Astronomy and Astrophysics, volume 486, 985-993 (2008/8-2)
MgI emission lines at 12 and 18µm in K giants.
SUNDQVIST J.O., RYDE N., HARPER G.M., KRUGER A. and RICHTER M.J.
Abstract (from CDS):
The solar mid-infrared metallic emission lines have already been observed and analyzed well, and the formation scenario of the MgI 12µm lines has been known for more than a decade. Detections of stellar emission at 12µm have, however, been limited to MgI in very few objects. Previous modeling attempts have been made only for Procyon
and two cool evolved stars, with unsatisfactory results for the latter. This prevents the lines' long predicted usage as probes of stellar magnetic fields. We want to explain our observed MgI emission lines at 12µm in the K giants Pollux
, and Aldebaran
and at 18µm in Pollux and Arcturus. We discuss our modeling of these lines and particularly how various aspects of the model atom affect the emergent line profiles. High-resolution observational spectra were obtained using TEXES at Gemini North and the IRTF. To produce synthetic line spectra, we employed standard one-dimensional, plane-parallel, non-LTE modeling for trace elements in cool stellar 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 numbers in statistical equilibrium. The MgI emission lines at 12µm in the K giants are stronger than in the dwarfs observed so far. We present the first observed stellar emission lines from MgI at 18µm and from AlI, SiI, and presumably CaI at 12µm. We successfully reproduce the observed MgI emission lines simultaneously in the giants and in the Sun, but show how the computed line profiles depend critically on atomic data input and how the inclusion of energy levels with n≥10 and collisions with neutral hydrogen are necessary to obtain reasonable fits.
stars: atmospheres - stars: individual: Pollux - stars: individual: Arcturus - stars: individual: Aldebaran - stars: late-type - line: formation
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