Raman Scattered O VI $λ$ 6825 and the Accretion Disk Emission Model in the Symbiotic Stars V1016 Cygni and HM Sagittae

Abstract

We present the high resolution spectra of the D type symbiotic stars V1016 Cygni and HM Sagittae obtained with the Bohyunsan Optical Echelle Spectrograph (BOES), and investigate the double-peaked asymmetric profiles of the Raman scattered O VI 6825. By adopting a wind accretion disk model, we assume that the O VI emission region is described by a Keplerian thin disk. The Raman scattering occurs in a neutral region near the giant, taking in the form of a slow stellar wind, part of which is ionized by the strong UV radiation from the hot white dwarf. Using a Monte Carlo technique, we compute the line profiles that are modulated by the slow spherical stellar wind from the giant component with the ionization front approximated by a hyperboloid. In order to account for the asymmetry and the existence of a central dip in the profiles, we add an O VI resonance scattering region between the hot white dwarf and the giant star which hinders the incidence of slightly blue O VI photons upon the H I region. Overall good fits to the observed data are obtained from our model, which lends support to the accretion disk emission model in these objects. The best fitting parameters for V1016 Cyg are $v_o=30{\rm km s^{-1}}$, $v_\infty=11{\rm km s^{-1}}$, and $v_{c}=10{\rm km s^{-1}}$, where $v_o$, $v_\infty$ and $v_{c}$ are the velocity of the outer disk rim, the terminal velocity of the giant wind, and the velocity component of the resonance scattering O VI region along the binary axis, respectively. Similar fitting parameters $v_o=27{\rm km s^{-1}}$, $v_\infty=10{\rm km s^{-1}}$ and $v_{c}=9{\rm km s^{-1}}$ are obtained for HM Sge. We also investigate the effect of a hot spot in a disk that is well known in accretion disks in cataclysmic variables.