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

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 $λλ$ 1032,1038 angstrom, which often show strongly polarized features in the visible spectrum. Similar Raman scattering of Ly$β$ 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$α$ 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$β$ photons, falling at the H$α$ 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$α$ at different orbital phases corresponding to the epochs of observations. The possibility of such strong variable H$α$ polarization, being caused by Raman scattering of Ly$β$, would thus open up avenues of exploring such effects in various other astrophysical situations having similar morphology.