Including Mg I IR (<72000 Å) lines in solar and stellar models

Considering the imminent launch of the James Webb Space Telescope with IR spectroscopy capabilities, our ability of calculating reliable synthetic stellar spectra in this spectral range must be improved. An important species with strong features in the spectra of late-type stars is the neutral Mg. In the present work we have improved our previous Mg I atomic model adding 128 strong transitions (log(g f)>−1), up to 72000 A, and updated several atomic parameters. One of the most important changes is our innovative treatment of the Effective Collision Strengths (ECS) parameters, which describe excitation by collision with electrons. We employed the latest R-matrix computations by Barklem et al. (2017) up to level 25. For transitions involving levels between 26 and 54, we compared the influence in the spectrum of two different sets of ECS parameters: the widely used combination of Seaton (1962) (allowed transitions) plus van Regemorter (1962) (forbidden transitions) formulas, and new ECS obtained in this work using the multi–configuration Breit–Pauli distorted–wave (DW) method. We found an overall better agreement between observed and synthetic IR spectral lines when the combination R-matrix and the new DW calculations for the ECS were used. This result opens the possibility of a new and more reliable approach than the formulas of Seaton and van Regemorter (1962) as methods of calculating ECS for Mg I and other species in the stellar atmospheres, when the R-matrix method is not available