| Abstract: | The partial frequency redistribution function for zero natural line width with dipole scattering (RI) has been considered in obtaining the simultaneous solution of the statistical equilibrium and line transfer equations in
the comoving frame of the expanding gas. We have considered a non-LTE two level atom in an expanding spherical medium whose
outer radii are 3, 10 and 20 times the stellar radius with a total optical depthT ≃ 2 × 103. In all the cases, we have calculated the population ratio of the two levels N2/N1 and compared these results with those obtained by using different expansion velocities and geometrical extensions. Initially,
the upper level population (N2) is set equal to zero. The converged simultaneous solution shows that the upper level population is enhanced considerably
from the initial value. Variation in velocity gradients seem to have little effect on the ratio N2/N1 when the geometrical thickness of the medium is 3 or 10 times the stellar radius. However, when the thickness is increased
to 20 times the central radius, the velocity gradients change the ratio N2/N1 considerably in the region where log T ≤ 2. The effect of variation of geometrical thickness is to reduce the N2/N1 ratio atτ = 0. |
