I do not intend to try to summarize the various excellent papers that have been presented in this Joint Discussion on the accuracy of the HR diagram and related parameters. I will content myself with saying that I found them uniformly well-prepared and clear, and all very interesting. I want to thank the various speakers for doing their work so well!

Instead, I propose - since we are in the Netherlands, after all - to assume the role of the good Dutch pastor for a few minutes and draw a few morals from what we have heard and seen. I think the papers presented point to several important recommendations to all of us, and it is these points that I would like to emphasize.

The first moral that I find in this Joint Discussion is a very encouraging one for those of us who are interested in stellar astronomy and astrophysics. It is clear from this JD that our field is alive and well at the 22nd General Assembly of the IAU. This JD drew more poster papers than any of the other 19 JD’s held during this General Assembly, with 54 papers listed in the programme, compared to a mean of about 19 per JD. In addition, these poster papers come from five of the six inhabited continents, and if we allow a paper from New Zealand to stand in for Australia, then we have papers from all six. Not a bad representation, I think.

This Joint Discussion (Number 13), took place on August 22, 1994 at The Hague, in connection with the XXII General Assembly of the IAU. At the one-day long meeting, there were presentations by 15 invited speakers and 15 posters.

The Joint Discussions had been organized in response to the considerable progress made in this field of research during the previous decade. Although it had long been known that the prevailing mixing length theory (MLT), used extensively and very successfully in Astrophysics for several decades had become needlessly limited, until recently it was impractical to contemplate more realistic approaches. The situation has changed recently as a consequence of advances in numerical techniques and computational capabilities, and thus JD 13 was organized to discuss the advances, and perhaps to understand the strengths and weaknesses of each approach.

There were two presentations which addressed the main issues in convection theory (E. Schatzman), and the astrophysical implications (P. Demarque). Several talks covered current numerical codes, which included deep convection in a rotating reference frame (K. Chan), convection in the presence of magnetic fields (P. Fox), and shallower solar convection simulations on a wide range of spatial scales (A. Nordlund). Although these approaches have enriched (and are continuing to enrich) our understanding of the physics of convective fluids, they are much too detailed (both in space and in time) to be integrated in the study of stellar evolution. To overcome this shortcoming, S. Sofia described a technique developed together with Lydon and Fox to use relationships between dynamical and thermodynamic properties of convective flows derived in numerical models to be applied in stellar structure and evolution codes by performing small modifications of the standard MLT formalism. The advantage of this technique is that it does not contain a mixing length or any other arbitrary parameter, and it was used successfully in modeling the evolution of the Sun and other solar analogues. V. Canuto also presented a formulation of convection both amenable to be used in stellar evolution studies, and not requiring an arbitrary mixing length-like parameter. His formulation uses the Reynolds stress method, which has the advantage of modeling the full eddy spectrum of the turbulence, rather than the narrow wave number range for energy containing eddies assumed in the MLT. Additionally, this technique can address the problems of non-locality and overshoot. M. Stix also addressed non-locality and overshoot by presenting results of a non-local mixing length model of the Sun derived from the Shaviv and Salpeter model.

The exact title of the JD 11 was: “stellar and interstellar lithium and primordial nucleosynthesis”. The large amount of work recently done about lithium provided an incentive for a discussion among the members of several commissions of the IAU. Lithium is a peculiar element. Since it is not produced in supernovae (at least such a production is not proven and would be quite different from the production of the other elements) its presence in old material is a legacy of the primordial nucleosynthesis. But lithium is a fragile element, and from a theoretical point of view, there are arguments tending to conclude that, in old stars, this legacy has been depleted. This difficult problem is areal challenge, an has been the motivation for many different works. The analysis of the lithium behavior in well known stars, of all kind of ages, metallicities, structures, peculiarities etc. is therefore extremely useful in order to understand the physicalprocesse at work for lithium depletion, and the reader will find here many up-to-date data. The analysis of lithium in interstellar material provides an essential information. Many works about lithium are in progress throughout the world on the different points of interest, so that the General Assembly of the IAU was an excellent occasion to have a review of the recent progresses made in different areas. A summary of (nearly) each communication made during the Joint Discussion 11 may be found hereafter. Longer summaries of the talks, and summaries of the posters will be published in a forthcoming volume of the Memorie of the Società Astronomica Italiana.

