Introduction

Axisymmetric outflows are associated with many nebulae (e.g. He 2-36, BI Cru, My Cn 18, IC 4406, K 3-72, Corradi & Schwarz 1993a, b, c; OH 17.7–2.0, La Bertre 1986; R Aquarii, Burgarella & Paresce 1992) and with Be stars. I will concentrate in the present review mainly on planetary nebulae (PNe).

An examination of the catalogue of narrow band images of Schwarz, Corradi and Melnick (1992) and other images in the literature reveals a few interesting morphologies. In some cases, almost perfect rings are observed (e.g. ScWe 3, ScWe 2, Schwarz, Corradi & Melnick 1992; Hen 1357, Bobrowsky 1993).

Introduction

If a supernova progenitor has undergone significant mass-loss then the expanding supernova ejecta will eventually collide with this circumstellar material (CSM). Shock waves arising from the collision will compress and heat both the ejecta and the CSM. The emission from the shocked material depends strongly on the density distributions of the ejecta and the CSM, thereby providing important information about the nature of the CSM.

SN 1987A

Images from the European Southern Observatory (ESO) (Wampler et al. 1990) and the Hubble Space Telescope (HST) (Jakobsen et al. 1991) revealed the presence of a ring-like structure at ∼ 6 × 1017 cm from SN 1987A. The outermost part of the supernova, ejecta is expanding at ∼ 104 km s−1 (Shigeyama & Nomoto 1990) and so is expected to collide with the ring ∼ 10 years after the explosion.

Hydrodynamical model

The progenitor of SN 1987A went through a red supergiant (RSG) phase, and then contracted to a blue supergiant (BSG) before the explosion (for reviews, see Arnett et al. 1989, Hillebrandt & Höflich 1989, Podsiadlowski 1992, and Nomoto et al. 1993a). This evolutionary scenario implies that the SN 1987A environment was formed as follows: the progenitor blew a stellar wind with a velocity ∼ 10 km s−1 and a mass loss rate ∼ 10−5M⊙ yr−1 during the RSG stage, and with corresponding values of ∼ 550 km s−1 and ∼ 10−6M⊙ yr−1 during the BSG stage (Lundqvist & Fransson 1991).

Introduction

The dusty envelopes of late type stars are fascinating objects on their own; they are also interesting for what they teach us about IS chemistry. From their velocity field and density profile, we can study the mass loss during a crucial phase of stellar evolution: since the gas expands at nearly constant velocity in all, but the innermost envelope, the velocity maps yield a 3-D view of the molecule spatial distribution; the distributions of the different molecular species show how photochemical, molecule-molecule and grainsurface reactions proceed with time in a well behaved environment.

The closest massive envelopes lie a few hundred parsecs away and have small angular sizes. The construction of large millimeter-wave interferometers, in particular the IRAM Plateau de Bure interferometer (Guilloteau et al. 1992), has provided a major breakthrough in their investigation.

Molecular emission in IRC+10216

The most remarkable and probably closest massive envelope surrounds the bright IR object IRC+10216 (CW Leo). Its outer radius, observed in the mm lines of 12CO, the most abundant molecule and the best shielded from photodissociation after H2, is R = 3′ (Guélin & Cernicharo, in preparation).

Introduction

The LMC star S119 is a member of the group of Ofpe/WN9 stars listed by Bohannan and Walborn (1989). The Ofpe/WN9 category, first defined by Walborn (1982), identifies peculiar supergiants whose spectra combine the typical Of characteristic emission lines of He II and N III with equally strong lower ionization emission features, such as those of He I and N II, and are believed to represent a transition phase in the evolution between massive O stars and WR stars.

High Resolution Echelle Observations

We have observed S119 with the high resolution echelle spectrograph EMMI, coupled to the NTT, ESO La Silla, on September 18, 1991. The spectra cover the wavelength range 4100Å- 7800Å, with a spectral resolution of 0.089 Å/pixel at 6563 Å. The selected slit width was 1.5″ × 5″, with a plate scale on the detector of 0.345″/pixel. In the spectrum, previously undetected nebular lines of Hα, Hβ, [NII], [SII] appear strong and spatially extended, an indication that S119 is surrounded by a bright gaseous nebula. We detect clear splitting of all the observed nebular lines. In Figure 1 we show the radial velocity map obtained from the [NII] 6583 Å line profile. During the observation the slit was oriented EW, and the star was not centered in the aperture, so that only the eastern portion of the nebula lies completely within the slit, while the western region is marginally covered (≃ 2″).

