Orbital motion in symbiotic Mira systems

Abstract

We present new spectropolarimetry of the Raman lines for seven symbiotic Miras, combine it with earlier data and review the polarimetric changes. All systems show slow and usually steady angular variations which are most easily interpreted as a reflection of the orbital motion. We measure angular rotation rates of typically 1 to 5. Small position angle changes are seen in Hen2-106 and RR Tel, while Hen2-38 and Hen2-127 have high rotation rates. Although our database spreads over a time span of almost 10 years, we still cover only a fraction of a typical D-type orbit. Since we expect elliptical orbits, it is too early to extrapolate to an orbital period for any individual object. In a statistical sense, however, our mean angular rotation rate shows that an average orbital period for a symbiotic Mira is 150 years. We also determine the position angle of the binary axes as projected onto the sky. Our polarimetry finally suggests that the binary axes of the systems V1016 Cyg and Hen2-38 are not far from parallel to the line of sight (conjunction), whereas the binary axes of RR Tel and AS 210 lie approximately at an angle of 90° to the line of sight.