Firstly, a brief account is given on the recent progresses in model assumptions, which influence the evolution of surface abundances in massive stars. A concise overview of this chemical evolution is presented.

We discuss in detail the surface chemistry of WN and WC stars, in particular, the effects of the initial metallicity Z and mass loss rates on the resulting abundances. The abundances in WN stars are not very sensitive to model assumptions and are mainly a test on nuclear cross sections. Conversely, the chemical abundances in WC stars are very much model dependent. Major differences in the composition of WC stars (and subtype distributions) are expected according to their initial metallicity. The various models proposed to explain the N/C enrichments in blue supergiants and the properties of the progenitor of SN 1987A are reviewed.

The abundances of C, N, O and the C and O isotopic ratios of stars undergoing the first dredge up phase of convective mixing are summarized in this paper. It is shown that rough agreement between predictions and observations of the surface abundances may be achieved for young Pop I giants, but that the observations deviate increasingly from the predictions in old disk field giants and especially in Pop II field and globular cluster giants. Recent investigations containing new information on the correlations among these elements in globular cluster giants are discussed.

Abundances of Population II stars of the field and globular cluster systems are reviewed. Emphasis is placed on [Fe/H] ratios determined from analyses of moderate- and high-resolution spectroscopic data. The status of analyses of metal-deficient stars is examined from the viewpoint of number of analyses and number of stars, overall distribution of the [Fe/H] ratios, and systematics of the [Fe/H] ratios.

A brief review is given of the structure of asymptotic giant branch (AGB) stars and of the characteristics of the thermal pulses which these stars experience. Following a pulse, model AGB stars with a large core mass easily dredge up fresh carbon, which is the main product of incomplete helium burning, and s-process isotopes, which are made as a consequence of the activation of the 22Ne neutron source. Model AGB stars of small core mass activate the 13C neutron source and produce s-process isotopes in nearly the solar system distribution. They also dredge up fresh carbon and s-process isotopes, but only if overshoot or some other form of “extra” mixing beyond the lower boundary of the convective envelope is invoked.

Recent results on low mass AGB stars are presented. Observed amounts of AGB mass loss imply that thermal pulses will only be encountered for stars of initial mass less than about 4M⊙ for Pop I and 3 M⊙ for Pop II. Mc – L, Me – τif, and Mc – Tb relations are summarized. Carbon dredge-up has been found in low mass stars of both Pop I and Pop II; the mixing length parameter α is crucial to dredge-up, and its value must be normalized according to each author's opacities and mixing length treatment (e.g., via the Sun's Te and L). The “carbon star mystery” is nearing a solution, but a new “s-process mystery” has appeared: only in a narrow range of mass and metallicity have theoretical models been found that encounter the semiconvective 13C s-process mechanism.

A brief overview is presented of the s-process of nucleosynthesis. Special emphasis is put on some of its nuclear physics aspects, and in particular on the uncertainties that still affect the rates of the 13C(α,n)16O and 22Ne(α,n)25Mg neutron producing reactions. We also briefly review the so-called “classical” s-process model, and put it in perspective with recent s-process calculations conducted in the framework of detailed models for massive stars or for thermally pulsing AGB stars.

This review discusses the chemical composition of AGB stars in the light of predictions for intermediate-mass (3-8 M⊙, 22Ne(α,n) = the neutron source) and low-mass (< 3 M⊙, 13C(α,n) = the neutron source) stars. LM-AGB models can be constructed with envelopes having a composition quite similar to that of solar system material, the SiC grains recently discovered in meteorites, and real AGB stars in the sequence of spectral types M → S → C. Stellar counterparts of the IM-AGB models have yet to be discovered.

The quantitative determinations of technetium abundances are discussed. Up to now Tc abundances were found for nearly 30 stars which all are thermally pulsing AGB stars. The technetium abundance values for these AGB stars atmospheres, where it was found to be present, cluster around −1.0 ÷ 1.7. The fact that Tc is not omnipresent in the atmospheres of AGB stars, which are highly enriched in other heavy elements produced in s-process, is discussed.

