Lithium abundances of halo dwarfs based on excitation temperature I. Local thermodynamic equilibrium

Context. The discovery of the Spite plateau in the abundances of ⁷Li 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_Bh², there was a discrepancy between observationally determined and theoretically determined abundances in the case of ⁷Li. One of the most important uncertain factors in the calculation of the stellar 7Li 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 ⁷Li 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 7Li 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 Ha 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 ⁷Li and metallicity or temperature are present in these metal-poor stars.