Ba II stars are red giants showing an enhancement of carbon and s-process elements. The elucidation of their nature seems to require a mass transfer, either by wind or Roche lobe overflow, during their past evolution. Were it really the case, all Ba II stars would be binaries with a white dwarf as companion. To better understand the exact role of their binarity, more orbits are definitely needed. They can be obtained by monitoring the radial velocity variations of those stars. However, a quicker way to find new Ba II stars with orbital elements would be to search for their existence among known spectroscopic binaries. This would also crucially test whether mass transfer is a necessary and sufficient condition to explain Ba II stars. If it is indeed the case, then all spectroscopic binaries, made of a giant and a white dwarf, in a reasonable range of periods, would exhibit the Ba II pecularity. However, the discovery of a peculiar giant+main sequence binary system would imply a revision of our ideas about Ba II stars. To this end have we begun a systematic spectral survey of spectroscopic binaries with orbital periods in the range characteristic of known Ba II stars and containing a red giant. The realization that some stars of the catalogue we compiled were already identified as semibariium stars encourages us to pursue our investigation. Coude spectra were taken with the 152 cm telescope, at a dispersion of 12 Å mm−1 . Until now, 2 stars out of a sample of 31 present a slight enhancement of s-process elements (their anomaly being in the range Ba 0.3 to 0.5), and 2 more appear to be good candidates. The study of a larger sample is currently in progress. A discussion of the nature of the companion to the 2 newly discovered semibarium stars is presented on grounds of their mass function and photometric indices.

Simulated populations of white dwarfs and N type carbon stars were generated for a Salpeter initial mass function and constant stellar birth rate history. The effect of very strong mass loss on the mass distribution of white dwarfs and the luminosity distribution of carbon stars is discussed and the results are compared with observations. A significant mass loss by stars on the TP-AGB occurs besides regular stellar wind and planetary nebulae ejection. Thus it is possible to explain the luminosity functions of carbon and M stars in the Magellanic Clouds (with very few stars brighter than Mbol = -6.0), the very narrow mass distribution of white dwarfs, and the very small number of white dwarfs with M > 1.0 MΘ. The luminosity of some AGB stars in the SMC is so high that they may be supernova of type 1 1/2 precursors. There are no such stars in the LMC. Comparison of the theoretical and observed luminosity distributions of high-luminosity AGB stars in the Magellanic Clouds shows that the mass-loss rate of these stars in the LMC is about an order of magnitude larger than in the SMC. In the Galaxy carbon stars may form only from stars with initial mass less than 1.5 MΘ due to the relatively small initial heavy element abundance in these stars; this is perhaps the main reason for the absence of carbon stars in open clusters in the Galaxy.

The evolution of a Population I star of 7 MΘ with mass loss has been followed. The mass-loss rates of the star are calculated by using the empirical relation of Waldron (1984) and the formula introduced in this paper. In the red supergiant stage there occur thermal pulses of the He-burning shell source either for the evolution with constant mass or for the evolution with the mass loss rate calculated by Waldron's relation. The rate of mass loss will become very large before the occurence of thermal pulses of He-burning shell source and the envelope of the star can be lost in a short period of time, if the introduced formula of the mass loss rate is used. The effects of mass loss on the internal structure and the evolution of the star, especially on the He-burning shell source are considered in detail.

It is generally believed that red giant mass loss plays some role in the formation of planetary nebulae (PNe), but the connection is not clearly understood. To investigate this issue we have undertaken an extensive search for molecular gas in PNe, which is likely to be remnant material of the red giant wind not yet ionized by the central star. The search has been carried out with the NRAO 12 m telescope in the 1.3 mm line of 00 which is widely observable in the molecular winds of red giants.

Stellar evolution calculations confirm that semiconvection will occur below the convective envelope of a low-mass, low-metallicity AGB star, after a thermal pulse. These calculations show how semiconvection leads to the creation of a “13C layer” in the star, which can provide a potent source of neutrons (via the 13C[a, n]160 reaction) in a convective shell during later evolution. The rate at which neutrons are released is largely determined by the rate at which the 13C layer is introduced into the convective shell. The 13C neutron source maintains neutron densities of 109-1010 n/cm3 for ∼ 10 years. This provides a neutron exposure r=0.15 mb“1 during most of the pulses calculated. Because of the strong filtering effect by light elements, only 10—20% of the neutrons produced will be captured by iron-seed nuclei, each such nucleus capturing 4–5 neutrons per pulse.

