INFRARED RYDBERG EMISSION-LINES IN EARLY-TYPE STARS .1. MG-II

Non-LTE radiative transfer calculations are performed in atmospheric m odels appropriate for B star photospheres to demonstrate that the high -l Rydberg transitions of Mg II are in emission. The emission arises f rom small deviations in the populations of these Rydberg levels from t heir thermodynamic equilibrium values. This mechanism is the same as t he one presented by Chang et al. (1991) and by Carlsson, Rutten, & Shc hukina (1992) to explain the emission from the Mg I Rydberg transition s 6g-7h and 6h-7i in the solar spectrum near 12 mu m. Our work predict s wide-ranging infrared emission from the Mg II Rydberg levels, spanni ng nearly the entire range of B spectral types. Small divergences betw een the Rydberg departure coefficients enhance stimulated emission, wh ich produces rising monochromatic source functions (dS(v)/dh > 0) and emission. Flux profiles of the Mg II high-l alpha-transitions (Delta n = +1) from n = 4 and 5 show an emission peak superposed on a wider ab sorption trough, similar in form to the solar Mg I lines, while for hi gher n, the profiles are in full emission. The emission strengths incr ease strongly for lower surface gravities where the rates of thermaliz ing collisions are lowest. Maximum emission is predicted for the alpha -transitions from n = 5, 6, and 7, reaching F-lambda/F-c approximate t o 1.15 at line center (no rotation) and W-lambda approximate to -0.1 A ngstrom. Transitions from higher n exhibit progressively lower continu um contrasts because of the steep rise of the continuous opacity with wavelength in the infrared and because of increased Stark broadening. The largest source of uncertainty affecting the predicted Mg II infrar ed emission strengths is the scale of the collisional excitation rates between the Rydberg levels. However, reasonable variation of these ra tes does not eliminate the emission.