Abstract

The HIPPARCOS mission will permit a decisive step forward in the comparison between observed and predicted global properties of stars, in producing distances and apparent magitudes with accuracies more than one order of magnitude higher than before. Nearby stars of intermediate and low mass will allow for statistical tests on the validity of the equation of state, like for instance the steepness of the main sequence.

La mission HIPPARCOS va permettre un pas en avant fondamental dans les tests des propriéés thermodynamiques des étoiles de masse intermédiaire en fournissant des distances et des magnitudes apparentes beaucoup plus précises que celles obtenues au sol.

Introduction.

Tests of the physical description of stellar interiors rely on a theory vs observation comparison. The stellar evolution theory predicts the variation with time of the state of the interior of a star and, also, of its fundamental, observable parameters, i.e. luminosity, surface temperature, for a given mass. The HIPPARCOS (High Precision PARallax COllecting Satellite) mission will permit a decisive step forward in this confrontation by producing distances and apparent magnitudes with accuracies more than one order of magnitude higher than before (Baglin, 1988).

For a description of the mission see for instance Perryman et al., 1992, and the ‘Hipparcos Input Catalogue’ (Turon et al., 1992).

Distances measurements.

HIPPARCOS measures parallaxes i.e. distances, and proper motions. Aproximately 120 000 stars brighter than mv ≈ 12.5 are observed; the survey is complete up to the apparent magnitude 7.5.

Abstract

The evolution of White Dwarf stars along their cooling sequences is governed not only by their thermal content, but also by the rate at which heat flows through the external, partially degenerate and non-isothermal layers. In particular, cooling is found to be largely influenced both by the optical atmosphere, and by the convective envelope. The first one, in fact, determines the internal density stratification, down to the point at which electron degeneracy takes over, while the second one affects the temperature stratification in the same layers. The reliability of the present generation of models of White Dwarf envelopes is discussed, on the grounds of the main physical inputs (thermodynamics, opacity, convection theory), for both H-rich and He-rich surface chemical compositions. The conclusion is that, below LogL/L⊙ ≤ –3, we can build little more than test models.

L'évolution des naines blanches le long de leur séquence de refroidissement est gouvernée non seulement pas leur contenu thermique, mais aussi par la vitesse à laquelle la chaleur s'échappe à travers les couches externes, non-isothermes et partiellement dégénérées. En particulier, le refroidissement est largement influencé à la fois par l'atmosphére optique et par l'enveloppe convective. La premiére détermine la stratification interne en densité jusqu'à ce que la dégénérescence électronique prenne le dessus, alors que la seconde affecte la stratification en température dans les mêmes couches.

Abstract

This paper is organized in two parts. First, phase diagrams for dense binary mixtures are computed with the density functional theory (DFT). The method of calculation is reviewed and the different approximations which are used are clearly stated. The DFT is then applied to several mixtures of astrophysical interest. A comparison is made between several existing phase diagrams and the origin of some discrepancies among them is discussed. In a second part, the consequences of these phase diagrams on the cooling of white dwarfs are presented in a pedagogical way starting from the simple Mestel theory. The importance of the partial separation of carbon and oxygen at crystallisation is emphasized and the possible effect of minor species such as 22Ne or Fe is also considered. The separation of carbon and oxygen adds 1 – 2 Gyr to age of the galactic disk estimated from the white dwarf luminosity function while the delay resulting from the presence of minor species is probably negligible when the chemical evolution of the Galaxy is properly taken into account.

Cet article est organisé en deux parties. Tout d'abord, les diagrammes de phase des mélanges binaires denses sont calculés à l'aide de la théorie de la fonctionnelle de densité. La méthode de calcul est détailleé et les différentes approximations utilisées sont clairement expliquées. Le théorie est ensuite appliquée à plusieurs mélanges d'intérêt astrophysique.

