We have performed an abundance analysis for F‐ and G‐ dwarfs of the Galactic thick‐disc component. A sample of 176 nearby (d≤ 150 pc) thick‐disc candidate stars was chosen from the Hipparcos catalogue and subjected to a high‐resolution spectroscopic analysis. Using accurate radial velocities combined with the Hipparcos astrometry, kinematics (U, V and W) and Galactic orbital parameters were computed. We estimate the probability for a star to belong to the thin disc, the thick disc or the halo. With a probability P≥ 70 per cent taken as certain membership, we assigned 95 stars to the thick disc, 13 to the thin disc, and 20 to the halo. The remaining 48 stars in the sample cannot be assigned with reasonable certainty to one of the three components. Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, Ce, Nd and Eu have been obtained. The abundances for the thick‐disc stars are compared with those for the thin‐disc members from Reddy et al. The ratios of α‐elements (O, Mg, Si, Ca and Ti) to iron for thick‐disc stars show a clear enhancement compared to thin‐disc members in the range −0.3 < [Fe/H] < −1.2. There are also other elements – Al, Sc, V, Co, and possibly Zn – which show enhanced ratios to iron in the thick disc relative to the thin disc. The abundances of Na, Cr, Mn, Ni and Cu (relative to Fe) are very similar for thin‐ and thick‐disc stars. The dispersion in abundance ratios [X/Fe] at given [Fe/H] for thick‐disc stars is consistent with the expected scatter due to measurement errors, suggesting a lack of ‘cosmic’ scatter. A few stars classified as members of the thick disc by our kinematic criteria show thin‐disc abundances. These stars, which appear older than most thin‐disc stars, are also, on average, younger than the thick‐disc population. They may have originated early in the thin‐disc history, and been subsequently scattered to hotter orbits by collisions. The thick disc may not include stars with [Fe/H] > −0.3. The observed compositions of the thin and thick discs seem to be consistent with the models of galaxy formation by hierarchical clustering in a Lambda cold dark matter (ΛCDM) universe.

Very high quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived one-dimensional, non-LTE 7 Li abundances from the Li i 670.8 nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the Li i 670.8 nm line and the Li i 610.4 nm line. The estimated primordial 7 Li abundance is 7 Li /H ¼ (1:1 1:5) ; 10 À10 , which is a factor of 3-4 lower than predicted from standard big bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6 Li is detected in 9 of our 24 stars at the !2 significance level. Our observations suggest the existence of a 6 Li plateau at the level of log 6 Li % 0:8; however, taking into account predictions for 6 Li destruction during the pre-main-sequence evolution tilts the plateau such that the 6 Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6 Li in the very metal-poor star LP 815À43. Such a high 6 Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic-ray spallation and -fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6 Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of 6 Li limits the possible degree of stellar 7 Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.

Photospheric abundances are presented for 27 elements from carbon to europium in 181 F and G dwarfs from a differential local thermodynamic equilibrium (LTE) analysis of high-resolution and high signal-to-noise ratio spectra. Stellar effective temperatures (T eff ) were adopted from an infrared flux method calibration of Strömgren photometry. Stellar surface gravities (g) were calculated from Hipparcos parallaxes and stellar evolutionary tracks. Adopted T eff and g values are in good agreement with spectroscopic estimates. Stellar ages were determined from evolutionary tracks. Stellar space motions (U , V , W ) and a Galactic potential were used to estimate Galactic orbital parameters. These show that the vast majority of the stars belong to the Galactic thin disc.Relative abundances expressed as [X/Fe] generally confirm previously published results. We give results for CEu. The α elements -O, Mg, Si, Ca and Ti -show [α/Fe] to increase slightly with decreasing [Fe/H]. Heavy elements with dominant contributions at solar metallicity from the s-process show [s/Fe] to decrease slightly with decreasing [Fe/H]. Scatter in [X/Fe] at a fixed [Fe/H] is entirely attributable to the small measurement errors, after excluding the few thick disc stars and the s-process-enriched CH subgiants. Tight limits are set on 'cosmic' scatter. If a weak trend with [Fe/H] is taken into account, the composition of a thin disc star expressed as [X/Fe] is independent of the star's age and birthplace for elements contributed in different proportions by massive stars (Type II supernovae), exploding white dwarfs (Type Ia supernovae) and asymptotic red giant branch stars.By combining our sample with various published studies, comparisons between thin and thick disc stars are made. In this composite sample, thick disc stars are primarily identified by their V LSR in the range −40 to −100 km s −1 . These are very old stars with origins in the inner Galaxy and metallicities [Fe/H] −0.4. At the same [Fe/H], the sampled thin disc stars have V LSR ∼ 0 km s −1 , and are generally younger with a birthplace at about the Sun's Galactocentric distance. In the range −0.35 [Fe/H] −0.70, well represented by present thin and thick disc samples, [X/Fe] of the thick disc stars is greater than that of thin disc stars for Mg, Al, Si, Ca, Ti and Eu. [X/Fe] is very similar for the thin and thick disc for -notably -Na and iron-group elements. Barium ([Ba/Fe]) may be underabundant in thick relative to thin disc stars. These results extend previous ideas about composition differences between the thin and thick disc.

