We use Hipparcos parallaxes to determine the evolutionary status and the nature of the so-called Li-rich giants. These informations clearly support the hypothesis of internal nuclear lithium enrichment by an extra-mixing process at two distinct evolutionary phases.

We have carried out a survey to detect Lithium-rich giants (LRGs). In Castilho et al. (1998) we reported the discovery of five red giants showing a strong Li I (λ670.78 nm) line and six other ones showing a moderate Li I line. In this work we present the main results of a detailed analysis of 10 stars from our survey and 6 LRGs selected from the literature.

The super-Li-rich (SLR) stars are rare AGB stars containing enhanced Li/H ≈ 2000 times the ISM Li/H. In order to determine if mass loss from SLR stars is a significant source of ISM Li, we measured their Li/H and the mass loss rate. From the weak Li I 8126 Â line in 2/4 SLR C stars and 6/8 SLR S stars we obtained Li/H = (0.2 — 5) x 10—6 for SLR C stars and Li/H = (0.2 — 5) x 10—7 for the SLR S stars. We determined dM/dt from the expansion velocity and line profiles of the circumstellar CO. The dM/dt = 1.3 × 10—5M⊙/yr, 2.6 × 10—7M⊙/yr, and 2.6 × 10—8M⊙/yr for the C stars IY Hya, T Ara, and WZ Cas; and average dM/dt = 5.5 × 10—7M⊙/yr for the S stars. Thus mass loss from SLR stars is a significant source of Galactic Li.

In this work we present the stellar parameters and the Li abundances, obtained by spectral synthesis, for a sample of 13 weak-line T Tauri stars.

The solar beryllium abundance is important because it provides a constraint on the depth to which mixing has occurred below the surface convective zone. Unlike helioseismology which only maps the present-day Sun, the solar beryllium abundance provides an integrated picture of mixing over the entire history of the Sun. In this review I outline the logic involving the ‘missing UV opacity’ that required that the solar beryllium abundance be re-determined. A brief summary of the empirical process of estimating the “missing UV opacity” is given along with a confirmation based on a recent re-calculation of the Fe I bound-free opacity. The addition of this opacity resulted in our finding that the solar beryllium abundance was meteoritic. The implications of this result in the context of mixing in solar-type stars is discussed.

The abundance of beryllium has been determined in unevolved stars over a range metal abundances in order to enhance our understanding of the chemical evolution of our Galaxy, cosmic-ray theory, and cosmology. Observations of 27 stars have been made with Keck I with HIRES at high spectral resolution (45,000) and high signal-to-noise ratios (60 - 110 typically). We find a remarkably linear relationship between log N(Be/H) and [Fe/H] with a slope of 0.96 (±0.04). Similarly, the relationship between log N(Be/H) and [O/H] is linear with a slope of 1.45 (±0.04). Beryllium increases at the same rate as Fe, but much faster than O. This provides constraints for and insights into models of Galactic chemical evolution. There is some evidence for an intrinsic spread in Be at a given [O/H] or [Fe/H]. There is no evidence of a plateau in Be at the lowest metallicities down to log N(Be/H) = —13.5.

We discuss the early evolution of beryllium and oxygen in our Galaxy by comparing abundances of these elements for halo and disk metal-poor stars. Both, O and Be rise as we go progressively to more metal-rich stars, showing a slope 0.41 ± 0.09 ([Be/O] vs [Fe/H]) for stars with [Fe/H]≤ —1. This relationship provides an observational constraint to the actually proposed Galactic Cosmic Ray theories.

Boron, together with lithium and beryllium, belongs to the group of the so-called light elements, the importance of which ranges from providing important tests to Big Bang nucleosynthesis scenarios to being useful probes of stellar interiors and useful tools to further constrain the chemical evolution of the Galaxy.

Since it became operative in the late eighties, the Hubble Space Telescope (HST) and its high- and medium-resolution spectrographs have played a key role in analyzing boron. Boron has now been observed in several stars and in the interstellar medium (ISM), providing important information in different fields of astrophysical research (nucleosynthesis, cosmic-ray spallation, stellar structure). In particular, determinations of boron in unevolved stars of different metallicity have allowed to study how boron evolves with iron.

After a general review of the current status of boron observations and of the major uncertainties affecting the measurements of its abundance, I will mainly concentrate on unevolved stars and discuss the ‘evolutionary’ picture emerging from the most recent analyses and how its interpretation compares with theoretical expectations. A brief discussion on future prospects will conclude this contribution, showing how the field may evolve and improve.

Although the behavior of boron versus metallicity has been probed in a fairly large sample of halo dwarfs with HST, it is only very recently that boron abundances have been derived systematically in solar metallicity dwarfs. This effort began with a re-analysis of the solar spectrum with modern atomic data and model atmospheres so that the Sun could be adopted as a standard for the calibration of a line list in the region of the B I transition at 2497 Â. The solar analysis indicates that boron is not depleted in the solar photosphere. From a subsequent study of a sample of 14 field F/G-dwarfs with roughly solar metallicities, it is found that the behavior of boron versus [Fe/H] follows the linear trend that is observed for the halo stars. The average B/Be obtained for solar metallicity stars is 27±5 compared to the solar ratio of 23. The determination of boron abundances in the young B-type and G-type stars of the Orion association reveals a behavior of boron and oxygen in Orion that is opposite of the positive correlation which is observed for the field stars: the boron and oxygen abundances are anticorrelated.

Recent detailed simulations of Galactic Chemical Evolution have shown that the heavy elements, in particular [Fe/H], are expected to exhibit a weak, or absent, correlation with stellar ages in the early Galaxy due to the lack of efficient mixing of interstellar material enriched by individual Type II supernovae. A promising alternative ‘chronometer’ of stellar ages is suggested, based on the expectation that the light elements Be and B are formed primarily as spallation products of Galactic Cosmic Rays.

