Supernova 1993J in M81 (NGC 3031) was discovered by Spanish amateur astronomers on 28.86 March 1993 (Ripero & Garcia 1993). The first IUE spectra were taken on 30.2 March at VILSPA a few hours after notification of the discovery (Wamsteker et al. 1993) and the supernova was regularly observed by IUE over the next several weeks. This paper summarizes the principal results of the IUE observations (see Fransson & Sonneborn 1994 and Sonneborn et al. 1994 for observational details and more extensive discussion).

The first photographic detection of the supernova was on 28.30 March at magnitude 13.6 (Merlin & Neely 1993). Modelling of the supernova V light curve indicates that the explosion occurred on 27.8 March and that shock breakout should have occurred at ∼ 28.0 March (Shigeyama et al. 1994). Careful analysis of pre-outburst plates and images has identified the progenitor and shown that its colors are consistent with a late-type supergiant (cf. IAU Circular No. 5739) and the supernova's Type II classification.

In Fig. 1 we show the first UV spectra of SN 1993J from 30.2 March to 3.5 April, where the very rapid cooling of the exploding photosphere is readily apparent. On 30.2 and 31.2 March the temperature was ∼22,500K and ∼14,500K, respectively. Here we have assumed a Galactic extinction law and EB–V = 0.18, as determined from a fit of the 2200 Å dust feature on 30.2 March.

Introduction

In several elliptical PNe, a number of structures have been observed which are called : rims, shells, caps, ansae, knots, etc. in addition to the haloes. Some are macrostructures (rims, shells, haloes) constituting in a sense the bulk of the nebula itself. They can either be homogeneous or contain themselves smaller structures. Others are microstructures, and some of them qualify as FLIERs (see below).

Balick et al. (1993; hereafter paper I) explored the spectra of microstructures in three elliptical PNe : NGC 3242, 7662, and IC 2149. They found that NGC 3242 and 7662 contain pairs of low ionization knots moving at supersonic velocity relative to the ambient gas. They have been named FLIERs (fast low ionization emitting regions). Various interpretations were discussed, but a convincing explanation was not found.

Here we present a preliminary study of microstructures in three more elliptical PNe : NGC 6543, 6826 and 7009.

Observations

The observations were performed using the Palomar 5-m telescope and double spectrograph at dispersions of 2.1 Å/pixel in the blue (3400-5150 Å) and 3.1 Å/pixel in the red (5150-7600 Å). The effective resolution corresponds to about 300 km s−1. Along the slit each pixel is 0″.58 for the red and 0″.78 for the blue. Typical seeing was about 1.5 arcsec.

Abstract

In this review, we consider a quantum Coulomb fluid made of charged point particles (typically electrons and nuclei). We describe various formalisms which start from the first principles of statistical mechanics. These methods allow systematic calculations of the equilibrium quantities in some particular limits. The effective-potential method is evocated first, as well as its application to the derivation of low-density expansions. We also sketch the basic outlines of the standard many-body perturbation theory. This approach is well suited for calculating expansions at high density (for Fermions) or at high temperature. Eventually, we present the Feynman-Kac path integral representation which leads to the introduction of an auxiliary classical system made of extended objects, i.e., filaments (also called “polymers”). The familiar Abe-Meeron diagrammatic series are then generalized in the framework of this representation. The truncations of the corresponding virial-like expansions provide equations of state which are asymptotically exact in the low-density limit at fixed temperature. The usefulness of such equations for describing the inner regions of the sun is briefly illustrated.

Abstract

Dans cette revue, nous considérons un fluide coulombien quantique constitué de charges ponctuelles (typiquement des électrons et des noyaux). Nous décrivons différents formalismes s'appuyant sur les premiers principes de la mécanique statistique. Ces méthodes permettent de calculer les quantités d'équilibre de manière systématique dans des limites particulières. La méthode des potentiels effectifs est d'abord évoquée, ainsi que son application aux développements à basse densité.

Abstract

We present the results of numerical experiments aimed at demonstrating how the g-mode period spectra of pulsating DA white dwarfs depend on the various components of the input physics. We take advantage of recent developments on many fronts of physics (equation of state, opacity, convection) to compare the theoretical pulsation periods of models with different pieces of the constitutive physics, but with otherwise fixed values of their stellar parameters. This exercise is necessary to assess the reliability of the pulsation analyses of white dwarfs which have started to come out.