The surface C/N abundance ratios of many cluster and field G and K giants following the 1st dredge-up phase are much lower than predicted from standard stellar evolution modeling. The occurrence of substantial mass loss, either during or immediately after the main-sequence phase would both reduce the mass fraction of the unprocessed envelope necessary to contaminate with CN-cycle products, as well as allow CN-processing of a greater amount of core material during the earlier high-mass phase. Willson, Bowen and Struck-Marcell (1987) have proposed that a combination of pulsation and rapid rotation could drive substantial mass loss in main-sequence stars of initial mass 1-3 MΘ. We evolved a grid of 16 mass-losing models from the zero-age main sequence through 1st dredge-up. The models have initial masses of 1.25, 1.5, 1.75 and 2.0 MΘ, and exponentially decreasing mass-loss rates with e-folding times 0.2, 0.4, 1.0 and 2.0 Gyr; all models evolve toward a final mass of 1.0 M". Since the mass-loss epoch is short-lived, most of the models reach 1.0 M0 rapidly, and follow the evolutionary track of a standard 1 MΘ model redward away from the main sequence and up the 1st giant branch. The convecuve envelope deepens during 1st dredge-up to homogenize the outer 3/4 of the star's final mass.

Conventional theory explains the origin of carbon stars as due to dredge up of carbon enriched material from the stellar core during helium flash events late in the life of solar mass AGB stars (e.g. Boothroyd and Sackmann 1988). This relatively efficient process however, seems to produce a larger C/O ratio than observed (Lambert et al. 1987). A secondary effect which could contribute to the appearance of carbon stars, is the selective removal of oxygen from the atmosphere by radiative force expulsion of oxygen rich dust grains (e.g. silicates like [Mg, Fe2SiO4]). We present calculations for this scenario which evaluate the degree of momentum coupling between the grains and gas under the thermodynamical conditions of AGB star atmospheres.

We suggest that the shape of a young asymmetric planetary nebulae may be influenced by a close binary star located at its center. This binary is a relic of the common envelope phase, presumably through which the asymmetric planetary nebula evolved. We assume that for a short period of time, shortly after the cession of the slow wind and long before the fast wind becomes effective, the binary ejects a small amount of mass, mainly in the equatorial plane. In this work we do not discuss the exact mechanism for the ejection of this pulse of mass. In the case in which the cooling is very efficient, (i.e., high-Mach-number isothermal flow), we can solve the problem analytically by using a few simplifying assumptions. In this case the high density region is shaped like a ring. We use two-dimensional hydrodynamics for the more general case. We find that at late times the high density region has a “horseshoe” shape, as viewed in the symmetry plane. There is an instability in the maximum density region. Finally we compare our results with the shape of the planetary nebula M2-9.

IRAS 21282+5050 is very compact planetary nebula with a substantial molecular shell. It is carbon-rich and exhibits infrared features attributed to PAH molecules. It has a far infrared color temperature higher than almost all other planetary nebulae, with IRAS flux ratios similar to low color temperature evolved stars. We present VLA images of IRAS 21282 at wavelengths of 2 and 6 cm (with FWHM beamsizes of 0.5” and 0.9”, respectively).

Binaries with very wide spearations are thought to evolve to small separations through a catastrophic form of mass exchange/loss known as common-envelope evolution. The theory of this process is fairly well developed, but proper tests remain elusive. Simply put, the theory argues that the rapidly shrinking Roche lobe of the mass losing giant will strip away the giant's main-sequence companion. Loss of mass from the system during the process carries away orbital angular momentum, thereby strengthening the effect.

We define the relationship between nitrogen enrichment in planetary nebulae and the mass of the nucleus. N/O remains flat at about 0.3 (double solar) from a core mass of 0.55 M(sun) to 0.8 M(sun), whereupon it rises quickly to values that approach and may exceed 2. The rate of increase of N/O with core mass exceeds that predicted for giant stars by standard dredge-up and mass-loss theories.

The possible existence of a hot CN-cycle at the bottom of the AGB convective envelopes (“hot-bottom burning” or HBB) encounters some revealing difficulties with observations (e.g. Reid and mould, 1985). On the other hand, existing models disagree with each other about it.

We examine the results of s-Processlng occurring in a low mass star of low metallicity during the pulsed He-instability in AGB phases by comparing them with the s-Classical analysis. Neutron exposures are provided by the C13(Alpha, N)016 reaction, according to the mechanism suggested by Iben and Renzini (1983) for the formation in the interpulse phase of a small zone rich of C13 and its subsequent ingestion in the next pulse.

