Several lines of evidence point to a scenario in which Tc-poor S stars are the cooler analogs to the Ba II stars, i.e. they are binary systems where the peculiar atmospheric composition of the primary star is due to mass accreted from a secondary star long ago. Cases have been found where such S stars have WD or main-sequence companions, but an increasing number are found to be in interactive, symbiotic-like systems. Evidence of wind shocks, gas-streaming, and/or accretion disks in these systems attest to the current proximity of the components and provide striking evidence that accretion can be an important mechanism in their chemical evolution. Interactive effects are not as prominent in Ba II stars, presumably because of the lack of a strong wind from the primary star.

We discuss GHRS spectra of single and binary late-type stars and describe in detail the spectra of α TrA and of ζ Aurigae obtained at ten orbital phases. The wind properties of α TrA are derived using a complete redistribution radiative transfer code, and we describe the properties of a new code, PRISMA, that we are building to fit line profiles using partial redistribution in a spherically-symmetric geometry. The ζ Aur spectra show that the mass loss process is variable on the timescale of several months, the wind density structure does not repeat from orbit to orbit, and the wind ionization structure is complex.

We examine whether dust forms in the photospheres of carbon-rich stars by referring to the case of red and brown dwarfs for which some observational clues on dust formation are now known. Dust may form in the photospheres of dwarf carbon stars and produce significant effects on both their structure and spectra. In carbon-rich asymptotic giant branch stars, dust probably forms in the photosphere, if not in the circumstellar envelope, and radiation pressure on dust is sufficient to expel the matter directly from the photosphere. This fact may play some role in mass-loss from cool luminous stars in general, including non-pulsating stars for which no successful mechanism of mass-loss was known.

A new mechanism is proposed to explain an asymmetric mass loss of carbon-rich AGB stars where slow stellar rotation modifies a wind due to the non-linear behavior of the dust formation process. This effect leads to a preferential mass loss with higher velocities in the equatorial plane and also provides a simple explanation for the widely-observed asymmetries in the shapes of planetary nebulae.

Chemical models of the inner circumstellar envelope of a typical carbon-rich AGB star are presented. The effect of pulsation-driven shocks on the gas close to the stellar photosphere is considered. The chemistry of dust condensation nuclei formation is described and applied to the gas layers close to the star. We derive formation yields for polycyclic aromatic hydrocarbon (PAH) species and their dimers and discuss their role in the condensation of dust.

The problem of dust formation in the circumstellar envelopes of Asymptotic Giant Branch stars is reviewed. Special emphasis is put on the consistent modelling of the dust-forming circumstellar shell, where due to a strong coupling the dust formation process governs the dynamical behavior of the object.

Inside carbonaceous chondrite meteorites are tiny dust particles which, when heated, release noble gases with an isotopic composition different from what is found anywhere else in the solar system. For this reason it is believed that these grains are (inter)stellar dust which survived the collapse of the interstellar cloud that became the solar system. We will describe here why we believe that the most abundant of these grains, micro-diamonds, were formed in the atmospheres of carbon stars, and explain how this theory can be tested observationally.

The circumstellar shells of M stars produce emission features due to amorphous silicates peaking around 10 and 18 μm, with additional emission at 11 μm due to crystalline olivine and at 13.1 μm (unknown carrier). C stars are associated with SiC dust emission at 11.2 μm with additional emission around 8.9 μm, whereas S stars have a relatively weak 10.5 μm emission feature which is due neither to silicates nor to SiC.

Spectroscopy and infrared photometry of carbon stars show three distinct forms of circumstellar matter. IRAS 12311-3509 probably has an edge-on disk and the spectrum is dominated by resonance emission from atoms and molecules in the vicinity. The long-period variables V Hya and R Lep are undergoing deep fadings, apparently caused by dust formation around the star, while variable emission from circumstellar gas is seen. The semiregular variable T Mus showed absorption bands from very cool material during an unusual episode in 1994.

We present dynamical models of circumstellar dust shells around long-period variables which include time-dependent hydrodynamics and a detailed treatment of dust formation, growth and evaporation. Important effects due to the complex interaction between the dynamics of the pulsating atmosphere and the dust complex are demonstrated.

The spectral energy distributions of 16 AGB stars in the LMC are fitted. The 6 known oxygen-rich stars are well fitted with silicate dust. Due to the completely different absorption properties of silicate and carbonaceous dust we argue that of the 10 stars of unknown spectral type, 1 is probably O-rich while the other 9 are probably C-rich.

The Period-Luminosity relation is discussed. Both the O- and C-rich LPVs in this study appear to lie on extensions of P-L relations derived for stars with much shorter periods.

