Abstract

The EVRIS experiment is an exploratory mission devoted to stellar seismology. It will observe approximately ten bright stars, for 20 days each, during the cruise of the Russian MARS 94 mission. The photometer will be able to detect amplitudes of modes as small as a few 10−6 magnitude. Some objects of masses lower than the solar one will allow to test the thermodynamics.

EVRIS est la première expérience dévouée à la sismologie des étoiles. Elle sera lancée par la mission Russe MARS 94. Elle observera une dizaine d'objets, chacun pendant une vingtaine de jours, avec un seuil de détection de quelques 10−6 magnitude. Plusieurs étoiles de masse plus faible que celle du Soleil devraient permettre des tests significatifs de leur thermody-namique.

An exploratory instrument for asteroseismology.

Scientific objectives and strategy.

After the success of detection, measurements and interpretation of the eigemodes of the Sun, it is tempting to try to achieve similar progress on other stars and to allow for a comparative and differential study of the seismical stellar behavior.

The major difficulties when going from the Sun to stars is the lack of photons and the lack of spatial resolution. The rationale for such an aim has already been developed several times (i.e. Hudson et al. 1986, Praderie et al. 1988).

The need to go to space has also been extensively documented (see i.e. Mangeney and Praderie 1984, Hudson et al. 1986, Baglin 1990, Weiss 1992).

Abstract

We present briefly a new generation of white dwarf models incorporating the latest developments of the constitutive physics. These are static models especially designed for accurate seismological studies.

Introduction

The main goal of asteroseismology is the determination of the internal structure of a pulsating star through the analysis of its observed pulsation properties. One way to fulfill this goal is by producing a stellar model that reproduces to high accuracy the observed periods of oscillation. This is generally not possible through full evolutionary calculations as the parameters of a model must be tuned rather finely to satisfy the requirement of accuracy. However, computations of static models can be used with profit here. We have therefore developed the capacity to rapidly build complete static models of stratified H-rich (DA) or He-rich (DB) white dwarfs, especially suited for asteroseismological studies, by specifying the stellar mass, the H-layer thickness, the He-layer thickness, the convective efficiency and the effective temperature.

Method

To build our models, we integrate with the help of a Runge-Kutta technique the equations of stellar structure and stellar grey atmosphere (see, e.g., Cox & Guili 1968 and Mihalas 1978) from the high atmosphere (ρ ≲ 10−13) down to the center of the star. We iterate this procedure until we find a model with Mr = 0 at r = 0. To have a good spatial resolution both in the interior and the external regions, we use the integration variable x[≡ ln(r/P)].

Abstract

The solution of the exact integral equation for the liquid pair-structure in the asymptotic strong coupling limit for the plasma, as mapped on the Onsager charge-smearing optimization for the energy lower bound, features “Onsager atoms” and “Onsager molecules”. The universal properties of this asymptotic limit make it a natural reference starting point for an asymptotic strong coupling expansion for the fluid structure and thermodynamics, playing the role of an “ideal liquid” state. In particular, the leading strong coupling terms for the potential energy, direct correlation functions, and screening potentials for the Coulomb and Yukawa mixtures (corresponding to classical plasmas and electron screened classical plasmas), with full thermodynamic consistency, are presented. These are in complete agreement with the Alastuey-Jancovici analysis of early simulations data by Hansen in strong coupling, and with recent highly accurate simulations data of Ogata, Iyetomi, and Ichimaru. Data analysis errors lead Ogata, Iyetomi, and Ichimaru to incorrect results for the short range screening potentials in strong coupling. Their calculations for the short range screening potentials, bridge functions, and enhancement factors for nuclear reaction rates in strongly coupled plasmas, should be revised.

La solution de l'équation intégrate exacte pour la structure du liquide dans la limite asymptotique de couplage fort pour le plasma, calquée sur l'optimisation de charge d'Onsager pour la limite inférieure de l'énergie, met en évidence des “atomes d'Onsager” et des “molécules d'Onsager”.

Abstract

The importance of low temperature opacities in stellar calibrations led us to compute new sets of Rosseland mean opacities for different Z-values. For the solar metallicity, these tables have been compared to those of Alexander (1975), Cox (1983), Sharp (1991) and Kurucz (1992).

Introduction

Opacities in the atmospheric layers are generally not considered of great importance in the calculation of theoretical evolutionary tracks since the atmosphere of a star only comprises a tiny part of its mass (see however, section 1.2).