Since the work of Spite & Spite (1982), halo main sequence turn-off stars have usually been considered a site where Li synthesized in the Big-Bang is wholly preserved and uncontaminated by other Li sources. However, the interpretation of halo dwarf Li abundances has become more complex as new samples of halo star Li I observations have become available. Linking the Li plateau abundance identically with the cosmological Li fraction is no longer warranted. At present, observational evidence favors a scenario where most—but not all—of the plateau abundance is cosmological in origin and a non-negligible amount is contributed subsequently by Galactic nucleosynthesis; severe, uniform Li depletion is constrained but not yet completely ruled out.

The lithium observed in the atmospheres of unevolved halo stars is believed to be an essentially unproccessed element which reflects the primordial yields. The primordial nature of Li is inferred from the presence of a constant Li abundance for all the halo dwarfs where convection is not effective (Teff ≥ 5600 K). Such an uniformity is taken as evidence for the absence of any stellar depletion during the formation and the long life of the halo stars and also as evidence for the absence of any production mechanism acting either before or at the same time of the formation of the halo population. The existence of a real plateau has been recently questioned by Thorburn (1994), Norris et al (1994) and Deliyannis et al (1993). These results open the possibility of substantial depletion by rotational mixing where a certain degree of dispersion is foreseen for different initial angular momenta of the stars and/or to a significant Galactic Li enrichment within the first few Gyrs.

Lithium abundances in a sample (39) of halo dwarfs have been redetermined by using the new Teff derived by Fuhrmann et al (1994) from modelling of the Balmer lines. These Teff are reddening independent, homogeneous and of higher quality than those based on broad band photometry. The sample represent a significant fraction of the presently available Li determinations, and form an unique sample with a good and homogeneous Teff Fuhrmann et al are also able to provide errors in the Teff for individual stars and in most cases they are as good as ± 50 K, which is a factor 2 smaller than the grossly estimated errors for photometric-based Teff. The Li equivalent widths have been taken from the literature and the theoretically derived random errors from Deliyannis et al (1993) or computed following their prescriptions. Abundances have been derived by generating new atmospheric models by using the ATLAS-9 code by Kurucz (1993) with enhanced α-elements and without the overshooting option. If we use the Kurucz 1993 grid which includes overshooting the Li abundance is increased by ≈ 0.08 dex.

The lithium isotope ratio in stars can be determined from high resolution observations of the profile of the Li I 6707 Å absorption line. Earlier studies of old F and G stars (Andersen et al. 1984, Maurice et al. 1984, Pilachowski et al. 1989) have led to upper limits of 6Li/7Li ranging from 0.05 to 0.10. Recently, Smith, Lambert & Nissen (1993) seem to have detected 6Li in HD 84937 - a metal-poor turnoff star with Teff ⋍ 6200 K and [Fe/H] ⋍ —2.4. An isotope ratio 6Li/7Li = 0.05 ± 0.02 was determined (see Fig. 1) The detection has been confirmed by Hobbs & Thorburn (1994), who derived 6Li/7Li = 0.07 ± 0.03. The main contribution to the quoted (1σ) errors comes from the noise in the spectrum (S/N = 400) and possible errors in the Doppler broadening of the Li line (Nissen 1994). This broadening is due to stellar rotation and macro-turbulent motions in the stellar atmosphere and can be determined from the profiles of unblended metallic absorption lines.

As discussed in detail by Steigman et al. (1993) the presence of 6Li in the atmosphere of HD 84937 is consistent with the measured Be abundance (Boesgaard & King 1993) within the context of i) Standard Big Bang nucleosynthesis, ii) Pop. II cosmic ray nucleosynthesis and iii) standard (non-rotating) models for Li depletion. In particular, Steigman et al. derive D6 > 0.2, where D6 is the depletion factor for 6Li. As shown by Chaboyer (1994) standard stellar evolution models with new opacities predict D6≃ 0.4 for turnoff stars and subgiants with Teff > 5900 K. The same models predict D7 ≃ 1.0, i.e. no 7Li depletion for main sequence stars as well as subgiants with Teff ≥ 5800 K.