Why stars become red giants has been a subject of investigation in many contexts, most recently in the discovery of the progenitor of the Supernova 1987A. Sanduleak 69°202 was found to be a blue supergiant star although it was generally presumed that type II SNe arise from red supergiants. Immediately after SN1987A, it was suggested that the blue spectral nature was due to lower metallicity in the LMC (Z = ZΘ/3 - ZΘ/4) although the existence of many red supergiants in the 30 Doradus region where SN1987A took place and in particular the observation of low-velocity nitrogen-rich gas presumed to be a circumstellar shell indicates that mass loss may also have played a significant role in bringing SK 69°202 from red to blue. We report here work in progress on the evolution of massive stars in the LMC with and without mass loss which can ultimately produce type II SNe.

The study of long period variable stars has been transformed in recent years by two observational developments. Large samples of stars have been observed at infrared wavelengths, providing knowledge of the intrinsic properties of the star as well as of circumstellar dust shells, and these observations have been extended to the variables in well defined stellar systems to allow their properties to be studied in relation to the stellar population to which they belong. Spectroscopic determinations of chemical composition have also provided several crucial insights.

Aperiodic intrinsic variability becomes increasingly evident for a greater number of red giant and supergiant stars as observations become more technically refined and extended over a longer time. All red variables show an erratic temporal behavior to some degree. Even Miras, with their temporal changes in maximum brightness and irregular fluctuations in the cycle length, are not perfect clocks.

The pulsation of a long-period variable star generates shock waves which dramatically affect the structure of the star's atmosphere and produce conditions that lead to rapid mass loss. Numerical modeling of atmospheres with periodic shocks is being pursued to increase our understanding of the processes involved and of the evolutionary consequences for the stars. It is characteristic of these complex dynamical systems that most effects result from the interaction of various time-dependent processes. For example, rapid mass loss in the models is a joint consequence of the enormous extension of the atmosphere caused by shocks, and of radiation pressure on grains formed in the cool outer region; it is also affected by thermal relaxation processes, which determine the temperature distribution. The progress and significance of these modeling calculations will be reviewed.

For many years, it has been commonly believed that oxygen-rich (M) stars evolve first to S stars and then to carbon (C) stars. However, the details of the transition are not understood. It is now accepted that the overabundance of carbon ([C/O] > 1) in some asymptotic giant branch (AGB) stars is due to the dredge up of products of a capture and s-process elements after a number of thermal pulses (Iben 1975). Effects of convective overshooting and semiconvection in the dredge up process have also been considered (Castellani et al. 1985, Lattanzio 1986). The dredge up of carbon into the photosphere leads to the formation of carbon-based molecules, which absorption bands become the basis of spectral classification.

During the past decade, it is recognized that the carbon-richness of a star not only manifests itself in the photospheric spectrum, but also in the circumstellar environment as well. Probes of the circumstellar envelopes in the infrared and radio regions provide new means to characterize the chemical properties of the star. These methods are particularly useful in cases where the photosphere is heavily obscured by circumstellar absorption. Table 1 summarizes the photospheric and circumstellar spectral characteristics of M and C stars.

A large sample of 177 Miras, comprising 164 M and 13 S stars, has been examined to determine the dependence of 9.7 μm silicate emission, as revealed by their IRAS LRS Spectral class, on the visual light curve asymmetry factor, f. It is found that the silicate feature occurs not only in M (Vardya et al. 1986; Onaká & de Jong 1987) but in S Miras also only for f ≤ 0.45. This, however, is only a necessary condition, as about one fifth of Miras with f ≤ 0.45 do not show the 9.7 μm emission. This non-detection shows dependence on other parameters like the mean visual light amplitude. Non-detection is highest in the region 0.43 < f ≤ 0.45, as well as when mean amplitude is ≤ 5m.0. Though strong emission features in M Miras may occur for any value of f, very weak features are absent for small values of f, and the strongest feature tends to appear for large values of f. Infrared excess tends to increase with increase in the strength of the silicate emission and with decrease in the value of f.

R CrB has been placed on the program of the Automatic Photometric Telescope Service (Genet et al. Pub. A.S.P. 99, 660, 1987) for continual monitoring while near maximum light UBV data are obtained on almost every clear night during the star's season and will be published in batches every few years