Until recently, the most commonly used “atmospheric” or “low-T” opacity tables were those of Cox & Stewart (1970), Alexander (1975) and Cox (1983) but there are rather large discrepancies between these different tables for typical T and ρ ranges encountered in stellar atmospheres of solar type stars.

Furthermore, for pop I stars, low-T opacities are calculated for very few values of the metallicity, Z, and the solar chemical composition is generally used in the calculation of tracks, whatever the actual value of Z.

Low-T opacities and stellar calibrations

Theoretical evolutionary tracks depend on mass, age, chemical composition on the zero age main sequence and convection parameter, α (ratio of mixing length to pressure scale height in the convective layers). Calibrating a star consists in computing evolutionary models that reproduce, at given age, chemical composition on ZAMS and convection parameter, the observed values of the luminosity and effective temperature of the star.

Abstract

The internal structure of a white dwarf may be changed by a strong magnetic field. A local model of the electrons is constructed within a thermal density matrix formalism, essentially a Heisenberg magnetism model. This results in a matrix Fermi function which is used to construct an isothermal model of the electron crystal. The central density of the crystal is 108kg/m3 independent of the magnetic field within the plasma and therefore lower than the relativistic density, whereas this density is constant until the Fermi momentum xf = 0.3 * me * c. Chandrasekhar masses up to 1.44 * 1.4M0 are possible for polarizations of the plasma zone lower than 0.5, if the temperature is close to the Curie point, whereas the crystal itself destabilizes the white dwarf dependent on temperature.

Introduction

From the theory of magnetic phase transitions of solid state physics (Grosse 1988) it is expected, that the structure of a single white dwarf is changed drastically by a magnetic field. The polarized electrons throughout the star may interact due to a magnetic field. The nonlinear influence of a crystallization transition and the crystal itself may change the mass and radius of a white dwarf. We construct a thermal Heisenberg model of the electrons which results in a Fermi matrix function, which predicts a plasma crystal phase transition. This Fermi function is used within the standard white dwarf theory.

Abstract

This review attempts a brief summary of the numerous and diverse searches for the so-called brown dwarfs, substellar objects having masses between giant planets and the lowest mass M dwarf stars.

Cette revue donne un bref aperçu de l'état actuel des diverses recherches de naines brunes, objects substellaires ayant des masses comprises entre les planètes géantes et les naines M de faible masse.

Introduction

Between the giant planets such as Jupiter (10−3M⊙) and stars at the bottom of the hydrogen-burning main sequence (≤0.1M⊙) – spanning more than two orders of magnitude in mass – the sequence of brown dwarfs has yet to be discovered and analyzed in detail. The previous sentence carries the positive bias of this author that – despite the current lack of a single, unambiguous example for me to discuss at this meeting – the flurry of searches now underway by a variety of techniques will identify at least some genuine brown dwarfs during the present decade. Our motivation for thinking and speaking positively is to encourage advances in the theory of both the interiors and atmospheres of such gaseous objects, in order to make possible positive identifications among the candidates found by observers. Indeed, numerous candidates exist of different kinds, some with measured masses, luminosities and temperatures which straddle the stellar mass limit (SML) near 0.08 M⊙.

Abstract

This spatial experiment is under construction and has been defined as a 2 years mission on board SOHO, a satellite dedicated to the Sun which will be launched in mid 95. The main objectives are the detection of solar low degree acoustic modes and solar gravity modes for improving our knowledge of the solar nuclear region.

Introduction

The spatial experiment, GOLF (Global oscillations at Low Frequencies), has been accepted by ESA in March 1988 and should be boarded on the SOHO (SOlar and Heliospheric Observatory) satellite (Damé et al 1988, Gabriel et al 1989). This satellite will be launched by NASA in mid 1995. The objectives of this experiment is to enhance our knowledge of the solar interior by the measurement of the low degree acoustic modes: 1=0, 1, 2, 3, i.e the most penetrating ones, and by the possible measurement of the gravity modes. These different types of modes correspond to frequencies between some 10−6 and 8 10−3 Hz. On the same satellite there will be two other helioseismic experiments: VIRGO and MDI, the first one is a variability solar irradiance measurement in different wavelengths which allows to reach acoustic modes of degree 1= 0, 7. The second one, using a Michelson Doppler imager, is a complementary experiment which must be able to detect degree acoustic modes up to 1= 4500.

The success of such mission is largely dependent on the stability of the measurements, which requires a pointing stability of the satellite better than 1 arc sec per 15 minutes.