We present the results of s-process nucleosynthesis calculations for Asymptotic Giant Branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M ⊙ ) and comparisons of them with observational constraints from high resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13 C(α,n) 16 O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13 C is manufactured locally by proton captures on the abundant 12 C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge up episode, when a chemical discontinuity is established between the convective envelope and the He-and Crich zone. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22 Ne(α,n) 25 Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13 C formation, the abundance of 13 C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or 'hs') and that of the Zr-peak ones (light s, or 'ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g. of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s path at low metallicities drains the Zr-peak and Ba-peak and builds an excess at the doubly-magic 208 Pb, at the termination of the s path. We then discuss the effects on the models of variations in the crucial parameters of the 13 C pocket, finding that they are not critical for interpreting the results.The theoretical predictions are compared with published abundances of s elements for AGB giants of classes MS, S, SC, post-AGB supergiants, and for various classes of binary stars, which supposedly derive their composition by mass transfer from an AGB companion. This is done for objects belonging both to the Galactic disk and to the halo. The observations in general confirm the complex dependence of neutron captures on metallicity. They suggest that a moderate spread exists in the abundance of 13 C that is burnt in different stars. Although additional observations are needed, it seems that a good understanding has been achieved of s-process operation in AGB stars. Finally, the detailed

Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B 2 FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments. [S0034-6861(97)

We carried out a comprehensive far-UV survey of 12 CO and H 2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H 2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH + , and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density ( 100 cm À3) and higher density diffuse clouds. A plot of log N (CO) versus log N (H 2 ) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N (CO; cm À2 ) ¼ 14:1 and log N (H 2 ) ¼ 20:4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N ¼ 14:1, 13.0), CH + versus H 2 (13.1, 20.3), and CH + versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C 2 and CN molecules also reside, reactions involving C + and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH + , our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH + and CO.

We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster u Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are deÐned as a function of the standard metallicity indicator iron. The lowest metallicity stars in u Cen have [Fe/H] D [1.8, and the initial abundance distribution in the cluster is established at this metallicity. The stars in the cluster span a range of [Fe/H] D [1.8 to [0.8. At the lowest metallicity, the heavy-element abundance is found to be well characterized by a scaled solar system r-process distribution, as found in other stellar populations at this metallicity. As iron increases, the s-process heavy-element abundances increase dramatically. Comparisons of the s-process increases with recent stellar models Ðnds that s-process nucleosynthesis in 1.5È3 asymptotic giant branch stars (AGB) Ðts well the heavy-element abundance M _ distributions. In these low-mass AGB stars, the dominant neutron source is 13C(a, n)16O. A comparison of the Rb/Zr abundance ratios in u Cen Ðnds that these ratios are consistent with the 13C source. The reason u Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGB ejecta, because of their low velocity winds, are more efficiently retained in the cluster relative to the much faster moving Type II supernova ejecta. SigniÐcant s-process enrichment relative to Fe, from the lower mass AGB stars, would require that the cluster was active in star formation for quite a long interval of time, of the order of 2È3 Gyr. The AGB ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of a-rich elements shows that no signiÐcant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] D [0.6 (derived here for the Ðrst time in this cluster) Ðnds a plausible interpretation.

ABSTRACT. A new high-resolution cross-dispersed echelle spectrometer has been installed at the coudé focus of the McDonald Observatory 2.7-m telescope. Its primary goal was simultaneously to gather spectra over as much of the spectral range 3400 À to 1 /¿m as practical, at a resolution R=X/AX -60,000 with signal-to-noise ratio of -100 for stars down to magnitude 11, using 1-h exposures. In the instrument as built, two exposures are all that are needed to cover the full range. Featuring a white-pupil design, fused silica prism cross disperser, and folded Schmidt camera with a Tektronix 2048X2048 CCD used at either of two foci, it has been in regularly scheduled operation since 1992 April. Design details and performance are described.