Introduction

This is a short summary of several calculations of the evolution of supernova remnants in a constant high density medium n0 ≥ 106−8 cm−3 and an abundance in the range 0.01Z⊙≤ Z ≤ 10Z⊙. The main difference found when comparing them with the standard calculation of a supernova evolving into a constant density medium n0 = 1 cm−3 is that radiative cooling becomes important very early in the life of the remnants. The radiative phase starts well before the ejecta is fully thermalized and while the expansion velocities are still in the range of several thousands of km s−1. Consequently, the remnants miss their Sedov evolutionary phase and, unlike the standard case, in these calculations full thermalization of the ejecta is only completed long after the moment of thin shell formation (see Terlevich et al. 1992, 1994a; hereafter referred to as papers I and II). The cooling event leads to large luminosities (≥ 109 L⊙) in spans of time of only a few years, causing a major rapid depletion of the supernova's stored thermal energy, in only a few weeks or months. Strong radiative cooling leads to an ionizing spectrum (see paper I) and thus to an HII region with multiple components, as it photoionizes the recombining, rapidly-moving swept-up gas and the outer unperturbed matter. The ionizing radiation is also absorbed by the still unshocked and dense expanding ejecta.

Such remnants, hereafter termed “compact SNRs”, are capable of producing the strong ionizing flux that makes them appear as Seyfert I impostors (Filippenko 1989) when occurring in dense regions far away from the nucleus of galaxies.

Introduction

Rapid progress has been made over the last few years in the study of the neutral gas in planetary nebulae (PNe), and it is now well established that at least some PNe are surrounded by massive envelopes of neutral gas (see, e.g., the review by Huggins 1993). The neutral envelopes provide a new perspective on the formation and evolution of the ionized nebulae, and allow the study of a range of circumstellar processes with different characteristics than those found in other circumstellar environments. In this paper we summarize the results of a recent survey of millimeter CO emission in PNe to study the molecular component of the neutral gas, and we comment on some of the issues raised by the observations.

The Molecular Gas in PNe

Millimeter CO emission has proved to be an especially useful probe of the neutral gas in PNe, since it can be used to determine the structure and kinematics of the envelopes, and to estimate the mass of the molecular component. In order to systematically study the differences between PNe, particularly evolutionary effects, we have undertaken extensive survey work in the 230 GHz CO (2–1) line using the IRAM 30 m and SEST 15 m telescopes. These provide access to both northern and southern PNe, with angular resolutions of 12″–24″. Our observations considerably extend the earlier survey work by Huggins ≈ Healy (1989), and are up to a factor of ≈ 6 times more sensitive, depending on the angular size of the PNe.

Introduction

It is well established that the bulk of mass loss from low and intermediate mass stars occurs during the asymptotic giant, branch (AGB) stage of evolution, leading to the well-defined sequence of mass-losing stars in the IRAS two-colour diagram (van der Veen and Habing 1988) and the formation of planetary nebulae (Abell and Goldreich 1966; Renzini 1981). However, a reliable theoretical understanding of the causes of mass loss is still not available, although progress is being made. An additional complication is that the time history of mass loss during AGB evolution is quite complex since AGB evolution is modulated by helium shell flashes which control the surface luminosity and thereby the mass loss rate. In this paper, mass loss rates from AGB stars are discussed and the effects of helium shell flashes on the mass loss are described and compared with observations. Time dependent winds produced by AGB stars are reviewed. Finally, the evolution of AGB stars that lose mass in binary mass transfer events is briefly described.

AGB mass loss rates

IRAS observations of stars in the solar neighbourhood indicate that those stars with substantial circumstellar shells - those with high mass loss rates - are nearly all AGB stars undergoing large-amplitude pulsation (Habing 1990).

Introduction

Practically all diffuse media of astrophysical significance are clumpy media which are responding to energy sources. The most important distinction between flows initiated in clumpy as opposed to homogeneous media, is that in the former, there is mass, momentum and energy interchange at clumptenuous plasma boundaries, i.e. in boundary layers. The consequences are major (Hartquist & Dyson 1993). This interchange reacts back on the dynamical, physical and even chemical state of the global tenuous plasma flow; conversely, the state of the global flow influences the interchange process.

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.