Infrared emission from radiatively heated dust possesses general scaling properties. The spectral shape is independent of overall luminosity when the inner boundary of the dusty region is controlled by dust sublimation; the only relevant property of the heating radiation is its frequency profile. For a given type of dust grains, the emission from red giants and supergiants is essentially controlled by a single parameter - the overall dust optical depth. This leads to tight correlations among different spectral properties that explain many available observations and enable systematic studies of large data bases.

Infrared emission from the dust shell around IRC+10216 is analysed in detail, employing a self-consistent model for radiatively driven winds around late-type stars that couples the equations of motion and radiative transfer in the dust. The resulting model provides agreement with the wealth of available data, including the spectral energy distribution in the range 0.5-1000 μm, and visibility and array observations. Previous conclusions about two dust shells, derived from modelling the data with a few single-temperature components of different radii, are not supported by our results. The IR properties vary with the stellar phase, reflecting changes in both the dust condensation radius r1 and the overall optical depth τ - as the luminosity increases from minimum to maximum, r1 increases while τ decreases. We find that the angular size of the dust condensation zone varies from 0.″3 at minimum light to 0.″5 at maximum. The shortage of flux at short wavelengths encountered in previous studies is resolved by employing a grain size distribution that includes grains larger than ∼ 0.1 μm, required also for the visibility fits. This distribution is in agreement with the one recently proposed by Jura in a study that probed the outer regions of the envelope. Since our constraints on the size distribution mostly reflect the envelope's inner regions, the agreement of these independent studies is evidence against significant changes in grain sizes through effects like sputtering or grain growth after the initial formation at the dust condensation zone.

A link between stellar and interstellar molecules is discussed with particular reference to optical and infrared emission from the Red Rectangle.

Circumstellar emission in the Na I and K I resonance lines has been detected from three carbon stars using high-resolution spectroscopy. Some properties of the circumstellar envelopes are discussed.

This review discusses the large-scale geometry and kinematics of envelopes around evolved stars, as inferred from radio line observations of circumstellar molecules, as a function of the evolutionary stage of the central object. In particular, the drastic change in morphology from largely spherical envelopes around asymptotic giant branch stars to distinctly non-spherical neutral envelopes around planetary nebulae is addressed. In addition, the small-scale structure of the envelopes, i.e., whether the circumstellar medium is smooth or clumpy, is discussed. Finally, a scenario for the morphological evolution, based on the assumption of a highly clumped medium, is presented.

We investigate the structure of extended dust shells around optical carbon stars in the far-infrared and in optical light. In the optical we have discovered that R Scl and U Ant are associated with circularly extended emission, the radii of which are about 20″ and 58″, respectively. The emission is probably scattered light of the central star by dust grains in their circumstellar shells. In the far-infrared we have discovered a double shell structure surrounding U Ant in high resolution IRAS images, which is direct evidence of a periodic change of mass-loss on a time-scale of the order of 104 years in the AGB evolution. Relating the two shells to two consecutive thermal pulses allows for a self-consistent determination of the interpulse period, core-mass, luminosity, and distance. Direct mapping of Y CVn at 90 and 160 μm with ISOPHOT on board ISO has revealed a very extended detached dust shell around this star. The mass-loss rate is found to have decreased drastically by two orders of magnitude 1.4×104 years ago, which should be an important clue to the understanding of J-type stars.

Departures from spherical symmetry are known to exist around Luminous Red Variables (LRVs). One of the earliest pieces of evidence came from the discovery that light from these objects is polarized. Additional evidence comes from other techniques and observations of related objects. This talk briefly reviews these data and focuses on the mechanisms that produce polarization in LRVs. Emphasis is given on recent spectropolarimetric monitoring data of selected key objects and what we have learned about the LRV environment from such data.

The production of certain neutron-rich elements heavier than iron occurs during He shell-burning on the asymptotic giant branch (AGB). These neutron captures occur at rather low neutron densities and, thus, the resulting heavy-element nucleosynthesis is characterisitic of the so-called s-process. Abundance analyses of the s-process elements in AGB stars can reveal details of the neutron densities and the stage of AGB evolution at which s-processing occurs, as well as the nature of the neutron source. These details derived from observations can constrain models of stars evolving along the AGB. Recent results concerning the nature of the s-process as a function of metallicity and the nature of the neutron source are reviewed.

We discuss nucleosynthesis within 6 M⊙ models with Z = 0.02, 0.008 and 0.004. The emphasis is on the AGB phase of evolution, with particular reference to thermal pulses and Hot Bottom Burning. We find strong CN cycling, with substantial Al production, especially at low metallicities.