Recent studies of lithium abundances in solar-type stars suggest that second order effects, such as rotation and/or mass-loss, might affect the rate at which lithium is depleted at the surface of cool stars. This conclusion mainly stems from the dispersion of lithium abundances observed in late-type stars with otherwise similar mass and age. The dispersion of lithium abundances is relatively small in pre-main sequence (PMS) stars [1], reaches a maximum of about 2 dex in ZAMS stars (Soderblom, this volume), and decreases later during main sequence evolution, although significant dispersion may still be present by the age of the Sun [2], and perhaps even during the giant stage (Balachandran, this volume).

Such a pattern of lithium dispersion is difficult to understand in the framework of conventional models in which lithium burning occurs at the base of the convective zone of non-rotating, non-mass losing stars. That two stars of the same age and mass can exhibit different lithium content strongly suggests that other, so far neglected, mechanisms play a role in lithium depletion.

In this context, it may be relevant to stress that the pattern of lithium dispersion varies with age in much a similar way than does the dispersion of stellar rotation, namely: the dispersion in both rotation rates and lithium abundances is relatively small in the PMS stage, then strongly increases from the PMS to the ZAMS, and subsequently decreases on the main sequence. The parallel evolution of the range of lithium abundances and of rotation rates observed in solar-type stars is strongly suggestive of a physical link between rotation and lithium depletion.

This paper discusses the lithium abundances of late-F through mid-F dwarfs in the nearby open clusters. That means that I am considering young disk stars, of course, with metallicities similar to the Sun’s. I will not discuss the chasm seen among mid-F dwarfs. Li is not easily studied because only one spectroscopic feature is available and that feature is often weak. Thus high spectroscopic resolution and good signal to noise are needed. But Li is still the most easily studied of the light-element pentateuch (D, He, Li, Be, B).

The few clusters now accessible to large telescopes at high spectroscopic resolution include the Hyades, Pleiades, a Persei, as well as smaller samples in a few others (such as Coma, M67, NGC 752, the Ursa Major Group). The samples within clusters are often substantial, and allow one to examine differences in Li between stars of the same mass, age, and composition. However the total number of clusters observed is still small, so one must assume that each cluster is fully representative for its age and composition.

Much recent work on Li in stars has been for Population II objects because of the importance of Li for cosmology. We study Li in Pop I stars for three reasons: First, to understand how the Galaxy has come to be enriched in Li since the Pop II stars formed. Second, to understand the stellar physics behind Li depletion in Pop I stars as a means to understanding what happens in the outer layers of such a star over time. Third, to apply that understanding of the stellar physics to Pop II stars so we can correctly infer the Big Bang Li abundance.

In the standard stellar model, lithium burning does not occur during the first-ascent giant branch phase. The surface lithium abundance in a first-ascent or clump giant is thus the product of the lithium abundance of its main sequence progenitor and subsequent dilution by the deepening convective envelope. Iben’s (1967a, b) early calculations predicted that the dilution factor would vary between a factor of about 28 in a 1 M⊙ star to a factor of 60 in a 5 M⊙ star. More recent calculations with improved physics show remarkably little change in these predicted dilution factors (Pinson-neault, pvt. comm.). Since Li only survives in the outer 2-3 % by mass of a main sequence star, the maximum redgiant ∼ abundance is predicted to be ∼ log e(Li)=1.5 to within a factor of two. Even lower abundances will result if surface Li is destroyed during the main sequence phase. Lithium depletion on the main sequence has been well documented in stars of spectral type mid-F and later and have been explained by a variety of non-standard mechanisms (e.g., rotation, diffusion and turbulence). Mixing induced by such mechanisms is not expected in high-mass stars (M⊙ > 1.5) which have insignificant surface convection. Yet recent observations reveal that non-standard processes may be at work in these stars as well.

Lithium abundances far below the predicted maximum value are seen in high-mass cluster giants, and in some clusters large spreads in Li are observed in giants of roughly the same mass (Gilroy 1989). If, in concordance with the standard-model prediction, one assumes that Li burning on the giant branch is unlikely, then both the destruction and the scatter of Li must be accounted for on the main sequence. Since the surface convection zones are insignificant, other forms of Li-loss (e.g. mass loss, meridional circulation) may have to be resorted to. Unfortunately the main sequence progenitors of most of these giants are too hot to have a measurable Li I feature, but observations of field late-A stars by Burkhart and Coupry (1991) reveal some evidence for surface Li-loss. Follow-up observations of Li inclusters, and Be and B abundance determinations would be useful. Lithium burning on the giant branch should be investigated theoretically.

