Raman Scattered He II $λ$ 6545 Line in the Symbiotic Star V1016 Cygni

Abstract

We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m Canada-France-Hawaii Telescope, in order to illustrate a method to measure the covering factor of the neutral scattering region around the giant component with respect to the hot emission region around the white dwarf component. In the spectrum, we find broad wings around H$α$ and a broad emission feature around 6545${\rm Å}$ that is blended with the [N II]$ λ$ 6548 line. These two features are proposed to be formed by Raman scattering by atomic hydrogen, where the incident radiation is proposed to be UV continuum radiation around Ly$β$ in the former case and He II $λ$ 1025 emission line arising from $n=6\to n=2$ transitions for the latter feature. We remove the H$α$ wings by a template Raman scattering wing profile and subtract the [N II] $λ$ 6548 line using the 3 times stronger [N II] $λ$ 6583 feature in order to isolate the He II Raman scattered 6545 Åline. We obtain the flux ratio $F_{6545}/F_{6560}=0.24$ of the He II $λ$ 6560 emission line and the 6545 Åfeature for V1016 Cyg. Under the assumption that the He II emission from this object is isotropic, this ratio is converted to the ratio $Φ_{6545}/Φ_{1025}=0.17$ of the number of the incident photons and that of the scattered photons. This implies that the scattering region with H I column density $N_{HI}\ge 10^{20}{\rm cm^{-2}}$ covers 17 per cent of the emission region. By combining the presumed binary period $\sim 100$ yrs of this system we infer that a significant fraction of the slow stellar wind from the Mira component is ionized and that the scattering region around the Mira extends a few tens of AU, which is closely associated with the mass loss process of the Mira component.