Accretion disk emission and Raman scattering in symbiotic stars

Symbiotic stars, known as binary systems of a giant with heavy mass loss and a white dwarf accompanied by an emission nebula, are believed to form an accretion disk around the white dwarf component by attracting the slow but heavy stellar wind around the giant companion. About a half of symbiotic stars exhibit mysterious broad emission features around 6830 A and 7088 A, which have been identified by Schmid (1989) as the Raman scattered features of the O VI 1032 A and 1038 A doublet by atomic hydrogen. The scattering incoherency results in very broad profiles and strong polarization. Spectroscopic and polarimetric observations show that the Raman scattered features exhibit double or triple peak profiles and a polarization flip in the red wing part. In the accretion disk emission model, it is expected that the Raman features are polarized perpendicular to the binary axis and show double peak structures in the profile, because the neutral scatterers located near the giant component views the accretion disk i...