Discovery of variable polarization in H$\alpha$ profile of symbiotic star Y Gem: A case for orbital-phase dependent variation of Raman-scattered Ly$\beta$ emission

TL;DR

This study reports the rare detection of variable polarization in Hα of Y Gem, attributed to Raman scattering of Lyβ photons, confirmed through simulations and orbital modeling over 22 months.

Contribution

It provides the first evidence of strong, variable polarization in Hα caused by Raman scattering of Lyβ in a symbiotic star, supported by simulations and orbital analysis.

Findings

Detected strong variable polarization in Hα over 22 months

Confirmed Raman scattering of Lyβ as the polarization cause

Validated polarization variation with orbital phase modeling

Abstract

The geometry and morphology of symbiotic stars are conducive to exhibit a variety of scattering phenomena. The prominent among them is the Raman scattering of O VI doublet $\lambda \lambda$ 1032,1038 angstrom, which often show strongly polarized features in the visible spectrum. Similar Raman scattering of Ly$\beta$ photons has also been predicted to occur in symbiotic stars, though with fewer detections and with weak polarization amplitudes. Here, we present the discovery of strong variable polarization in the H$\alpha$ profile of a recently established symbiotic system Y Gem, over a period of nearly 22 months. This is, most likely, a very rare detection of the strongly polarized Raman scattered Ly$\beta$ photons, falling at the H$\alpha$ emission. Monte-Carlo simulations have been conducted to confirm the underlying Raman scattering process causing the polarized line profile, and a simple orbital model is constructed with typical parameters available in the recent literature along with a complementary low-resolution spectroscopic data. These simulations and models are then used to validate the observed polarization variation of H$\alpha$ at different orbital phases corresponding to the epochs of observations. The possibility of such strong variable H$\alpha$ polarization, being caused by Raman scattering of Ly$\beta$, would thus open up avenues of exploring such effects in various other astrophysical situations having similar morphology.