LINE FORMATION OF NEUTRAL ALUMINUM IN THE SUN

We investigate the formation of neutral aluminium lines in the solar p hotosphere using an atomic model containing 58 levels plus the ground state of Al II connected via radiative and collisional interaction. Sy nthetic line flux and intensity profiles are compared with the solar s pectrum to study the relevant kinetic processes and their influence on level populations and line profiles. For neutral aluminium with its e xtremely large ground state photoionization cross-section near 2071 An gstrom the reduction of the ultraviolet radiation field due to metalli c line absorption has to be taken into account using Kurucz' (1992) OD F opacities. In the photosphere of a cool star excitation and ionizati on due to collisions with neutral hydrogen can outweight electronic co llisions. The influence of different types of collisional interactions with electrons and neutral hydrogen is therefore examined. As expecte d, the non-LTE effects in most of the solar Al I lines are small, irre spective of the details of the atomic model. The cores and innermost p arts of the wings of the resonance lines at 3944 and 3961 Angstrom are affected by only small deviations from the observed profiles, part of which is due to the uncertainty connected with the proper choice of t he continuum flux in the region of the Ca II H+K lines. Since the firs t excited state, 4s S-2, is slightly overpopulated with respect to bot h the 3p P-2(o) ground state and to 4p P-2(o) the strongest evidence f or non-thermal excitation is found in the infrared lines at 1.3 and pa rticularly at 12 mu m. Empirical evidence for the necessity to include neutral particle collisions in the kinetic equilibrium of aluminium a rises from comparison of these lines with observations. General agreem ent with solar line profiles in the infrared and in the visible is fou nd for an atomic model with both electronic collisions and a strongly reduced amount of neutral particle collisions. The solar model will se rve as a reference for the investigation of cool metal-poor stars in w hich both the reduced electronic collision rates and the enhanced UV i ntensities lead us to expect more pronounced deviations from LTE.