Nous présentons les résultats de simulations numériques pour déterminer comment les périodes de pulsation (type g) des étoiles naines blanches DA dépendent des différentes composantes de la physique constitutive. A cet effet, nous avons utilisé des résultats récents au niveau de la physique de base (équation d'état, opacité, convection) pour comparer les périodes de pulsation de modèles stellaires ayant des paramètres fixes, mais qui différent au niveau de leur physique constitutive. Notre démarche est essentielle afin de pouvoir quantifier les premiers résultats d'analyses d'étoiles pulsantes qui commencent à être publiés.

Introduction

It is now well established that white dwarf stars become intrinsically variable during certain phases of their evolution. For the majority of them, the so-called DA white dwarfs (with atmospheres dominated by hydrogen), luminosity variations are observed when the stars have effective temperatures in the rather narrow interval 13,000 K ≳ Teff ≳ 11,000 K (Wesemael et al. 1991).

Abstract

The numerous complexities underlying large tables of thermodynamic quantities act as a deterrent to a careful evaluation of their reliability. As a consequence, equations of state are often used as black boxes. To clarify this situation, some of the more critical issues of equation of state physics are discussed from the point of view of the user. They are illustrated by a comparison of four equations of state for hydrogen. The flaws and disagreements thus brought into light are explained and evaluated with simple physical arguments.

Les tables d'équations d'état utilisées en astrophysique découlent de modèles d'une complexité telle qu'il est souvent difficile d'en évaluer la fiabilité. Il en résulte une situation où les équations d'état sont souvent utilisées sans une analyse critique de leur contenu physique ni de leur précision. Dans le but de remédier à cette situation, une discussion des principaux éléments physiques des équations d'état est présentée dans l'optique de l'utilisateur. Quatre équations d'état de l'hydrogène développées pour être appliquées à des problèmes d'astrophysique stellaire sont comparées de façon critique. Cette comparaison illustre l'importance de certains éléments clés des équations d'état et la nature des problèmes qui subsistent. Les déefauts et les différences observés entre ces quatre équations d'état sont élucidés en termes de physique de base.

Introduction

The richness of stellar phenomena exposed by modern observational techniques calls for a quantitative understanding of more subtle, “second order” effects in stellar structure.

Abstract

We consider a simple model for the evolution of a poloidal magnetic field initally trapped in a region containing normal npe matter within the outer liquid core of a neutron star. We have performed numerical computations for neutron stars with masses of 1.4, 1.6, and 1.7 M⊙ that undergo very rapid cooling due to the direct Urea process. Because the timescale for the magnetic field decay is directly proportional to T2, such a cooling history produces a rapid decline in the magnetic-field strength B, even for B as low as ∼ 1012 G. In particular, we show that an initially quasi-homogeneous magnetic field of strength B = 1012 G declines during the first ∼ 1 Myr.

Introduction

The calculations of Baym, Pethick, and Pines (1969a) have shown that the electrical conductivity of matter in the core of a neutron star is too large to permit ohmic decay of the magnetic field within the age of the Universe. Recently, Haensel, Urpin, and Yakovlev (1990; hereafter HUY) have pointed out that the magnetic-field strength |B| ∼ 1012 G typical of pulsars is sufficiently strong that the anisotropy of the transport coefficients cannot be neglected and that the “resistivity” for current flow perpendicular to B is many orders of magnitude larger than that for current flow parallel to B. Using a simple “toy” model, they found that internal fields B ≥ 1013 G can decline to ∼ 1012 G in times ∼ 107 years, but that fields ≤ 1012 G remain practically unchanged on this timescale.

Abstract

This 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 oK) for miscibility of He in ionized H at 10 Mb.

Resume

Ce papier resume l'ouvrage recent sur le OCP à fort couplage et sur l'equation d'état et d'autres quantites thermodynamiques pour le melange binarire ionique aux conditions interieure des nains blancs. Les conditions pour seperation de phae dan les éléments divers dans l'état solide ou fluide sont très sensible aux deviations de regie lineaire qui donne l'equation d'état comme function additive sur celui du OCP.

