On the profiles and the polarization of Raman-scattered emission lines in symbiotic stars -- II. Numerical simulations

Abstract

A Monte Carlo method is used to calculate the profiles and the polarization of the Raman scattered O VI lines (λλ6827, 7088) in symbiotic stars, which are believed to be a binary system of a cool giant and a hot star with an emission nebula around it. A point-like isotropic UV radiation source is assumed and a simple spherical wind model is adopted for the kinematics of the scattering material from the cool giant.We first investigate the case where the incident line photons are described by a Gaussian profile having a width of 10 4 K. We subsequently investigate the effects of the extended ionized region and non-spherical wind models including a disk-type wind and a bipolar wind. The cases where the emission source is described by non-Gaussian profiles are briefly studied.Finally as an additional component for the kinematics of symbiotic stars the orbital motion of the hot component around the cool giant is included and the effect on the spectropolarimetry is investigated. In this case the polarization direction changes around the red part of the Raman-scattered emission lines, when the observer's line of sight is perpendicular to the orbital plane, and no such effect is seen when the line of sight lies in the orbital plane. Furthermore, complex peak structures are seen in the degree of polarization and the polarized flux, which have often been observed in several symbiotic systems including RR Tel.Brief observational consequences and predictions are discussed in relation to the present and future spectropolarimetry for symbiotic stars. It is concluded that spectropolarimetry may provide a powerful diagnostic of the physical conditions of symbiotic stars.

3D Grid-Based Monte Carlo Code for Radiative Transfer through Raman and Rayleigh Scattering with Atomic Hydrogen -- STaRS

“…Schmid (1992) performed Monte-Carlo simulations to investigate the formation of Raman-scattered O VI features in an expanding H I region with an assumption that a given line photon has an invariant scattering cross section as it propagates through the H I medium. Similar studies were presented by Lee & Lee (1997), who adopted a density matrix formalism to determine the physical information of scattered radiation including polarization. Chang et al (2015) investigated the formation of broad Balmer wings near Hα and Hβ in the unification scheme of active galactic nuclei and presented quantitatively the asymmetry of the wings formed in neutral regions with extremely high H I column density ∼ 10 23 cm −2 .…”

“…The medium for radiative transfer through Raman and Rayleigh scattering corresponds to a thick H I region that is easily found in the slow stellar wind from a red giant (Lee & Lee 1997;Lee et al 2019). No consideration on the thermal motion of neutral hydrogen is given because the variation of the cross section and the branching ratio in the scale of thermal speed is negligible (Chang et al 2015(Chang et al , 2018.…”

“…Analytic solutions are available for this case, against which our result obtained from STaRS is compared. The second case is reproduction of the result of Lee & Lee (1997), who investigated Raman scattering of O VI in symbiotic stars. In this case, the H I region is an expanding spherical wind around the giant component.…”

“…Lee & Lee (1997) presented their basic study of line formation of Raman O VI in a symbiotic star consisting of a white dwarf and a mass losing giant. The O VI emission region near the white dwarf component was assumed to be a point source broadened thermally with T = 10 4 K. For simplicity, the slow stellar wind from the red giant component is assumed to be entirely neutral ignoring the photoionization by the white dwarf.…”

“…where n 0 is the characteristic number density defined in Eq. 2.11 of Lee & Lee (1997) and v ∞ is the terminal velocity. As a second code test case, we revisit Raman O VI formation illustrated in Figs.…”