The Source Function in a Non-Equilibrium Atmosphere. I. The Resonance Lines.

The source function for the central core of resonance lines is investigated under the assumption that the core is formed in the chromosphere. The conclusions reached are as follows: (1) The absorption coefficient is Doppler over a core some three Doppler widths in extent. (2) The frequency dependence of the emission coefficient follows closely that of the absorption coefficient, the difference amounting to about a factor of 4 over the Doppler core, a region where the absorption coefficient varies by about 1O thus the mechanism of line formation is essentially formally equivalent to that of pure absorption or of completely non-coherent scattering, the formal distinction between the two fading (3) Completely generally, the source function lies intermediate between a value fixed by collisional excitation with radiative de-excitation and a value fixed by photoelectric ionixations with recapture. For the ydrogen and non-metal group and for the neutral metals, the collisional term dominates only for Te> 75OO looooe; for singly ionixed metals, the collisional term dominates for Te > 35OO . Discussions of eclipse data make it probable that the cores of the strong resonance lines have chromospheric origin, so the collisional component of the source function may predominate In this case, a rough knowledge of ne(re) permits an inference of Te( e) from the line profile in the region of the core.