Mg I emission lines at 12 & 18 μ m in K giants ⋆

Context. The solar mid-infrared metallic emission lines have alread y been observed and analyzed well, and the formation scenari o of the Mg I 12μm lines has been known for more than a decade. Detections of stellar emission at 12μm have, however, been limited to Mg I in very few objects. Previous modeling attempts have bee n made only for Procyon and two cool evolved stars, with unsat isfactory results for the latter. This prevents the lines’ long predic ted usage as probes of stellar magnetic fields. Aims. We want to explain our observed Mg I emission lines at 12 μm in the K giants Pollux, Arcturus, and Aldebaran and at 18 μm in Pollux and Arcturus. We discuss our modeling of these line s and particularly how various aspects of the model atom a ffect the emergent line profiles. Methods. High-resolution observational spectra were obtained usin g TEXES at Gemini North and the IRTF. To produce synthetic line spectra, we employed standard one-dimensional, plane -p rallel, non-LTE modeling for trace elements in cool stel lar atmospheres. We computed model atmospheres with the MARCS code, applied a comprehensive magnesium model atom, and used the radiative transfer code MULTI to solve for the magnesium occupation nu mbers in statistical equilibrium. Results. The Mg I emission lines at 12 μm in the K giants are stronger than in the dwarfs observed so fa r. We present the first observed stellar emission lines from Mg I at 18 μm and from Al I, Si I, and presumably Ca I at 12 μm. We successfully reproduce the observed Mg I emission lines simultaneously in the giants and in the Su n, b t show how the computed line profiles depend critically o n atomic data input and how the inclusion of energy levels with n ≥ 10 and collisions with neutral hydrogen are necessary to obt ain reasonable