On the centre shifts of Raman-scattered He ii and Hα wings in symbiotic stars

Abstract

Using a Monte Carlo technique, we investigate the centre shifts that are expected to occur for the broad Ha wings and the He II A6545 feature as a function of the neutral hydrogen column density in symbiotic stars. These two features are proposed to be formed via Raman scattering of ultraviolet (UV) continuum around the Lyβ and He II λ 1025 emission line by neutral hydrogen. The strengths of these two features are determined by a combination of various physical parameters including the covering factor and the neutral hydrogen column density (N H1 ) of the scattering region. The branching ratio of Raman scattering to Rayleigh scattering for UV radiation around Lyβ is a non-linearly increasing function of the wavelength, which results in enhanced Raman optical fluxes redward of the Ha line centre as N H I increases. However, we find that the amount of wing centre shift is quite small about 20 km s -1 as N H I increases from 10 20 to 10 21 cm -2 . Assuming that He II λ 1025 emission is characterized by a Gaussian profile, the Raman-scattered He II 6545 feature exhibits near Gaussian profiles with the peak shifted redward for N H I < 10 22 cm -2 . The redward centre shift amounts to 1 A for N H I ∼ 10 20 cm -2 and decreases as N H I increases up to N H I ∼ 10 22 cm -2 , above which no centre shift is observed. The redward peak shift is due to the fact that the incident emission profile is symmetric with respect to the He II λ 1025 line centre whereas the Raman conversion rate is increasing towards the line centre of Lyβ. We emphasize that the determination of N H I by locating the exact peak position of the He II 6545 feature will lift the degeneracy to allow a more accurate estimate of the covering factor of the neutral region, providing strong constraints on the mass-loss process occurring in symbiotic stars.