RAMAN-SCATTERED O VI λ6825 AND THE ACCRETION DISK EMISSION MODEL IN THE SYMBIOTIC STARS V1016 CYGNI AND HM SAGITTAE

摘要

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 λ6825 formed through Raman scattering of O VI λ1032 by atomic hydrogen. By adopting a wind accretion disk model, the O vi emission region is described by a Keplerian thin disk. The Raman scattering occurs in a neutral region near the giant, part of which is ionized by the strong UV radiation from the hot white dwarf. A Monte Carlo technique is used to 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 central dip in the profiles, we add an O VI resonance scattering region between the hot white dwarf and the giant 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. The best-fitting parameters for V1016 Cyg are υ_o = 30 km s~(-1), υ_∞ = 11 km s~(-1), and υ_c = 10 km s~(-1), where υ_o, υ_∞, and υ_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 υ_o = 26 km s~(-1), υ_∞ = 10 km s~(-1), and υ_c = 7 km s~(-1) are obtained for HM Sge.