Abstract

We present a free energy model for fluid hydrogen at high-density and high-temperature. This model aims at describing pressure dissociation and ionization, which occur in partially ionized plasmas encountered in the interiors of giant planets and low-mass stars. The model describes an interacting mixture of H2,H,H+ and e− in chemical equilibrium. The concentrations of H2+ and H− ions are found to be negligible for equation of state purposes. Our model relies on the so-called chemical picture approach, based on the factorization of the partition function into translational, internal and configurational degrees of freedom. The present model is found to be unstable in the pressure-ionization regime and predicts the existence of a first-order plasma phase transition (PPT) which ends up at a critical point given by Tc = 15300 K, Pc = 0.614 Mbar, and ρc = 0.35 gcm−3. The transition occurs between a weakly ionized phase and a partially ionized (∼ 50%) phase.

Nous présentons un modèle d'énergie libre pour l'hydrogène fluide à haute densité et haute température. Le but de ce modèle est de décrire la dissociation et l'ionisation en pression, telles qu'elles se produisent dans les plasmas partiellement ionisés rencontrés à l'intérieur des planètes géantes et des étoiles de faible masse. Le modèle décrit un fluide en interaction composé de H2,H,H+ et e− en équilibre chimique.

Abstract

Present available interior models of giant planets assume that the internal transport of energy is entirely convective and, accordingly, rule out any possibility of radiative transport. New opacity calculations at temperatures and densities occurring within the giant planets, taking into account H2-H2 and H2-He collision-induced absorption as well as infrared and visible absorption due to hydrogen, water, methane and ammonia are presented. These opacities are not high enough to exclude the presence of a radiative zone in the molecular H2 envelope of Jupiter, Saturn and Uranus.

Abstract

Les modèles de structure interne des planètes géantes développés actuellement supposent que le transport de l'énergie s'effectue entièrement par convection, ce qui élimine toute possibilité de transport radiatif. Des nou-veaux calculs d'opacité aux températures et densités caractéristiques des planètes étudiées, tenant compte de l'absorption induite par collisions H2-H2 et H2-He ainsi que de l'absorption dans l'infrarouge et dans le visible de l'hydrogène, l'eau, le méthane et l'ammoniaque, sont présentées. Ces opacités ne sont pas suffisamment élevées pour exclure la présence d'une zone radiative dans l'enveloppe d'hydrogène moléculaire de Jupiter, Saturne et Uranus.

Introduction

Since the estimations of the conductive and radiative opacities in Jupiter by Hubbard (1968) and Stevenson (1976) all the interior models of the four giant planets have been calculated under the assumption that the energy is transferred by convection through the entire hydrogen-helium envelope. Consequently, the thermal profile is assumed to be adiabatic at all depths.

Abstract

Transport 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.

Nous présentons une revue des propoiétés de transport dans les plasmas denses stellaires caractéristiques des intérieurs de naines blanches et d'étoiles à neutrons. L'accent est mis sur la précision des résultats numériques. Nous présentons la conductivité electrique et la conductivité thermique dans la matière dense. Les méthodes de calcul sont différentes dans la phase liquide et dans la phase cristalline. Nous donnons une revue générale des calculs des propriétés de transport dans la matière dense, et nous essayons de donner les meilleures instructions quant aux données disponibles actuellement.

Introduction

In recent years white dwarf asteroseismology opened up a new fertile land of astrophysics (Bradley & Winget 1991; Bradley, Winget, & Wood 1992). Consequently, the basic physics data which go into white dwarf models need to be sufficiently accurate that they should live up to the standard required by the asteroseismological data.

Abstract

Astrophysical objects of low mass, ranging from giant planets to extreme dwarf main-sequence stars, have a number of physical characteristics in common due to properties of their equations of state. Their luminosities are low (much less than the solar luminosity L⊙) and their evolutionary timescales are typically measured in Gyr. So far there are few observational examples of these objects, although they are undoubtedly numerous in the galaxy. The lower mass limit is set by the object's ability to retain hydrogen during accumulation (about the mass of Saturn), while the upper mass limit is set by the lifting of electron degeneracy by high internal temperature. Objects confined within this broad range, which extends up to about 0.1 M⊙, are governed by the thermodynamics of liquid metallic hydrogen. In this paper, we discuss the implications of this feature of their interior structure for their radii, interior temperatures, thermonuclear energy generation rates, and luminosities. We conclude with a brief assessment of the confrontation between observations and theory in galactic clusters and in the solar system.

