Polarization in the Raman-Scattered Emission from Symbiotic Stars

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Session 84 -- Variable Stars
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[84.09] Polarization in the Raman-Scattered Emission from Symbiotic Stars

P.M. Garnavich (DAO)

The mysterious 6830\AA\ emission band, seen exclusively in symbiotic stars, remained unidentified for over fifty years. Recently, Schmid (1989, {A\&Ap}, 211, L31) proposed that the 6830\AA\ line, along with a weak companion at 7088\AA , is a result of Raman scattering of O~VI photons by neutral hydrogen gas. This theory explains many of the observed properties of the 6830\AA\ band, such as its correlation with high ionization species and its substantial width when compared with direct emission lines. Because the mechanism producing the emission is scattering, Schmid predicts that these lines should be highly polarized.

Seven symbiotic stars were observed with the 1.8m Plaskett Telescope and the UBC polarization analyzer developed by N. Dinshaw and G. Walker. Polarization was detected in the 6830\AA\ feature for five of the stars and ranged between 4\%\ and 20\% . When detected, the polarization in the 7088\AA\ feature was generally less than half that of the 6830\AA\ emission. Changes in the polarization strength of V1329~Cyg and the polarization position angle of Z~And over a four week period were seen and probably resulted from the relative orbital motion of the binary components. Monitoring of these variations is continuing at DAO and it should be possible to determine the orbital period, inclination and orientation for the program systems.

The fraction of polarized light and polarization position angle were observed to vary with wavelength across the 6830\AA\ band. This complex behavior suggests that analysis of the Raman scattered emission is an excellent probe of gas dynamics in symbiotic systems. In V1329~Cyg, the polarization position angle abruptly jumps by 90$^\circ$ on the red tail of the emission, implying the cool giant wind has a velocity of 85 km~s$^{-1}$ and nearly surrounds the hot source (i.e. , the system has a small ionization parameter). The symmetry of the polarization structure in V1016~Cyg suggests that the neutral hydrogen causing the scattering is in the form of a mass-loss ring encircling the cool star.

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