Abstract

In this paper we review the behavior of growing stellar degenerate cores. It is shown that ONeMg white dwarfs and cold CO white dwarfs can collapse to form a neutron star. This collapse is completely silent since the total amount of radioactive elements that are expelled is very small and a burst of γ-rays is never produced. In the case of an explosion (always carbon-oxygen cores), the outcome fits quite well the observed properties of Type la supernovae. Nevertheless, the light curves and the velocities measured at maximum are very homogeneous and the diversity introduced by igniting at different densities is not enough to account for the most extreme cases observed. It is also shown that a promising way out of this problem could be the He-induced detonation of white dwarfs with different masses. Finally, we outline that the location of the border line which separetes explosion from collapse strongly depends on the input physics adopted.

Dans cet article on revise le comportement d'un noyau stellaire dégénéré qui grandit. On montre que les naines blanches d'ONeMg et celles de CO, froides et massives, peuvent s'effondrer pour former une étoile à neutrons. Cet effondrement est complètement silencieux puisque la quantité tot ale d'élements radioactifs expulsée est très petite et on ne produit pas d'eruption de rayons gamma. Dans le cas d'une explosion (toujours pour des noyaux de carbone-oxygène), le résultat des calculs reproduit assez bien les propriétés observées des supernovae de Type Ia.

Introduction

After the first success of helioseismology, it has been shown than new results could only be obtained from long set of continuous observations. Therefore different groups intented to set-up worldwide networks in order to observe the Sun 24 hours a day. This is the case of GONG and IRIS. Both are six-stations networks. Figure 1 shows the different sites of the two networks. They have one common site in Izaña.

IRIS has already 5 stations installed and running. The last site, in Australia, is under selection, and very probably will be in Culgoora, more accessible than the Western site selected by GONG team, in Learmonth. It will be set-up at the beginning of 1994. The operation, already started since 1990, will continue untill at least 2000. So far, the best piece of data obtained covers a period of three monthes, during Summer 1991, with a duty cycle of 60%, with only 3 instruments working. With 6 sites, a duty-cycle of at least 85% is expected.

The six instruments of the GONG network are constructed and under tests. The deployement is supposed to take place during the second half of 1994. The network will be fully operational in 1995. A duty-cycle higher than 90% is expected.

IRIS provides only full-disk velocity measurements, whereas GONG is designed to record images of the Sun, providing a spatial resolution. GONG will measure non-radial modes, from 1=1 to 1=200, when IRIS is only sensitive to radial and low degree non-radial modes (1=0 to 3).

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 to cool down to luminosities corresponding to the observed cut–off. This leads to estimates of the age of the galactic disk 1.5 to 2 Gyr older than the ones obtained in previous studies (about 9 Gyr). Furthermore, the presence of minor chemical species, in particular 22Ne, alters significantly the crystallization process, and produces extra delays of 2 to 3 gigayears. However, the detailed computation of the theoretical white dwarf luminosity function, taking into account a reasonable model of galactic chemical evolution, and including the effect of these species, shows that the location of the cut–off, and then the estimated age of the disk, is not modified significantly.

Le traitement détaillé du processus de solidification revêt une importance cruciale dans l'étude du refroidissement des naines blanches carbone–oxygéne et la détermination de leur fonction de luminosité. La séparation du carbone et de l'oxygène lors de la cristallisation introduit un retard de plus de 2 109 ans pour atteindre les valeurs de la luminosité correspondant au cut–off observé. Ceci conduit à une estimation de l'âge du disque de 1.5 à 2 109 ans plus vieille que celles obtenues dans les études précédentes.

Abstract

The equation of state (EOS) of astrophysical plasmas is, for a wide range of stars, nearly ideal; with only small non-ideal Coulomb corrections. Calculating the EOS of an ionizing plasma from a ground state ion, ideal gas model is easy, whereas, fundamental methods to include the small Coulomb corrections are difficult. Attempts to include excited bound states are also complicated by plasma screening and microfield phenomena that weaken and broaden these states. Nevertheless, the high quality of current observational data, particularly seismic, dictates that the best possible models should be used. The present article discusses these issues and describes how they are resolved by fundamental many-body quantum statistical methods. Particular emphasis is placed on the activity expansion method that is the basis of the OPAL opacity code. Some comparisons with standard methods are given.