L'équation d'état des corps célestes de faible masse, qui vont des planètes géantes aux étoiles naines qui sont à la limite de la séquence principle, est a l'origine d'un ensemble commun de propriétés physiques. Leur Iuminosité est de beaucoup inférieure à celle du Soleil et leur temps caractéristique d'évolution se mesure en milliards d'années.

Abstract

New technics such as asteroseismology are able to sound the deep interior of stars and to provide the data that will constrain the modelisation of the core. This information will be combined with data collected from the stellar surface which give direct access to measurements of the radiative losses, angular momentum losses and distribution of active structures. From the two sets of data, the key role of the convection zone will be clarified, as the convection zone excites the waves that propagate through the whole star and generates the magnetic field that structures the stellar surface. The PRISMA mission was developed to collect the data needed for detecting the oscillations by very accurate photometry (micromagnitude) and to derive the surface activity and rotation from accurate ultraviolet spectroscopy. A short description of the model payload is given with the observational constraints related to the needed accuracy of measurements. Following the non-selection by ESA in may 1993, some following perspectives are described.

Introduction

The sounding of the stellar interior can be traced either by neutrino detection or by reconstruction of the path of travelling waves perturbing the surface. Asteroseismology is the study of such waves detected either in brillance or in velocity fluctuation. Up-to-now the use of such fluctuations (Grec et al, 1980; Frohlich and Toutain, 1992) has been proven to be a powerful diagnostic tool to modelise the solar interior (Gough, 1985).

Abstract

The properties of plasma in neutron star crusts with strong magnetic fields B = 1010 − 1013 G are reviewed: thermodynamic properties (equation of state, entropy, specific heat), transport properties (electron thermal and electrical conductivity of degenerate electron gas, radiative thermal conductivity of very surface nondegenerate layers) and neutrino energy losses. Classical effects of electron Larmor rotation in a magnetic field are considered as well as quantum effects of the electron motion (Landau levels). The influence of the magnetic fields on density and temperature profiles in the surface layers of neutron stars and on neutron star cooling is briefly discussed.

Nous présentons la revue des proprietés du plasma dans l'écorce des étoiles neutroniques avec des champs magnétiques forts B = 1010 − 1013 G: proprietés thermodynamiques (equation d'état, entropie, chaleur specifique), proprietés de transfer (conductivité electronique thermique et electrique du gaz electronique dégénéré, conductivité radiative thermique des couches non-dégénées superficielles), et les pertes dûes à l'énergie des neutrinos. Nous examinons des effets classiques de la rotation Larmor d'un electron dans le champ magnétique, et aussi des effets quantiques (niveaux de Landau). Nous discutons en bref l'influence des champs magnétiques sur la densité et la temperáture des couches des étoiles neutroniques et sur les taux de refroidissement des étoiles neutriniques.

Introduction

Neutron stars are the densest stars known in the Universe. Their masses are M ∼ 1.4M⊙, and radii R ∼ 10 km.

Abstract

Model sequences computed with the recently-published OPAL radiative opacities, Itoh et al. conductive opacities, and Itoh et al. neutrino rates are presented. Cooling times for DA model sequences are found to be independent of metallicity for Z ≤ 0.001.

Introduction

In the past decade, many improvements in the constitutive physics relevant to white dwarf evolutionary models have been published. These include improved radiative opacities (Rogers & Iglesias 1992; Iglesias & Rogers 1993), conductive opacities for pure (Itoh et al. 1993 and references therein) and mixed (Itoh & Kohyama 1993) compositions, and updated neutrino rates (Itoh et al. 1992 and references therein). We have incorporated these results into our white dwarf evolution code (=WDEC; see Lamb & Van Horn 1975, and Wood 1990), and present here selected C-core DA model sequences computed with the updated code. Stellar masses for the sequences are 0.4, 0.6, and 0.8 M⊙ and surface layer masses are log q(H) = −6 and log q(He) = −4. To determine the effect of metallicity on the evolutionary timescale, we computed parallel sequences with Z = 0.000 and 0.001.

Opacities

The radiative opacities used in WDEC in the past (Cox & Stewart 1970) had an unrealistically-high metallicity of Z = 0.001 (Zobs ≲ 10−5) The new OPAL opacities span a wide range of metallicities and compositions, and therefore allow the inclusion of more plausible composition profiles in the models. The OPAL opacities only extend to a minimum temperature of 6000 K, however, so for DA models WDEC references the pure-H opacities of Lenzuni et al. (1991) below this point.

