Polarized resonance line transfer with collisional redistribution

Abstract

In this paper we study the effect of depolarizing collisions on resonance line polarization in the absence of magnetic fields. The role of γ, the coherence parameter is investigated in determining the percentage of polarization across the spectral line profiles. We also try to find out whether the partial frequency redistribution function RIII(which describes the frequency redistribution due to collisions) can be replaced by the complete redistribution (CRD, i.e Φ(x)Φ(x'))in resonance line polarization calculations. We have chosen idealized models to illustrate the differences in polarization when CRD, RIII, and RII functions are used in the radiative transfer calculations. We considered isothermal, effectively thin and semi-infinite atmospheres. RIII function gives different polarization in the Doppler core compared to CRD when depolarizing collisions are neglected, but it gives the same polarization as CRD in the wings in all circumstances. It is proposed that any redistribution function which is different from Φ(x)Φ(x')at certain frequencies is likely to show different polarization compared to CRD at those frequencies even though the total specific intensities may not differ. We find that the percentage of polarization at the line centre to be a monotonic function of the parameter γ when depolarizing collisions are included in the calculations. This important aspect is absent if the depolarizing collisions are neglected. Therefore one may use this property for diagnostic purposes. The character of the wing polarization is altered when γ = 0.9 compared to γ = 1.0 (RII redistribution). Therefore γ is an important parameter while evaluating the wing polarization. We find that CRD may be a good approximation to RIII when depolarizing collisions are included in the approximation.