David L. Lambert

The 'Forbidden' abundance of oxygen in the Sun

We reexamine closely the solar photospheric line at 6300 A, which is attributed to a forbidden line of neutral oxygen and is widely used in analyses of other late-type stars. We use a three-dimensional time-dependent hydrodynamical model solar atmosphere that has been tested successfully against observed granulation patterns and an array of absorption lines. We show that the solar line is a blend with a Ni I line, as previously suggested but oftentimes neglected. Thanks to accurate atomic data on the [O I] and Ni I lines, we are able to derive an accurate oxygen abundance for the Sun: log (O) = 8.69 ± 0.05 dex, a value at the lower end of the distribution of previously published abundances but in good agreement with estimates for the local interstellar medium and hot stars in the solar neighborhood. We conclude by discussing the implication of the Ni I blend on oxygen abundances derived from [O I] λ6300 in disk and halo stars.

Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances

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 we present 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 13C(α, n)16O 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 13C is manufactured locally by proton captures on the abundant 12C; 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 C-rich zones. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22Ne(α, n)25Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13C formation, the abundance of 13C 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 [hs]) and that of the Zr-peak ones (light s [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 and Ba peaks and builds an excess at the doubly magic 208Pb, which is at the termination of the s-path. We then discuss the effects on the models of variations in the crucial parameters of the 13C 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 13C 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 abundance distribution including the light elements (CNO) of a few s-enriched stars at different metallicities are examined and satisfactorily reproduced by model envelope compositions.

A REAPPRAISAL OF THE SOLAR PHOTOSPHERIC C/O RATIO

An accurate determination of photospheric solar abundances requires detailed modeling of the solar granulation and accounting for departures from local thermodynamical equilibrium (LTE). We argue that the forbidden C i line at 8727 A u is largely immune to departures from LTE and can be realistically modeled using LTE radiative transfer in a time-dependent three-dimensional simulation of solar surface convection. We analyze the [C i] line in the solar flux spectrum to derive the abundance dex. Combining this result with our log e(C) p 8.39 0.04 parallel analysis of [O i] l6300, we find , in agreement with the ratios measured in the solar C/O p 0.50 0.07 corona from gamma-ray spectroscopy and solar energetic particles. Subject headings: convection — line: formation — Sun: abundances — Sun: photosphere

THE HIGH-RESOLUTION CROSS-DISPERSED ECHELLE WHITE PUPIL SPECTROMETER OF THE MCDONALD OBSERVATORY 2.7-M TELESCOPE

A new high-resolution cross-dispersed echelle spectrometer has been installed at the coude focus of the McDonald Observatory 2.7-m telescope. Its primary goal was to simultaneously gather spectra over as much of the spectral range 3400A to 1 micron as practical, at a resolution R = lambda/delta-lambda 60,000 with signal-to-noise ratio of ~100 for stars down to magnitude 11, using 1-hour 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 ross disperser, and folded Schmidt camera with a Tektronix 2048 X 2048 CCD used at either of two foci, it has been in regularly-scheduled operation since April 1992. Design details and performance will be described.

Carbon, nitrogen, and oxygen abundances in early B-type stars

We report on a survey of the C, N, and O abundances in a sample of early B-type stars which was undertaken to test Lyubimkovs claim that CN-cycled material is mixed to the surfaces of these stars during their core hydrogen-burning phase. We have obtained equivalent widths of generally weak lines using high signal-to-noise Reticon spectra of 39 stars in four spectral regions. We have derived effective temperatures and gravities for these stars using Stromgren dereddened color indices and Hy line profiles through a comparison with colors and line profiles from Kurucz line-blanketed atmospheres

Lithium isotopic abundances in metal-poor halo stars

Very high quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESOs VLT/UVES for the purpose of determining Li isotopic abundances. The derived one-dimensional, non-LTE 7Li 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 7Li abundance is 7Li/H = (1.1-1.5) ? 10-10, 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, 6Li is detected in 9 of our 24 stars at the ?2 ? significance level. Our observations suggest the existence of a 6Li plateau at the level of log ? 0.8; however, taking into account predictions for 6Li destruction during the pre-main-sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP 815-43. Such a high 6Li 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 6Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.

A search for lithium-rich giant stars

Lithium abundances or upper limits have been determined for 644 bright G-K giant stars selected from the DDO photometric catalog. Two of these giants possess surface lithium abundances approaching the cosmic value of the interstellar medium and young main-sequence stars, and eight more giants have Li contents far in excess of standard predictions. At least some of these Li-rich giants are shown to be evolved to the stage of having convectively mixed envelopes, either from the direct evidence of low surface carbon isotope ratios, or from the indirect evidence of their H-R diagram positions. Suggestions are given for the unique conditions that might have allowed these stars to produce or accrete new lithium for their surface layers, or simply to preserve from destruction their initial lithium contents. The lithium abundance of the remaining stars demonstrates that giants only very rarely meet the expectations of standard first dredge-up theories; the average extra Li destruction required is about 1.5 dex. The evolutionary states of these giants and their average masses are discussed briefly, and the Li distribution of the giants is compared to predictions of Galactic chemical evolution. 110 refs.

The Chemical Evolution of the Globular Cluster ω Centauri (NGC 5139)

We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster ω Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are defined as a function of the standard metallicity indicator iron. The lowest metallicity stars in ω Cen have [Fe/H] ~ -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] ~ -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 finds that s-process nucleosynthesis in 1.5–3 M⊙ asymptotic giant branch stars (AGB) fits well the heavy-element abundance distributions. In these low-mass AGB stars, the dominant neutron source is 13C(α, n)16O. A comparison of the Rb/Zr abundance ratios in ω Cen finds that these ratios are consistent with the 13C source. The reason ω 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. Significant 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 2–3 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 α-rich elements shows that no significant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] ~ -0.6 (derived here for the first time in this cluster) finds a plausible interpretation.

Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

We carried out a comprehensive far-UV survey of 12CO and H2 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 H2 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−3) and higher density diffuse clouds. A plot of -->log N(CO) versus -->log N(H2) 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 −2) = 14.1 and -->log N(H2) = 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 = 14.1, 13.0), CH+ versus H2 (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 C2 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.

THE SEGUE STELLAR PARAMETER PIPELINE. III. COMPARISON WITH HIGH-RESOLUTION SPECTROSCOPY OF SDSS/SEGUE FIELD STARS*

The authors report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which they compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R {approx} 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), they empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s{sup -1}, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. They estimate random errors for lower S/N spectra based on numerical simulations.