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Red giants are sometimes surrounded by envelopes, the result of the ejection of stellar matter at a large rate (Ṁ> 10 −7 M /yr) and at a low velocity (10 km/s). In this review the envelopes are discussed and the relation between stars and envelope: what stars combine with what envelopes?The envelope emits radiation by various processes and has been detected at all wavelengths between the visual and the microwave range. I review the observations of continuum radiation emitted by dust particles and of rotational transitions of molecules, where these molecules have been excited by thermal or by non-thermal ("maser") processes. I discuss mainly the oxygen-rich stars, those of spectral type M, and only briefly the closely related carbon-rich stars.By and large the density in the envelope is well described by spherically symmetric outflow at a constant velocity; on the time scale needed to flow from stellar surface to the outermost layers, i.e. 10 5 yr, the loss of mass is sometimes interrupted suddenly after which the envelope becomes "detached" from the star. The temperature decreases when moving outward; heat input is by friction between dust particles and gas and cooling occurs by line radiation by various molecules, especially by H 2 O. The molecular composition is determined by formation in an equilibrium process deep in the atmosphere and by destruction in the outer parts of the outflow by interstellar UV radiation (H 2 , CO, H 2 O) or by depletion due to condensation on dust grains (SiO); dust particles of silicate material solidify where the radiation temperature is decreased to about 1000 K, and this is at a few stellar radii.The various continuum spectra produced by the dust particles in different stars are well modelled by a simple model of the density and dust temperature distribution plus the assumption that the particles consist of "dirty silicate", i.e. silicate with Fe and Al ions added. A large range of optical depths, τ 9.7µm , is observed: from 0.01 to 10. In envelopes with large optical depth the star itself can no longer be detected directly. Model calculations also show that the momentum in the outflow, i.e.Ṁ .v out is provided by radiation pressure on the dust particles followed by the complete transfer of this momentum to the gas. The mass-loss rate itself,Ṁ , is not determined by radiation pressure but by dynamic processes in the region below the 98 H.J. Habing dust condensation layer. When τ 9.7µm is sufficiently large its measurement, that of the stellar luminosity, L * and that of the outflow velocity, v out , permit the determination ofṀ , i.e. the total outflow rate, without making assumptions about the abundance of the dust particles or of the molecular gases. Detached envelopes have been seen in a few cases.Thermal molecular radiation is faint compared to the maser emission but has been measured in distant stars, e.g. in stars near the galactic center. Different molecules outline different "spheres" around the star. The largest sphere (a radius of 0.1 pc) is outlined by an emission line...

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Abstract. We present photometric ISO 60 and 170 um measurements, complemented by some IRAS data at 60 µm, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Fifty stars were detected at 60 µm; 36 of these emit a flux expected from their photosphere while 14 emit significantly more. The excess emission we attribute to a circumstellar disk like the ones around Vega and β Pictoris. Thirty four stars were not detected at all; the expected photospheric flux, however, is so close to the detection limit that the stars cannot have an excess stronger than the photospheric flux density at 60 µm. Of the stars younger than 400 Myr one in two has a disk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their disk in a similar way: some stars as old as the Sun still have significant disks.

This article compares the distribution of K s magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon-and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5−6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in Paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in Paper III of this series.

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Lithium and zirconium abundances (the latter taken as representative of s-process enrichment) are determined for a large sample of massive Galactic O-rich AGB stars, for which high-resolution optical spectroscopy has been obtained (R ∼ 40 000−50 000). This was done by computing synthetic spectra based on classical hydrostatic model atmospheres for cool stars and using extensive line lists. The results are discussed in the framework of "hot bottom burning" (HBB) and nucleosynthesis models. The complete sample is studied for various observational properties such as the position of the stars in the IRAS two-colour diagram ([12] − [25] vs.[25] − [60]), Galactic distribution, expansion velocity (derived from the OH maser emission), and period of variability (when available). We conclude that a considerable fraction of these sources are actually massive AGB stars (M > 3−4 M ) experiencing HBB, as deduced from the strong Li overabundances we found. A comparison of our results with similar studies carried out in the past for the Magellanic Clouds (MCs) reveals that, in contrast to MC AGB stars, our Galactic sample does not show any indication of s-process element enrichment. The differences observed are explained as a consequence of metallicity effects. Finally, we discuss the results obtained in the framework of stellar evolution by comparing our results with the data available in the literature for Galactic post-AGB stars and PNe.