The present situation concerning the observations of lithium in subgiant and giant stars is reviewed. These observations, compared to the determinations of lithium in main-sequence stars, can be used to obtain more stringents constraints on the dynamics of the stellar gas in the main-sequence as well as in the subgiant and giant stages. The macroscopic motions expected in stars are discussed from a theoretical point of view and compared to the observations, with special focusing on the theories of rotation-induced turbulence.

Objects that may be in a post-AGB evolutionary stage include high-latitude supergiants, UU Her and RV Tau variables, and IRAS sources identified with A- to G-type supergiants. Photospheric abundance analyses of such objects typically reveal moderate to extreme iron-group deficiencies, consistent with membership in the thick-disk or halo populations, and with having arisen from low-mass progenitors. The photospheric CNO abundance patterns in such stars are distinctly atypical of normal Population I supergiants, and appear to indicate the presence of helium-burning products at the stellar surface. However, enhancements of s-process elements are typically not seen, suggesting that most of the stars have avoided the AGB dredge-up. A subset of the stars, typified by HR 4049, show ultra-low iron-group abundances and extreme enhancements of C, N, O, and S. They appear to constitute a new class of chemically peculiar stars, with severely depleted photospheric metals. Grain formation is proposed as the depletion mechanism.

About one half of all stars are close binaries whose components can efficiently interact during their evolution. The interaction of the components significantly influences the evolution and changes the chemical composition of their atmospheres. This review is devoted to the discussion of the binarity role in the creation of stars with an anomalous chemical composition of their envelopes: WR, OBN, OBC, RCrB, Ba, sdBO, Am, Fm, Ap, Bp, helium stars, CO stars, Na giants.

Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars and PG 1159 stars are reviewed. These classes of stars trace stellar evolution from the horizontal branch towards the white dwarf cooling sequence. The impact of Non-LTE model atmospheres for the analyses of sdO and PG 1159 stars is outlined. The resulting atmospheric parameters and abundances are used to obtain constraints on the evolutionary status of the different classes of stars. The sdB stars form a homogeneous group and can be identified with models of the extended horizontal branch. Abundance anomalies (deficiency of helium and some metals, enrichment of 3He) observed in Horizontal Branch stars and sdB stars do not give hints to the stars' evolutionary history but are caused by atmospheric diffusion. The class of subluminous O stars is much less homogenous and two subclasses can be defined: the “compact” sdO stars probably evolved from the extended horizontal branch and are hence successors of the sdBs, whereas some sdOs of relatively low gravity are in a post-AGB stage of evolution. The third class, the PG 1159 stars, are amongst the hottest stars known with effective temperatures exceeding 100,000 K and are immediate progenitors of the white dwarfs. The abundance pattern is dominated by carbon and helium with a significant admixture of oxygen, whereas hydrogen and nitrogen are trace elements only. As in the case of the (helium rich) sdO stars these abundances indicate that nuclear processed material is exposed at the stellar surface. The origin of the PG 1159 stars as well as of the “low gravity” sdO stars can be explained by the born-again post-AGB star scenario of Iben et al. (1983).

The available information about spectral characteristics and photospheric abundances of 115 CSPN is presented. About 33% of CSPN in the sample are H-deficient. Many (but not all) H-deficient CSPN show extremely high C abundances. There is also some evidence of a variety of C abundances among H-rich CSPN. The available evidence suggests that most CSPN leaving the AGB with a H-rich photosphere will become DA white dwarfs. The “late He shell flash + fast wind” (born-again) mechanism may offer a satisfactory way of explaining some of the H-deficient CSPN. However, at least in some cases, the complete ejection of the H-rich stellar envelope, and exposure of He-burned matter at the remnant stellar surface, must happen already at the time of departure from the AGB.

The mixing and nucleosynthetic processes which occur in the main-sequence and red giant stages of evolution and which can affect measured PN abundances are discussed. It is suggested that samples of planetary nebulae contain a sufficient range of progenitor stars' initial metallicities, angular momenta, binarity, etc. so as to explain the large scatter always seen in abundance diagrams for PN samples. CNO abundances in the Galactic disk and in the Magellanic Clouds are reviewed. New results for nebular abundances and central star properties in the Clouds are given. The current red giant population in the Clouds mix out more 12C per star than their Galactic disk counterparts.

