Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...461..972s&link_type=abstract
Astrophysical Journal v.461, p.972
Astronomy and Astrophysics
Astronomy
11
Infrared: Stars, Line: Formation, Radiative Transfer, Stars: Early-Type, Stars: Emission-Line, Be
Scientific paper
Non-LTE radiative transfer calculations are performed in atmospheric models appropriate for B star photo spheres to demonstrate that the high-l Rydberg transitions of Mg II are in emission. The emission arises from small deviations in the populations of these Rydberg levels from their thermodynamic equilibrium values. This mechanism is the same as the one presented by Chang et al. (1991) and by Carlsson, Rutten, & Shchukina (1992) to explain the emission from the Mg I Rydberg transitions 6g-7h and 6h-7i in the solar spectrum near 12 μm.
Our work predicts wide-ranging infrared emission from the Mg II Rydberg levels, spanning nearly the entire range of B spectral types. Small divergences between the Rydberg departure coefficients enhance stimulated emission, which produces rising monochromatic source functions (dSv/dh > 0) and emission. Flux profiles of the Mg II high-l α-transitions (Δn = +1) from n = 4 and 5 show an emission peak superposed on a wider absorption trough, similar in form to the solar Mg I lines, while for higher n, the profiles are in full emission. The emission strengths increase strongly for lower surface gravities where the rates of thermalizing collisions are lowest. Maximum emission is predicted for the α-transitions from n = 5, 6, and 7, reaching Fλ/Fc ≍ 1.15 at line center (no rotation) and Wλ ≍ -0.1 Å. Transitions from higher n exhibit progressively lower continuum 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 infrared emission strengths is the scale of the collisional excitation rates between the Rydberg levels. However, reasonable variation of these rates does not eliminate the emission.
Lester John B.
Sigut Thomas Allan Aaron
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