Tuesday 17 November at 3pm

Thomas Lowe - Joint Astronomy Centre

"Monte Carlo Simulations of Raman Scattered Emission Lines in Symbiotic Stars"

ABSTRACT:"Symbiotic stars are binary systems consisting of a hot compact object (usually a white dwarf) and a red giant. Over half of the known symbiotic stars exhibit strong broad emission features at 682.5 nm and 708.8 nm. These lines remained a mystery for over fifty years until H.M. Schmid in 1989 proposed that they were due to Raman scattering of the OVI resonance doublet at 103.2 nm and 103.8 nm. In this process an incident OVI photon excites a hydrogen atom from its ground state to a virtual state near n=3 from where it decays to the n=2 state resulting in the emission of an inelastically scattered line. The atom subsequenty decays to the ground state with the emission of a Lyman alpha photon. By energy conservation the energy of the scattered photon is given by the difference in energy between the incident OVI photon and the Lyman alpha photon. Since Raman scattering is a dipole type process and the scattering geometry in a symbiotic system is anisotropic these lines can exhibit a high degree of polarization. In addition since the scattering geometry varies in a systematic way the polarization in these emission lines is also expected to vary in a systematic way. Following the pioneering work of Schmid, Monte Carlo simulations have been carried out which illustrate how the variation in the polarization of Raman scattered emission lines is a diagnostic for the orbital elements of the binary system. In principle by comparing these theoretical models with a time series of spectropolarimetry of these lines one should be able to determine the orbital period, inclination and the longitude of the ascending node of these binary stars."