Abstract

The rate of nuclear reactions depends on the influence of the surrounding particles that compose the plasma. At high densities the situation is far from being satisfactory and the influence of electron polarization has not been completely elucidated. In particular, it is shown that the possibility of an accretion induced collapse of a carbon-oxygen white dwarf instead of a supernova explosion completely depends on the screening factors and pycnonuclear rates that are adopted. Similarly, the possibility of detecting isolated neutron stars that accrete matter from the interstellar medium depends on the adopted pycnonuclear rates. Low rates allow the formation of a metastable layer that can release energy explosively and produce aγ-ray burst. Nevertheless, current rates seem to prevent such a situation.

Le taux des réactions nucléaires dépend de l'influence exercée par les particules voisines qui composent le plasma. A haute densité, la situation est loin d'être satisfaisante et l'influence de la polarization electronique n'est pas sufisamment claire. En particulier, on montre que la possibilité d'obtenir un collapse non explosif d'une naine blanche de carbone oxygène dépend des facteurs d'écrantage et des taux pycnonucléaires adoptés. Egalement, la possibilité de détecter des étoiles à neutrons isolées dépend des taux pycnonucléaires adoptés. Des petites valeurs favorisent la formation d'une couche metastable qui peut libérer de l'energie explosivement et produire une éruption gamma. Quand-même, les taux actuels semblent empêcher cette situation.

In the Internal Constitution of the Stars, published in 1926, Eddington gave a central temperature of white dwarfs of several billions degrees. The Fermi-Dirac statistics appeared just one year later, in 1927, and the new equation of state for degenerate matter provided the explanation of white dwarfs.

The solution of the problems we have to consider presently are probably not relevant of the same kind of intellectual jump. But who knows! Anyhow, the time of the perfect gas law is definitely over. It is possible, in many cases, to get astrophysical orders of magnitude, using simple or oversimple relations betwen physical quantities. However, modeling correctly observational results has become, nowaday, more and more difficult. Data are of a better precision and provide more information, would it be chemical abundances, evolutionary tracks or those wonderful helioseismological data. An elementary statement is that, in order to look Inside the Stars (the title of a recent colloquium), we need more accurate descriptions of basic physical laws: equation of state, opacities, thermonuclear reactions.

We are still facing many difficulties in the field, and we can give a few examples: we do not have a theory which decribes consistently both the equation of state of a plasma and the level population of the atoms; we still have to improve the theory of screening effects in dense plasmas; we have now a description of cold, dense, weakly ionized matter of brown dwarfs, but it is still incomplete.

Abstract

The DENIS survey will survey the southern sky in the near-IR J (1.2 micron) and K (2.2 microns) bands at 3” resolution and to limiting magnitudes in J and K of respectively 16 and 14.5 (lmJy in both cases), and at 1” resolution in the red I band (0.9 microns). Astrophysical motivation is provided by basic problems concerning structure and evolution of galaxies, of types ranging from our own to active galaxies, and concerning specific stellar populations including stars with low temperature photospheres, those still embedded in their protostellar envelopes, and those currently losing mass on the AGB.

Scientific objectives

The release of large 2D detector arrays sensitive in the near infrared provides the first opportunity to undertake a deep survey of the sky in the non-thermal infrared range (1 to 2.5 microns). This underexplored spectral range will provide crucial insights into fundamental problems in stellar and galactic astrophysics. Theere is no recent all-sky atlas of data between the visible and the IRAS 12 microns band. The 25 year-old IRC catalog remains the state of the art effort in the near IR despite its limitations. Our objective is to carr y a 3 colour (IJK) survey of the complete southern sky, improving on the pioneering IRC sensitivity by 4 orders of magnitude and improving on its spatial resolution by a factor of 20.

There are two main motivations for a deep near IR sky survey: the near IR brightness is the best tracer of mass in stellar form, and the interstellar extinction is reduced by a factor of 10 with respect to the visible V band.

Abstract

We report the results of a nonlinear inversion of solar oscillation data that enable us to detect nonideal Coulomb interactions between particles, including pressure ionization, in the solar convection zone.

Introduction

Precise measurements of solar oscillation frequencies provide data for accurate inversions for the sound speed in the solar interior. Except in the very outer layers, the stratification of the convection zone is almost adiabatic. There, the sound-speed profile is governed principally by the specific entropy, the chemical composition and the equation of state, being essentially independent of the uncertainties in the radiative opacities. The inversions thus reveal, via tiny effects on the adiabatic compressibility of the solar plasma, physical processes that influence slightly the equation of state. We have carried out a nonlinear inversion based on a recent accurate asymptotic description of intermediate- and high-degree solar p modes (Brodsky & Vorontsov 1993; Gough & Vorontsov 1993), using the observational data of Libbrecht, Woodard & Kaufman (1990).

