Raman-scattered He II λ6545 line in the symbiotic star V1016 Cygni

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 Hα and a broad emission feature around 6545 Å that is blended with the [N II] λ6548 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 Lyβ in the former case and the He II λ1025 emission line arising from n = 6 → n = 2 transitions for the latter feature. We remove the Hα wings by a template Raman scattering wing profile and subtract the [N II] λ6548 line using the 3 times stronger [N II] λ6583 feature in order to isolate the He II Raman-scattered. 6545 Å line. We obtain the flux ratio F₆₅₄₅/F₆₅₆₀ = 0.24 of the He II λ6560 emission line and the 6545 Å feature for V1016 Cyg. Under the assumption that the He II emission from this object is isotropic, this ratio is converted to the ratio Φ₆₅₄₅/Φ₁₀₂₅ = 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_{H I} ≥ 10²⁰ cm^-2 covers 17% of the emission region. By combining the presumed binary period of ∼100 yr for 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. It is argued that the Raman-scattered He II λ6545 line is an important and useful tool to investigate the mass-loss process occurring in the late stage of stellar evolution.