Effects of partial frequency redistribution with dipole scattering on the formation of spectral lines in expanding media

Abstract

Line formation in expanding spherical atmospheres using partial frequency redistribution with dipole scattering has been studied by using a non LTE two level atom model. Lines with zero natural line widths are treated by using the angle dependent and angle independent redistribution function R1 (see Unno 1952, Field 1959, Hummer 1962, and Mihalas1978). Lines formed by partial and complete redistribution with isotropic scattering also have been calculated for the sake of comparison with those formed by dipole scattering. The ratios of outer to inner radii of the atmosphere have been taken to be 1, 10 and 100 so that the effects of sphericity are clearly separated from those of plane parallel approximation. Velocities upto 2 thermal units are considered in the rest of the star. Two cases have been considered: (1) є=10/sup-3 and β=0 and (2) є= β=10/sup-3 where є is the probability per scatter that the photon is destroyed by collisional de-excitation and β is the ratio KC/KL of opacity due to continuous absorption per unit interval of frequency to that in the line. The total optical depth TL at the line centre is taken to be approximately 10/sup-3.

Several important differences have been observed among the lines calculated using the five redistribution functions. However, for all parameters є β, B/A and v, the differences between the lines formed by the angle independent and angle dependent R1 with dipole scattering are substantially small so that it is not possible to resolve them graphically. When the velocity at the outermost layer is 2, the P Cygni type profiles are obtained i.e., with red emission blue absorption. This effect is more pronounced in the extended spherical medium than in the plane parallel situation. However, in all situations, the lines formed by dipole scattering show less emission and absorption.