarf induced by the rotation, the strain amplitude is found to be ~10-27 for a white dwarf at ~50 pc. The Galactic population of these sources is estimated to be ~107 and may produce a confusion-limited foreground for proposed advanced detectors in the frequency band between space-based and ground-based interferometers. Nearby oscillating white dwarfs may provide a clear enough signal to investigate white dwarf interiors through gravitational wave asteroseismology.

Calibration of White Dwarf Cooling Sequences: Theoretical Uncertainty

White Dwarf luminosities are powerful age indicators, whose calibration should be based on reliable models. We discuss the uncertainty of some chemical and physical parameters and their influence on the age estimated by means of white dwarf cooling sequences. Models at the beginning of the white dwarf sequence have been obtained on the base of progenitor evolutionary tracks computed starting from the zero age horizontal branch and for a typical halo chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear reaction rates, convection, mass loss and initial chemical composition are discussed. Then, various cooling sequences for a typical white dwarf mass (M=0.6 Mo) have been calculated under different assumptions on some input physics, namely: conductive opacity, contribution of the ion-electron interaction to the free energy and microscopic diffusion. Finally we present the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much effort has been spent to extend the equation of state down to the low temperature and high density regime. An analysis of the latest improvement in the physics of white dwarf interiors is presented. We conclude that at the faint end of the cooling sequence (log L/Lo=-5.5) the present overall uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr. We suggest that this uncertainty could be substantially reduced by improving our knowledge of the conductive opacity (especially in the partially degenerate regime) and by fixing the internal stratification of C and O.

Dense astrophysical plasmas

We briefly examine the properties of dense plasmas characteristic of theatmospheres of neutron stars and of the interior of massive white dwarfs. Theseastrophysical bodies are natural laboratories for studying respectively theproblem of pressure ionization of hydrogen in a strong magnetic field and thecrystallization of the quantum one-component plasma at finite temperature.

Measuring12C(α, γ)16O from White Dwarf Asteroseismology

During helium burning in the core of a red giant, the relative rates of the 3α and 12C(α, γ)16O reactions largely determine the final ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in the 3α reaction at stellar energies due to the extrapolation from high-energy laboratory measurements is relatively small, but this is not the case for the 12C(α, γ)16O reaction. Recent advances in the analysis of asteroseismological data on pulsating white dwarf stars now make it possible to obtain precise measurements of the central ratio of carbon to oxygen, providing a more direct way to measure the 12C(α, γ)16O reaction rate at stellar energies. We assess the systematic uncertainties of this approach and quantify small shifts in the measured central oxygen abundance originating from the observations and from model settings that are kept fixed during the optimization. Using new calculations of white dwarf internal chemical profiles, we find a rate for the 12C(α, γ)16O reaction that is significantly higher than most published values. The accuracy of this method might improve as we modify some of the details of our description of white dwarf interiors that were not accessible through previous model-fitting methods.

Electron-Screened Pycnonuclear Reactions in Dense Matter Near Fluid-Solid Transition

Rates of nuclear fusion reactions in dense electron-screened plasmas are formulated for the entire density-temperature domain. As the temperature is lowered from the high-temperature, thermonuclear regime, the reaction rates are significantly enhanced over the binary Gamow rates, and eventually become insensitive to the temperature (fluid pycnonuclear rates) near solidification, because the effective interaction between the reacting nuclei is significantly reduced owing to electron screening and internuclear many-body correlation. We find that the resultant reaction rates join continuously into those in the solid phase across the solidification line; this is consistent with the intuition that the fluid phase already exhibits a strong short-range lattice-like order near solidification. The theory is applied to the interiors of white dwarfs, giant planets, brown dwarfs, and to low-temperature pressurized metallic hydrogen.