In the present review we summarize the problems relative to the chemical composition of the inner and outer layers of white dwarfs as expected from stellar evolution. We point out that there is a contrast between standard predictions and the indications deriving from studies of white dwarfs, as “massive” hydrogen remnat layers seem not to be present on single white dwarfs. We discuss a previously neglected feature of stellar models in the phase of thermal pulses -which occurs when the outer hydrogen envelope becomes very small- by which the progenitors of low total mass may get rid of practically the whole hydrogen envelope during the final phases of asymptotic giant branch evolution. We finally propose a new global scheme for the pre-white dwarf evolution, which depends mainly on the initial progenitor mass.

Various evidence indicates that the atmospheric compositions of white dwarfs may switch from hydrogen- to helium- dominated, and vice versa, over wide ranges of surface temperature (luminosity). This envelope evolution may be understood only by assuming that the hydrogen (DA) sequence retains only a tiny residual mass of hydrogen, many orders of magnitude less than that predicted by stellar evolution theory. A corresponding problem may exist for the helium envelopes of some lot stars with helium-rich atmospheres.

Of all stars in the Hertzprung-Russell diagram, white dwarfs are those for which the clues on past evolution given by photospheric abundances are probably the hardest to decipher. This is because the cooling phase of white dwarfs, a relatively uneventful phase from an evolutionary point of view, is, in contrast, a most active phase for the evolution of the chemical composition of the envelope. Indeed, it is now well established that the often puzzling variety of surface abundances observed in white dwarf stars can be traced to the simultaneous operation, in the outer layers of these stars, of a variety of physical processes which will also erase the abundances present in the photosphere at the onset of cooling.

Downward element diffusion in the intense gravitational field of the degenerate star is perhaps the mechanism which is the most closely identified with white dwarf stars. However, convective mixing, ordinary diffusion, radiative forces, winds, and accretion from the interstellar medium all are equally important processes which, at times, compete efficiently with the rapid element segregation expected in those stars. The various regions, along the cooling sequence of white dwarfs, where individual processes are expected to operate, have been summarized by Fontaine and Wesemael (1987). We illustrate here various combinations of these mechanisms which have been found in white dwarfs, and show how their competition affects the observed abundance patterns. The unity underlying these cases stems from the fact that, in many cases, progress in investigating these complicated situations has come only through the combination of evolutionary calculations with new and powerful numeriques techniques which have been developed at Montréal (Pelletier 1986; Pelletier, Fontaine, and Wesemael 1989).

Although the availability of large telescopes and more sensitive detectors has led to a much more complicated picture of white dwarf spectral classes and abundance patterns as compared to the situation ten years ago, it still remains true that the vast majority of white dwarfs show a pure hydrogen spectrum even in temperature regions where the presence of other elements with e.g. solar abundances would be easily revealed.

After a week of highly interesting discussions on ‘stellar evolution and the photospheric abundance connection’ we now come to resume the contributions to this Symposium. This task has been divided up among three of us, arranged according to the course of stellar evolution, and in alphabetical order of the speakers (Baschek, Lambert, van Horn). I begin with the essentially unevolved stars, recalling some results and problems, not systematically but rather selected from my own interests.

First I should like to briefly reconsider the accuracy of abundance determinations in view of the contributions following my review. Then some comments on stellar models are given, followed by a few remarks about peculiar A stars. Finally, the similarity of the oxygen abundance in Magellanic Cloud B stars and galactic population II stars is pointed out.

On Sunday morning, the day before our symposium opened here by the Black Sea, Georges Michaud asked me if I would be willing to be one of three speakers at this closing session. When he intimated that I was to be a stand-in for Wal Sargent who is unable to be here, I set aside all remnants of reluctance and agreed to entertain you for approximately twenty minutes. Wal was one of my earliest inspirations as I trod a path from solar physics into stellar spectroscopy.