Raman-scattered Ne vii λ973 at 4881 Å in the symbiotic star V1016 Cygni

Abstract

We present the high resolution spectra of the symbiotic star V1016 Cygni obtained with the Bohyunsan Optical Echelle Spectrograph in 2003 and 2005, from which we find a broad emission feature at 4881 \AA. We propose that this broad feature is formed from Raman scattering of Ne VII$\lambda973$ by atomic hydrogen. Thus far, the detection of Raman scattered lines by atomic hydrogen is limited to O VI$\lambda\lambda$1032, 1038 and He II$\lambda\lambda$940, 972 and 1025. With the adoption of the center wavelength 973.302 \AA\ of Ne VII$\lambda$973 and consideration of the air refractive index of $n_{air}=1.000279348$, the atomic line center of the Raman scattered Ne VII feature is determined to be 4880.53 \AA. The total cross section at the line center of Ne VII$\lambda973$ is computed to be $2.62\times 10^{-22}{\rm\ cm^2}$ with the branching ratio of 0.17. We perform Monte Carlo simulations to fit the Raman scattered Ne VII$\lambda973$. Assuming that the Ne VII and He II emission regions share the same kinematics with respect to the neutral scattering region, we find that the Raman scattered He II$\lambda$972 at 4850 \AA\ and Ne VII$\lambda$973 at 4881 \AA\ are excellently fitted. We also propose that the He II and Ne VII emission regions were stationary with respect to the H I region in 2003 but that they were receding from it with a velocity $\sim 20{\rm\ km\ s^{-1}}$ in 2005.