FLASH: An Adaptive Mesh Hydrodynamics Code for Modeling Astrophysical Thermonuclear Flashes

We report on the completion of the first version of a new-generation simulation code, FLASH. The FLASH code solves the fully compressible, reactive hydrodynamic equations and allows for the use of adaptive mesh refinement. It also contains state-of-the-art modules for the equations of state and thermonuclear reaction networks. The FLASH code was developed to study the problems of nuclear flashes on the surfaces of neutron stars and white dwarfs, as well as in the interior of white dwarfs. We expect, however, that the FLASH code will be useful for solving a wide variety of other problems. This first version of the code has been subjected to a large variety of test cases and is currently being used for production simulations of X-ray bursts, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and thermonuclear flame fronts. The FLASH code is portable and already runs on a wide variety of massively parallel machines, including some of the largest machines now extant.

Pycnonuclear Reactions in Dense Matter near Solidification

Rates of nuclear fusion reactions in dense matter and their enhancement due to electron screening and internuclear many-body correlation are calculated by explicitly taking into account dielectric functions of relativistic and nonrelativistic electrons, screening potentials based on the Monte Carlo simulations, and interaction free energies in dense electron-screened binary-ionic-mixture fluids. Pycnonuclear reactions are predicted near fluid-solid transitions at high densities, where the total reaction rates in fluid and solid phases take on comparable values that are virtually independent of the temperature. Analytic expressions for the reaction rates are presented and are applied to dense carbon-oxygen matter in the white dwarf interiors near ignition conditions and to dense proton-deuteron matter in giant planets and brown dwarfs. Possibility of a laboratory detection of pycnonuclear reactions in ultrahigh-pressure metallic hydrogen near solidification is explored.

Quasilocalized charge approximation in strongly coupled plasma physics

The quasilocalized charge approximation (QLCA) was proposed in 1990 [G. Kalman and K. I. Golden, Phys. Rev. A 41, 5516 (1990)] as a formalism for the analysis of the dielectric response tensor and collective mode dispersion in strongly coupled Coulomb liquids. The approach is based on a microscopic model in which the charges are quasilocalized on a short-time scale in local potential fluctuations. The authors review the application of the QLC approach to a variety of systems which can exhibit strongly coupled plasma behavior: (i) the one-component plasma (OCP) model in three dimensions (e.g., laser-cooled trapped ions) and (ii) in two dimensions (e.g., classical 2D electron liquid trapped above the free surface of liquid helium), (iii) binary ionic mixture in a neutralizing uniform background (e.g., carbon–oxygen white dwarf interiors), (iv) charged particle bilayers (e.g., semiconductor electronic bilayers), and (v) charged particles in polarizable background (e.g., laboratory dusty plasmas).

Seismology of the ZZ Ceti Stars

The pulsations of white dwarf stars are potentially a rich source of information about white dwarf structural properties. Extracting and applying this information to improve our knowledge of white dwarf interiors requires measuring individual eigenperiods in a complex power spectrum, and identifying the character of the eigenmodes they represent. This review will summarize observational progress in these areas for the ZZ Ceti pulsators.

The influence of crystallization on the luminosity function of white dwarfs

view Abstract Citations (72) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Influence of Crystallization on the Luminosity Function of White Dwarfs Hernanz, M. ; Garcia-Berro, E. ; Isern, J. ; Mochkovitch, R. ; Segretain, L. ; Chabrier, G. Abstract The inclusion of a detailed treatment of solidification processes in the cooling theory of carbon-oxygen white dwarfs is of crucial importance for the determination of their luminosity function. Carbon-oxygen separation at crystallization yields delays larger than 2 Gyr in the cooling time for a white dwarf to cool down to the observed cutoff luminosity, i.e., log (L/Lsun) approximately equal -4.5. When calculating the luminosity function, this leads to estimates of the age of the Galactic disk 1.5-2 Gyr older than the ones obtained when ignoring such separation processes. Furthermore, the presence of minor chemical species, in particular, Ne-22 and Fe-56, alters significantly the crystallization process and the cooling time and produces extra delays of 2-3 Gyr. However, the detailed computation of the theoretical white dwarf luminosity function, taking into account galactic chemical evolution, shows that, for minor species, this effect does not modify significantly the location of the cutoff, and thus the estimated age of the disk. The effect of Ne-22 crystallization is shown to produce a sharp peak in the luminosity function at log (L/Lsun) approximately equal -3.8. Though non-detectable with present-day observations, this peak can be used as a future observational test of the crystallization of minor chemical species in white dwarf interiors. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174767 Bibcode: 1994ApJ...434..652H Keywords: Carbon; Chemical Composition; Crystallization; Oxygen; Plasma Cooling; Stellar Interiors; Stellar Luminosity; Stellar Models; White Dwarf Stars; Chemical Evolution; Galactic Evolution; Iron; Neon; Stellar Mass; Astrophysics; EQUATION OF STATE; STARS: INTERIORS; STARS: WHITE DWARFS full text sources ADS |