We present the results of observations of the λ3130 interstellar absorption doublet of 9Be ii in the direction of ζ Per. The data were obtained at the Canada-France-Hawaii 3.6m telescope using the Gecko spectrograph at a resolving power ∼ 110000 and a signal-to-noise ratio ∼ 2000. The 9Be ii lines are not detected and we obtain an upper limit on the equivalent width W ≤ 30μÂ. This upper limit is 7 times below the lowest upper limit ever reported hitherto. The derived interstellar abundance is (9Be/H) ≤ 7 × 10—13; it corresponds to an upper limit δBe ≤ —1.5 dex on the depletion factor of 9Be.

Based on detailed 2D radiation hydrodynamics (RHD) simulations, we have investigated the effects of photospheric temperature inhomogeneities induced by convection on spectroscopic determinations of the lithium abundance. Computations have been performed both for the solar case and for a metal-poor dwarf. NLTE effects are taken into account, using a five-level atomic model for Li I. Comparisons are presented with traditional 1D models having the same effective temperature and gravity. The net result is that, while LTE results differ dramatically between 1D and 2D models, especially in the metal-poor case, this does not remain true when NLTE effects are included: 1D/2D differences in the inferred NLTE Li abundance are always well below 0.1 dex. The present computations still assume LTE in the continuum. New computations removing this assumption are planned for the near future.

The influence of stellar granulation on the line formation of the light elements in metal-poor stars has been investigated by means of 3D hydrodynamical model atmospheres. Due to a lucky (?) coincidence, the effects due to the lower photospheric temperatures with 3D models compared with classical 1D model atmospheres are almost exactly balanced by a pronounced over-ionization of Li i, as concluded from detailed 3D NLTE calculations. Additionally, the effects of convective line asymmetries on 6Li isotope determinations have been investigated and found to be of minor importance.

Theoretical aspects of modeling of spectra of late M- and L-dwarfs are discussed. We show, that the processes of formation of spectra of M- and L-dwarfs are basically different. Instead of the case of M-dwarfs, atoms of Ti and VO should be depleted into grains in the atmospheres of L-dwarfs. Overall shape of the L-dwarf spectra is governed by the K I + Na I resonance line wings of the huge strength. To fit lithium lines observed in spectra of the coolest dwarfs we used two additional suggestions: a) there are some extra depletions of molecular species absorbed in the optical spectra of L-dwarfs; b) there may be (a few?) additional (“dusty”?) opacity sources in their atmospheres. Problems of lithium line formation and the “natural” limitation of their use for the ‘lithium test’ for the case of L-dwarfs are discussed.

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the atomic diffusion of all important contributors to opacity. The radiative accelerations and the opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self-consistent stellar evolution models for A and F stars. In A and F stars an iron-peak convection zone appears.

The calculated abundance anomalies are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. To reduce the calculated anomalies to the observed ones, an additional source of turbulence (or some other hydrodynamical process) must be introduced. The mixed zone must extend about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out.

The LiBeB abundances observed in clusters give additional information. The abundances of the 28 species offer considerable constraints on the models. Various potential turbulence models have been introduced in a stellar evolution code and results of evolutionary calculations for Li gap stars are discussed in the light of the constraints offered by the abundances of LiBeB and metals. The radiative accelerations of LiBeB have also been recalculated taking the effect of changing metal abundances into account. This modifies the expected Li gap in the absence of turbulence.

The observations of light elements in the Sun and Solar type stars give special clues for understanding the hydrodynamical processes at work in stellar interiors. In the Sun 7Li is depleted by 140 while 3He has not increased by more than ≅ 10% in 3 Gyrs. Meanwhile the inversion of helioseismic modes lead to a precision on the sound velocity of about 1%. The mixing processes below the solar convection zone are constrained by these observations. Lithium is depleted in most Pop I solar type stars. In halo stars however, the lithium abundance seems constant in the “spite plateau” with no observed dispersion, which is difficult to reconcile with the theory of diffusion processes. In the present paper, the various relevant observations will be discussed. It will be shown that the μ-gradients induced by element settling may help solving the ‘lithium paradox’.

We propose to observe Asymptotic Giant Branch stars (AGB) with spectro-differential interferometric techniques in order to find new observational constraints to inner structure and evolutionary models of these stars. We examine the interferometric signatures created by possible heterogeneities on AGB stars surface due to local Li-enrichments or Li-rich spots and find that such heterogeneities, if they exist, could be detected with present or future interferometers.

The application to main-sequence stars of the rotation-induced mixing theory in the presence of μ-gradients leads to partial mixing in the lithium destruction region, not visible in the atmosphere. The induced lithium depletion becomes visible in the sub-giant phase as soon as the convective zone deepens enough. This may explain why the observed ‘lithium dilution’ is smoother and the final dilution factor larger than obtained in standard models, while the lithium abundance variations are very small on the main sequence.

Type I planetary nebulae (PNe), defined as those with high He (and N) abundances, generally present bipolar geometries and high stellar temperatures. The main goal of this paper is to check if the empirically derived He overabundance for Type I PNs is real, or if it is a consequence of geometrical effects due to the bipolarity and/or to the ionization stratification due to the high stellar temperature of these objects.

In this work we analyze the beryllium-iron chemical diagram from the point of view of non-LTE effects. Be abundances were re-calculated by considering non-LTE corrections in ionization equilibrium (logg) and Fe abundances ([Fe/H]). These corrections seem do not affect the linear relation between Be-Fe for metal-poor stars already found in the literature for LTE derived abundances.