A time-dependent “convective diffusion” algorithm for convective transport in the mixing-length framework has been coupled for the first time with a self-consistent full evolutionary computation, in order to investigate theoretically the creation of superrich lithium stars on the asymptotic giant branch. For intermediate mass stars in the mass range from 4 to 7 M⊙ with both Population I and II compositions, hot bottom burning in the convective envelope was found, with maximum temperatures Tce at the base of the convective envelope ranging from 20 to 100 million K, depending on stellar mass and mass loss rates. For Tce ≥ 40 million K, lithium-rich giants were produced (with log ε(7Li) ≳ 1, i.e., above the normal observed range in giants). For Tce ≥ 50 million K, superrich lithium giants were created, with log ε(7Li) ≳ 3 (i.e., larger than the present cosmic7Li abundance).

To solve the system of statistical balance equations and radiative transfer equations (NLTE problem) we followed the modification of linearization method proposed by Auer and Heasley (1976) and used 20-level model of the lithium atom. The 70 radiative and all possible collisional transitions were included into the rate matrix (Pavlenko 1994).

Lithium-7 is now generally accepted to originate in the hot Big Bang nucleosynthesis (BBN), with a primordial abundance (7Li/H)⋍ 10-10 in excellent agreement with the observed uniformity of the Li abundance in very metal deficient Pop II stars.

During the galactic evolution, both Li isotopes are created by spallation reactions of galactic cosmic rays (GCR) interacting with the ISM, that yield (7Li/H)⋍ 2 × 10-10 in 10 Gyrs, with a ratio (7Li/6Li)GCR=1.4. The major problem is then to explain the observed Pop I Li abundance, (7Li/H)PopI ∼ 10-9, of which only 30% is accounted for by BBN and GCR spallation, as well as the high 7Li/6Li ratio measured in meteorites, representative of the solar system formation epoch 4.6 Gyrs ago, (7Li/6Li)⊙=12.3, whereas the above mechanisms predict a ratio around 2.

The existence of an extra stellar source of Li has been suggested. GCR spallation alone tends to decrease the 7Li/6Li ratio with time, and one should observe today an interstellar ratio ≃5-6 without production of Li in stars, or ≳6 with a steuar production. Measuring this ISM ratio thus provides a key test for the models of lithium evolution. If it is found to be ≲5, then another scenario would have to be considered.

Under the assumption that the abundance of 7Li in Population II stars represents the primordial Li abundance (with perhaps a small contribution from GCR spallation) and that GCR spallation/fusion processes cannot contribute to more than ≃ 10 − 20% of the Li abundance observed in Pop. I stars and in the solar system, one must conclude that most of Li in Pop. I stars has a stellar origin.

Possible stellar Li producers are discussed: low mass AGB stars (2−5M⊙) (C-stars), high mass AGB stars (5 - 8M⊙), supernovae of type II (M > 10M⊙) and novae. The various problems connected with all of these sources are indicated: in particular, we discuss the Li production in AGB stars when evolutionary effects due to the metallicity are taken into account, and the fact that novae do not seem to be good candidate for Li production, as suggested by a recent nucleosynthesis study. We then calculate the yields from these stellar sources and predict the behavior of log N(Li) vs. [Fe/H] by means of a galactic chemical evolution model.

We conclude that, although a unique model cannot be found, due to the uncertainties still existing in the stellar nucleosynthesis, the most likely scenario is that Li is partly produced in type II supernovae (v-induced nucleosynthesis) and partly in massive AGB stars.

Richer & Michaud (1993) calculated a series of envelopes fully coupled to non-rotating, constant mass, stellar evolution models of hydrogen burning stars with masses in the range of 1.2 to 2.2 M⊙, typical of A and F main sequence stars. They included He settling. The location of the theoretically predicted gap of the Hyades agrees quite well with the observed one, a result obtained without the introduction of any free parameter. At temperatures above the gap, while the observed lithium abundances are within a factor of 2-3 of normal values, the theoretical calculated curve drops to very low values. Diffusion velocities being fairly small, any other physical process with larger or similar velocities can reduce the effect of diffusion and produce the observed results. Mass loss is one such process. Another difficulty with the present theory is the width of the gap. Observations show that the observed gap is wider than the calculated one in the Hyades. This also suggests that other physical processes play an important role.

