Context. The discovery of the Spite plateau in the abundances of 7 Li for metal-poor stars led to the determination of an observationally deduced primordial lithium abundance. However, after the success of the Wilkinson Microwave Anisotropy Probe (WMAP) in determining the baryon density, Ω B h 2 , there was a discrepancy between observationally determined and theoretically determined abundances in the case of 7 Li. One of the most important uncertain factors in the calculation of the stellar 7 Li abundance is the effective temperature, T eff . Aims. We use sixteen metal-poor halo dwarfs to calculate new T eff values using the excitation energy method. With this temperature scale we then calculate new Li abundances for this group of stars in an attempt to resolve the 7 Li discrepancy. Methods. Using high signal-to-noise (S /N ≈ 100) spectra of 16 metal-poor halo dwarfs, obtained with the UCLES spectrograph on the AAT, measurements of equivalent widths from a set of unblended Fe i lines are made. These equivalent widths are then used to calculate new T eff values with the use of the single line radiative transfer program WIDTH6, where we have constrained the gravity using either theoretical isochrones or the Hipparcos parallax, rather than the ionization balance. The lithium abundances of the stars are calculated with these temperatures. Results. The physical parameters are derived for the 16 programme stars, and two standards. These include T eff , log g, [Fe/H], microturbulence and 7 Li abundances. A comparison between the temperature scale of this work and those adopted by others has been undertaken. We find good consistency with the temperatures derived from the Hα line by Asplund et al. (2006, ApJ, 644, 229), but not with the hotter scale of Meléndez & Ramírez (2004, ApJ, 615, L33). We also present results of the investigation into whether any trends between 7 Li and metallicity or temperature are present in these metal-poor stars.

Context. Theory and observations of heavy element nucleosynthesis are in conflict with one-another. Theory states that in the most metal-poor stars, the rapid (r-) neutron-capture nucleosynthetic process would be dominant over the slow (s-) process. The most recent determinations of r-and s-process yields do not support this. Aims. We provide measurements of the Ba isotopic fractions for five metal-poor stars derived with a local thermodynamic equilibrium (LTE) analysis with 1D model stellar atmospheres. This increases the comparisons with heavy element nucleosynthesis theory. Methods. We use high resolution (R ≡ λ/Δλ = 90 000−95 000), very high signal-to-noise (S /N > 500) spectra to determine the fraction of odd Ba isotopes ( f odd ) by measuring subtle asymmetries in the profile of the Ba ii line at 4554 Å. We also use two different macroturbulent broadening techniques, Gaussian and radial-tangential, to model the Fe lines of each star, and propagate each technique to model macroturbulent broadening in the Ba 4554 Å line. We conduct a 1D non-LTE (NLTE) treatment of the Fe lines in the red giant HD 122563 and the subgiant HD 140283 in an attempt to improve the fitting. We determine [Ba/Eu] ratios for the two giants in our study, HD 122563 and HD 88609, which can also be used to determine the relative contribution of the s-and r-processes to heavy-element nucleosynthesis, for comparison with f odd . Results. We find mathematical solutions of f odd for HD 122563, HD 88609 and HD 84937 of −0.12 ± 0.07, −0.02 ± 0.09, and −0.05 ± 0.11 respectively. BD+26 • 3578 yielded a value for f odd = 0.08 ± 0.08. Only BD−04 • 3208 was found to have a physical f odd ratio of 0.18 ± 0.08. This means that all stars examined here show isotopic fractions more compatible with an s-process dominated composition. The [Ba/Eu] ratios in HD 122563 and HD 88609 are found to be −0.20 ± 0.15 and −0.47 ± 0.15 respectively, which indicate instead an r-process signature. We report a better statistical fit to the majority of Fe profiles in each star when employing a radial-tangential broadening technique during our 1D LTE investigation. Conclusions. With the increase of the number of stars for which the Ba isotope fraction f odd has been measured, and the nature of their results, there is now a stronger argument to suggest that other synthesis codes that employ alternative approaches to radiative transfer (e.g. 3D hydrodynamics) have to be considered to tackle the high level of precision required for the determination of isotopic ratios. We have shown that, from a statistical point of view, one must consider using a radial-tangential broadening technique rather than a Gaussian one to model Fe line macroturbulences when working in 1D. No improvement to Fe line fitting is seen when employing a NLTE treatment of the Fe lines.

Context. The plateau in the abundance of 7 Li in metal-poor stars was initially interpreted as an observational indicator of the primordial lithium abundance. However, this observational value is in disagreement with that deduced from calculations of Big Bang nucleosynthesis (BBN), when using the Wilkinson microwave anisotropy probe (WMAP) baryon density measurements. One of the most important factors in determining the stellar lithium abundance is the effective temperature. In a previous study by the authors, new effective temperatures (T eff ) for sixteen metal-poor halo dwarfs were derived using a local thermodynamic equilibrium (LTE) description of the formation of Fe lines. This new T eff scale reinforced the discrepancy. Aims. For six of the stars from our previous study we calculate revised temperatures using a non-local thermodynamic equilibrium (NLTE) approach. These are then used to derive a new mean primordial lithium abundance in an attempt to solve the lithium discrepancy.Methods. Using the code MULTI we calculate NLTE corrections to the LTE abundances for the Fe i lines measured in the six stars, and determine new T eff 's. We keep other physical parameters, i.e. log g, [Fe/H] and ξ, constant at the values calculated in Paper I. With the revised T eff scale we derive new Li abundances. We compare the NLTE values of T eff with the photometric temperatures of Ryan et al. (1999, ApJ, 523, 654), the infrared flux method (IRFM) temperatures of Meléndez & Ramírez (2004, ApJ, 615, L33), and the Balmer line wing temperatures of Asplund et al. (2006, ApJ, 644, 229). Results. We find that our temperatures are hotter than both the Ryan et al. and Asplund et al. temperatures by typically ∼110-160 K, but are still cooler than the temperatures of Meléndez & Ramírez by typically ∼190 K. The temperatures imply a primordial Li abundance of 2.19 dex or 2.21 dex, depending on the magnitude of collisions with hydrogen in the calculations, still well below the value of 2.72 dex inferred from WMAP + BBN. We discuss the effects of collisions on trends of 7 Li abundances with [Fe/H] and T eff , as well as the NLTE effects on the determination of log g through ionization equilibrium, which imply a collisional scaling factor S H > 1 for collisions between Fe and H atoms.