The equations of state (EOS) used in the analysis

In the reference models, we use the following equations of state. We are mostly brief, with the exception of the pressure-ionization model used in the helioseismic calibration.

• Saha EOS: a free-energy-minimization type realization for a mixture of reacting ideal gases, with ground-state-only partition functions of the bound species. Note that by assuming only ground states we are using the term ‘Saha’ in a rather restricted sense.

• […]

Abstract

Accurate measurements of observed frequencies of solar oscillations are providing a wealth of data on the properties of the solar interior. The frequencies depend on the solar structure, and on the properties of the plasma in the Sun. Except in the very outer layers, the stratification of the convection zone is almost adiabatic. There, the sound-speed profile is governed principally by the specific entropy, the (homogenous) chemical composition and the equation of state. It is therefore essentially independent of the uncertainties in the radiative opacities. The sensitivity of the observed frequencies is such that it enables to distinguish rather subtle features of the equation of state. An example is the signature of the heavy elements in the equation of state. This opens the possibility to use the Sun as a laboratory for thermodynamic properties.

Les fréquences observées des oscillations solaires constituent une base de données extrêmement riche qui nous permet d'étudier les propriétés de l'intérieur du soleil. Les fréquences dépendent de la structure solaire et des propriétés locales du plasma (surtout de la vitesse du son). Sauf dans les couches très exterieures, la structure de la zone convective du soleil est essentiellement adiabatique. Le profil de la vitesse du son est done donné par l'entropie spécifique, la composition chimique (homogène) et l'équation d'état. L'opacité radiative ne joue pas de rôle. Grace à la grande précision des fréquences observées on arrive à distinguer des phénomènes assez subtiles dans l'équation d'état, comme la signature faible des élements lourds.

Abstract

Coulomb interactions for the Free Helmholtz energy and the pressure are studied in a partial new formulation which described more exactly the numerical evaluation of many body theories.

Introduction

With regard to the EOS many activities have been developed to yield results which consider different phenomena, for instance pressure dissoziation and ionisation, degeneracy, relativity, Coulomb- and non-Coulombic interactions, pair production and charge mixing in different chemical compositions.

Various theoretical approaches are used in order to include exchange and correlation effects for fully ionized or partially ionized matter (see e.g. Salpeter and Zapolski 1967, Graboske et al. 1969, Hansen 1973, Pokrant 1977, Fontaine et al. 1977, March and Tosi 1984, Perrot and Dharmawardana 1984, Hubbard and Dewitt 1985, Dandrea et al. 1986, van Horn 1987, Kraeft et al. 1986, Ichimaru et al. 1987, Rogers and DeWitt 1987, Däppen et al. 1988, Ichimaru 1990, Eliezer and Ricci 1991, Saumon and Chabrier 1992).

For many applications (e.g. stellar evolution calculations or astroseismology) it is necessary either to have algebraic formulae for the EOS or extensive tables which supply the input, informations at any density and temperature. As a, first step we present an analytical EOS for fully ionized multicomponent plasmas covering a large density-temperature range. The EOS includes non-ideal effects due to exchange-correlation interactions of charged particles at any degeneracy and is applicable to any chemical mixture. Relativistic effects as well as ionic quantum corrections are taken into account.

Introduction

In Chapter 18, we showed that we can make a convincing case that the high energy electrons which are observed at the top of the atmosphere represent a sample of the high energy electrons present throughout the interstellar medium and which are responsible for the diffuse Galactic synchrotron radio emission. Our task in this chapter is to interpret these observations in terms of the propagation of these particles from their sources through the interstellar medium and the energetics of possible energy sources within the Galaxy. The key diagnostic tools are aging processes, which can result in features in the energy spectra of the electrons and estimates of the energy requirements of sources of synchrotron radiation. In this chapter, we will develop these tools in the context of the origin of the Galactic radio emission and the study of supernovae as sources of high energy electrons. These tools are, however, of very general applicability to the whole of high energy astrophysics. We will use them repeatedly in our discussion of the physics of radio sources and active galactic nuclei.

Energy loss processes for high energy electrons

High energy electrons are subject to a number of energy loss processes as they propagate from their sources through the interstellar medium. The loss processes cause distortions of the injection energy spectra of the particles from their sources and thus potentially provide information about the life histories of the high energy electrons.