Transport Processes in Dense Stellar Plasmas

AbstractTransport processes in dense stellar plasmas which are relevant to the interiors of white dwarfs and neutron stars are reviewed. The emphasis is placed on the accuracy of the numerical results. In this review we report on the electrical conductivity and the thermal conductivity of dense matter. The methods of the calculations are different for the liquid metal phase and the crystalline lattice phase. We will broadly review the current status of the calculations of the transport properties of dense matter, and try to give the best instructions available at the present time to the readers.

Strongly Coupled Ionic Mixtures and the H/He EOS

AbstractThis paper summarizes recent work on the strongly coupled OCP and Binary Ionic Mixture equation of state and other thermodynamic quantities in white dwarf interior conditions for both fluid and solid phases with the assumption of a uniform background. Conditions for phase separation of different elements in fluid or solid phases is strongly dependent on deviations from the linear mixing rule which gives the equation of state as an additive function of the OCP equation of state. These deviations turn out to be small (a few parts in 105) and always positive including the case where the fraction of the higher Z component approaches 0. Also the equation of state of strongly coupled light elements (H and He particularly) obtained from simulations with a linear response description of the electrons is given for conditions appropriate to brown dwarf star interiors. Recent Livermore work on a band structure calculation of the enthalpy of H and He mixtures under jovian conditions is discussed. This work leads to a prediction of a high temperature (15000 °K) for miscibility of He in ionized H at 10 Mb.

Equations of state and phase diagrams for dense multi-ionic mixture plasmas.

Equations of state for dense binary-ionic mixture (BIM) plasmas with charge ratios ${\mathit{R}}_{\mathit{Z}}$\ensuremath{\le}4.5 are evaluated for both fluid and solid phases on the basis of internal energies calculated by the Monte Carlo (MC) simulation method. The accuracy of the results has been confirmed through a sum-rule analysis of the free energy with respect to screening potentials at zero separation in one-component plasmas. Phase diagrams for various BIM's appropriate to white-dwarf interiors are constructed, including the effects of compressibility of the relativistically degenerate electrons. Eutectic diagrams, which result in significant chemical separation upon solidification, are predicted for BIM's with ${\mathit{R}}_{\mathit{Z}}$\ensuremath{\gtrsim}1.6. Free energies for dense ternary-ionic mixtures are also derived for both fluid and solid phases through comparison of the corresponding MC internal energies and by a sum-rule analysis using BIM screening potentials at zero separation. We apply the results to the process of solidification of $^{12}\mathrm{\ensuremath{-}}^{16}$O${\mathrm{\ensuremath{-}}}^{22}$Ne mixtures in the interiors of white dwarfs. We find that chemical separation does indeed occur at solidification, and we obtain the result that nearly pure Ne cores are formed in such white dwarfs, even though Ne is only a trace element in the mixture.

White dwarf and neutron star interior solutions in the nonsymmetric gravitational theory

Spherically symmetric, static, numerical solutions of the field equations of the nonsymmetric gravitational theory (NGT) for dense, homogeneous, cold matter with a stiff equation of state are presented in graphic form. As in Einstein's general relativity theory (GRT), two types of stable objects are predicted, corresponding to white dwarfs and neutron stars. The NGT makes these less stable as it effectively softens the equation of state, decreasing their mass and radius