Raman‐scattered Oviλ6825 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 k6825 formed through Raman scattering of O vi k1032 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 v o ¼ 30 km s À1 , v 1 ¼ 11 km s À1 , and v c ¼ 10 km s À1 , where v o , v 1 , 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 ¼ 26 km s À1 , v 1 ¼ 10 km s À1 , and v c ¼ 7 km s À1 are obtained for HM Sge.

The spectroscopic evolution of the symbiotic star AG Draconis

“…To increase the optical depth sufficiently in the UV requires column densities of order 10 24 cm −2 or higher and would be consistent with an expansion of the WD photosphere accompanied by an optically thick wind. Lee & Kang (2007) propose an accretion disk as the source for Raman scattered photons and model the inequality of the peaks as an optical depth effect by self-absorption, in the RG wind. It is possible that this is a contributor to the spectra, hydrodynamical modeling of symbiotic wind does produce disks (e.g.…”

A MONTE CARLO STUDY OF FLUX RATIOS OF RAMAN SCATTERED O vi FEATURES AT 6825 AND 7082 Å IN SYMBIOTIC STARS

“…According to Lee & Kang (2007), the velocity difference of the two peaks in the Raman O VIλ 6825 feature of the symbiotic star V1016 Cyg is about 60 km s −1 . This separation corresponds to the observed width of ∼ 10Å which is typical for most symbiotic stars exhibiting Raman O VI features.…”

“…Lee & Kang 2007 Seaquist & Taylor (1990) proposed that the mass loss rate is correlated with the spectral type in such a way that later spectral type giants tend to lose mass faster (see also Seaquist et al 1993). In the case 'D' type symbiotic star HM Sge, our result of N HI ∼ 10 22 cm −2 implies a relatioṅ M 7 /(v 10 A 14 ) ∼ 1.…”

“…A realistic neutral region is expected to take a shape that may be approximated by a hyperboloid (e.g. Taylor & Seaquist 1984;Lee & Kang 2007). However, in this work we adopt a much simpler geometry of a finite cylinder in order to focus on the issue of the flux ratio.…”

“…Combining this fact with the asymmetric accretion flow with the entering side much thicker than the opposite side, we expect that the blue part of Raman 7082 should be more suppressed compared to its red part than that of Raman 6825. Lee & Kang (2007) performed profile analyses of Raman O VI 6825 fluxes for two 'D' type symbiotic stars, V1016 Cygni and HM Sagittae. They successfully showed that the profiles are consistent with the Keplerian motion of the O VI emission region around the white dwarf component with a typical speed of ∼ 30 km s −1 .…”