Centre Shift of the Raman Scattered He II $λ$ 4850 in the Symbiotic Star V1016 Cygni

Abstract

We present our spectroscopic data around H$β$ of the symbiotic star V1016 Cyg obtained with the Bohyunsan Echelle Spectrograph, in order to secure the broad emission feature at around 4850Å, which is formed through Raman scattering of He II $λ$ 972. The total cross section around Ly$γ$ is approximately given by $σ(λ)\simeq 1.7\times 10^{-28}[λ_{Lyγ}/(λ-λ_{Lyγ})]^2{\rm cm^2}$, with $λ_{Lyγ}$ being the line centre wavelength of Ly$γ$. We find a centre shift redward by an amount $Δλ= +0.64{\rm Å}$ in the Raman scattered He II $λ$ 4850. This redward centre shift is exactly analogous to the effect for the Raman scattered He II $λ$ 6545 blueward of H$α$ discussed in our previous study. We compute the branching ratios of Raman scattering into the level $2s$ and levels $3s,$ and 3d, which are subsequently incorporated in our Monte Carlo code. Assuming that He II $λ$ 972 emission is characterized by a single Gaussian profile, the redward peak shift observed in the Raman scattered He II 4850 feature corresponds to the neutral column density $N_{HI}=1.2\times 10^{21}{\rm\ cm^{-2}}$. Assuming that the covering factor $\sim 0.1$ of the scattering region with respect to the He II emission region and adopting a simple spherical stellar wind model, we may place an upper bound $\dot M \le 3.6\times 10^{-7}{\rm M_\odot yr^{-1}}$ for the mass loss rate of the giant component of V1016 Cyg. Our estimate can be severely affected by the inematics of the scattering and He II emission regions and the exact atomic physics, about which brief discussions are presented.