Abstract:
We have investigated how an optically thin spherical shell with small velocities change the profiles and equivalent widths of the lines. We have employed several types of variations in density, velocity of expansion and source functions. In all the cases we find that the line centres are shifted to the blue side almost in proportion to the velocity of expansion. The shells moving with constant velocities shift the line centre the most, irrespective of the density variation. The velocity gradients shift the line centre the least. Maximum velocity of expansion considered is 6 mean thermal units and the total optical depth is taken to be equal to 5.
