Accretion disk emission and Raman scattering in symbiotic stars

Abstract

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 in the edge-on direction. Assuming the presence of scattering regions outflowing in the polar directions, we may explain the additional red wing or red peak structure, which is polarized parallel to the binary axis. Adopting the asymmetric accretion disk emission model, it is predicted that the blue peak strength relative to the red peak is larger in the 6830 A Raman feature than in the 7088 A Raman feature. It is concluded that Raman scattering is an important tool to investigate the physical conditions and geometrical configuration of the accretion disk in a symbiotic star.