Abstract

L'equation d'etat des plasmas astrophysiques est, pour un large domaine d'etoiles, pratiquement ideale; avec de petites corrections coulombiennes. Calculer l'equation d'etat d'un plasma ionise a partir d'un modele de gaz ideal d'ions dans leur etat fondamental est facile, alors que les methodes fondamentales pour inclure les petites corrections coulombiennes sont difficiles. Des tentatives pour inclure des etats lies excites sont aussi rendues difficiles par les effets d'ecran et le phenomene de microchamp qui affaiblissent et elargissent ces etats. Neanmoins, la haute qualite des observations actuelles, en particulier en sismologie, impose l'utilisation des tous meilleurs modeles.

Abstract

We discuss problems related to the electronic and ionic structure of fluid Hydrogen, for equation of state calculations in the domain where a “plasma phase transition” (PPT) may occur. It is argued that the ionization of an electron bound to a particular nucleus proceeds through a progressive delocalization involving “hopping” electron states (i.e. cluster states). A description of the plasma containing pseudoatoms, pseudomolecules and free electrons is proposed. The PPT, if it exists, might be a mobility edge transition across a percolation threshold. It is shown how the effect of electron density, field-particle distributions and temperature on the binding energy of these pseudoatoms and pseudomolecules, can be included. Finally the abundances of these objects is determined by a minimization which allows the self-consistent optimization of ionic as well as electronic parameters contributing to the total free energy.

On discute les problèmes associés à la structure electronique et ionique de l'Hydrogène en phase fluide, en vue de calculs d'équation d'état dans le domaine d'une éventuelle transition de phase vers l'état de plasma (TPP). L'argument essentiel est que l'ionization d'un électron lié attaché à un atome se produit par une délocalisation progressive mettant en jeu des “états de grappe” (cluster states). La TPP pourrait être une transition de la mobilité se produisant au seuil de percolation. On propose une description du plasma où “pseudoatomes”, “pseudomolécules” et électrons libres coexistent.

Abstract

The cyclotron and the one-photon annihilation emissions are investigated for a strongly magnetized thermal electron-positron plasmas. The annihilation spectral component is significant when the particle number density N exceeds some critical value, Ncr(T, B). For T ∼ 108 – 109 K and B ∼ 1012 – 1013 G, this condition can be fulfilled at N < 1022 cm−3, which is realistic for neutron star magnetospheres.

Introduction

The e−e+-plasma in strong magnetic fields of neutron stars can be thought to be responsible for X-ray and γ-ray radiation of radio pulsars and γ-ray bursters. In the emitting regions of these objects, the cyclotron emission and one-photon pair annihilation can be important. Separately, they have been investigated by many authors (see, e.g., Bezchastnov and Pavlov 1991, Harding 1986, 1991, and the references therein). However the comparison of these mechanisms has not been performed even for the simplest case of thermal plasmas. We consider the total emission spectra and find the domain of temperatures T and magnetic fields B where the annihilation component is significant for realistic particle number densities N < 1022 cm−3.

Spectra of radiation

Quantum cyclotron emission and one-photon pair annihilation are characterized by the emissivities (summed over polarizations) jc and ja, respectively.

Abstract

In the past decade, measurements of the properties of H2 and He systems at very high pressures have made great progress, now reaching density at the limit of the plasma phase transition of hydrogen. The potentialities and limits of static and dynamic methods will be reviewed. Then, a survey of the major experimental results is presented. It is the intention of this article to show how these measurements can bring information to model low-mass astrophysical objects. Three levels of usefulness are distinguished on selected examples: data for codes of planetary interiors, constraints for theoretical descriptions of dense matter, observations of unsuspected properties at very high density.

Abstract

De grands progrés ont été faits ces dix dernières années dans la mesure des propriétés des systèmes d'H2 et d'He sous très fortes pressions. Des densités à la limite de la transition de phase plasma de l'hydrogène peuvent maintenant être obtenues en laboratoire. Les possibilités et limites des méthodes dynamiques et statiques seront tout d'abord discutées. Ensuite, les principaux résultats expérimentaux seront présentés. Le but de cet article est de montrer comment ces études peuvent être utiles à la modélisation des intérieurs planétaires. Trois niveaux d'application seront dégagés: données pour les codes de structures internes; contraintes pour valider les descriptions théoriques; mise en évidence à très haute densité de comportements inhabituels.