Polarisation of the Broad H alpha Wing in Symbiotic Stars

Abstract

In many symbiotic stars there appear broad wings around H alpha, of which the formation mechanism proposed thus far includes a fast outflow, the inner accretion disc motion, electron scattering and Raman scattering of Ly beta. We adopt a Monte Carlo technique to simulate the Raman scattering of UV photons that are converted into optical photons around H alpha forming broad wings, and compute its polarisation. Noting that many symbiotic stars exhibit a bipolar nebular morphology and polarisation flip in the red wing part of the Raman scattered O {\tiny{VI}} features, we assume that the neutral scattering region is composed of the two components. The first component is a static cylindrical shell with finite thickness and the other component is a finite slab that is moving away with velocity v_p = 100 km s^{-1} along the symmetry axis of the first component. The cylindrical shell component yields polarisation in the direction parallel to the cylinder axis. The polarisation near the line-centre is weaker than in the far wing regions because of the large Rayleigh scattering numbers due to the large scattering cross sections near the line centre. The receding polar scattering component produces strong polarisation in the direction perpendicular to the cylinder axis. When the both scattering components coexist, the polarisation is characterised by weak parallel polarisation near the line-centre and strong perpendicular polarisation in the red part. We discuss the observational implications of our computation.