Non-LTE Balmer line formation in late-type spectra: Effects of atomic processes involving hydrogen atoms

Context. The wings of Balmer lines are often used as effective temperature diagnostics for late-type (F, G, K) stars under the assumption they form in local thermodynamic equilibrium (LTE). Aims. Our goal is to investigate the non-LTE excitation and ionisation of hydrogen and the formation of Balmer lines in late-type stellar atmospheres, to establish if the assumption of LTE is justified. Furthermore, we aim to determine which collision processes are important for the problem; in particular, the role of collision processes with hydrogen atoms is investigated. Methods. A model hydrogen atom for non-LTE calculations has been constructed accounting for various collision processes using the best available data from the literature. The processes included are inelastic collisions with electrons and hydrogen atoms, mutual neutralisation and Penning ionisation. Non-LTE calculations are performed using the MULTI code and the MACKKL semi-empirical solar model, and the relative importance of the collision processes is investigated. Similar calculations are performed for MARCS theoretical models of other late-type stellar atmospheres. Results. Our calculations show electron collisions alone are not sufficient to establish LTE for the formation of Balmer line wings. Mutual neutralisation and Penning ionisation are found to be unimportant. The role of inelastic collisions with neutral hydrogen is unclear. The available data for these processes is of questionable quality, and different prescriptions for the rate coefficents give significantly different results for the Balmer line wings. Conclusions. Improved calculations or experimental data are needed for excitation and, particularly, ionisation of hydrogen atoms in low-lying states by hydrogen atom impact at near threshold energies. Until such data are available, the assumption of LTE for the formation of Balmer line wings in late-type stars is questionable.