A well established property of late-type stars is that their rotation rate and their lithium abundance both decline with age. We understand why such stars are spun down while losing so little mass (Schatzman 1962), but we are still working on a theory which would explain all aspects of the observed lithium depletion. Although most agree that some kind of mixing must occur below the convective envelope of late-type stars, which transports 7Li to the depth where it is destroyed through nuclear burning, the physical process which is responsible for this transport is still in debate. However the most recent observations of the lithium abundance in close binary stars clearly show that there is a causal link between the destruction of lithium and the loss of angular momentum: consistently, tidally-locked binaries tend to display more lithium than single stars of the same effective temperature, as observed in the Hyades (Soderblom et al. 1990, Thorburn et al. 1993) and among old disk stars and halo stars (Spite et al. 1994).

The observations by Spite & Spite (1982), that the upper values of the lithium abundance in halo stars are one order of magnitude smaller than the ones observed in galactic stars, lead to a large debate about the primordial lithium abundance. The whole problem can be summarized by the following question: has lithium been depleted in the outer layers of halo stars, or has the original lithium been preserved at their surfaces since the beginning?

It seems difficult to maintain the original lithium abundance in halo stars during all their lifetime. Either lithium is depleted due to element separation, or it is destroyed by nuclear reactions. Computations by Proffitt & Michaud (1989) showed that nowhere inside halo stars the lithium abundance could have remained at its original value.

It was suggested by Vauclair (1988) that rotation-induced turbulence could lead to a nuclear destruction of lithium in halo stars large enough to explain their present abundances, with an original abundance equal to the present galactic one. It seemed possible that the “plateau shape” of the abundances be preserved if the turbulent diffusion coefficient decreased rapidly with radius, as in Zahn (1987) (see also Pinsonneault et al. 1992).

The nucleosynthesis of 6Li in the galaxy has become of considerable interest in the past year. This is largely because of the exciting developments relating to the first detection of this isotope outside of our own solar system. Not only has 6Li been recently detected in the atmospheres of halo dwarfs [1-2], but also in the local interstellar medium (ISM) [3-4].

It is well known that knowledge of the primordial abundance of the lithium isotopes has important ramifications for cosmology, through the constraints they impose on standard big bang nucleosynthesis (SBBN) [5], and those imposed on non-standard primordial nucleosynthesis models. Since a complete understanding of the galactic evolution of the lithium isotopes is a prerequisite to unambiguously determining their primordial abundance, a thorough interpretation of the new 6Li data in the context of chemical evolution models cannot be overstated.

It is a great pleasure for me to be able to start off this “Joint Discussion” on Extragalactic Planetary Nebulae. Many astronomers appear somewhat ambivalent in their attitudes towards planetary nebulae. A few of us regard planetaries as the spectacular “last hurrah” before stars slide off into oblivion as white dwarfs. Others think of them as soaring butterflies that evolved from stellar caterpillars. Finally a few less romantic astronomers simply ask “what can planetary nebulae do for us?” Perhaps surprisingly the answer is that they can do a great deal for the progress of various branches of astronomy. At this meeting George Jacoby will, for example, show us how planetary nebulae can be used to measure extragalactic distances. Mike Dopita, Ziaohui Hui and Ken Freeman will tell us that planetaries are powerful probes of the kinematics of galaxies. Furthermore Henry Ferguson will show how planetary nebulae can be used to probe the effects of metallicity on stellar evolution in elliptical galaxies.

We made a spatially complete and kinematically unbiased survey of M 31 for planetary nebulae (PNs). The survey covers a projected area of 20 by 40 kpc centered on M 31. Subsequently, we measured the radial velocities of over 800 PNs using the Hydra multi-object spectrograph on the KPNO 4-m telescope. With this large dataset, a detailed dynamical study of the stellar disk, bulge and halo populations in M 31 is currently underway. We are focusing on the dynamical transition between the disk, bulge and halo, and a comparison between the kinematics of the PNs and globular clusters.

Heavy element abundances, in particular those of oxygen, obtained from recent spectroscopic surveys of Magellanic Cloud planetary nebulae (PN), are reviewed and compared with those derived for H regions and objects in our own galaxy. These abundances have been based on collisionally excited lines and are very sensitive to the adopted electron temperature. There is increasing evidence that temperature or density fluctuations within nebulae lead to the electron temperatures being overestimated, with the corollary that the heavy element abundances have been underestimated.