Abstract

In the limit of short wavelengths, it has been shown that superfluidity significantly affects wave propagation in neutron stars. Here we abandon the short-wavelength restriction and extend these calculations to global oscillation modes. In the present analysis, the solid crust of the neutron star is divided into an outer crust and an inner crust, and a superfluid of neutrons coexists with the solid lattice in the inner crust. We have computed several low-order global spheroidal modes for l = 2 both with and without superfluidity. We find that superfluidity in the inner crust affects the frequency spectra of acoustic (p-) modes, shear (s-) modes, and interfacial (i-) modes, although the surface gravity (g-) modes are not affected at all.

Introduction

Most previous calculations of the non-radial oscillations of neutron stars have completely neglected the effects of superfluidity (cf McDermott, Van Horn, and Hansen 1988 and references therein). Epstein (1988) has previously considered superfluid effects, but only in the short-wavelength limit, where the length scales for variations in equilibrium quantities are all assumed to be much longer than the typical wavelength of an oscillation. In general, global oscillations may be either spheroidal or toroidal in character. Van Horn and Epsetein (1990) extended Epstein's short-wavelength results to include the global toroidal oscillation modes of neutron stars. More recently, Mendell (1991) and his colleagues have also considered the effects of superfluidity, but they employed simple models for neutron stars, and their analysis did not reflect the variety of oscillation modes of realistic, neutron stars.

Abstract

This paper reviews a new astrophysical subject: seismology of the giant planets. Seismology is dedicated to the sounding of the interior structure of any object; on the other hand, the interiors of the Jovian planets need to be constrained, in order to improve our knowledge of their structure and of their evolution, as well as the thermodynamical laws involved at high pressures and low temperatures. The relationship between Jovian seismology and, first, Jovian internal structure, and second, high pressure physics, is examined, in order to determine the task of “dioseismology” in the next years. We present then the seismological theoretical approaches developped since the pionnering work of Vorontsov et al. (1976), who calculated the frequencies of the Jovian eigenmodes. We report the first observational attempts for the detection of the oscillations of Jupiter. We discuss the observational results and examine what can be done in the future.

La sismologie des planètes géantes apparaît comme un centre d'intérêt astrophysique d'avenir. Elle doit permettre en effet – et il s'agit en fait du seul outil dont l'on dispose – de sonder les intérieurs de ces planètes, actuellement mal connus, mais dont la détermination représente un intérêt majeur. Cet article récapitule aussi bien les diverses approches théoriques développées depuis l'article précurseur de Vorontsov et al. (1976) que les diverses expériences menées pour détecter les oscillations de la planète Jupiter. L'accent est mis sur les liens reliant l'étude sismologique des planètes géantes avec d'une part leur structure interne, d'autre part la physique hyperbare gérant les équations d'état utilisées pour décrire le comportement de l'enveloppe fluide.

Abstract

In this paper I summarize some of the recent advances in studies of dense matter. Research on phase separation in the binary ionic mixtures (BIMs) that constitute the matter in white dwarfs has been motivated by the need to obtain accurate estimates for the ages of the faintest white dwarfs and thus of the disk of our Galaxy. Substantial age increases appear possible, but it is not yet clear whether such large increases occur in real white dwarfs. A second advance is the prediction, based on state-of-the-art physical calculations, that ionization of H at low temperatures and increasing densities may occur via a first-order “plasma phase transition” (PPT). Astrophysical consequences of this result are still being explored in an effort to test this prediction. Related to these equation-of-state calculations are calculations of the enhancement of nuclear reaction rates at high densities. New thermonuclear rates have been computed for C+C reactions in BIMs, although there is currently some controversy about results at the highest densities. New pycnonuclear reaction rates have also been calculated for BIMs, and it has been suggested that He-burning at T = 0 may occur through a first-order phase transition. Finally, calculations of the equation of state of matter in strong magnetic fields and of radiative opacities at high densities have undergone very recent and substantial improvements, which are just beginning to be utilized in astrophysical calculations.