RAMAN SCATTERED EMISSION-LINES IN SYMBIOTIC STARS - A SPECTROPOLARIMETRIC SURVEY

We observed spectropolarimetrically the emission lines at lambda6825 and lambda7082 in fifteen symbiotic stars. We find clear and sometimes very strong polarisation in these lines in agreement with their recent identification as due to Raman scattering. We examine and remove the contribution of the interstellar polarisation. We obtain polarisation maps with surrounding field stars and find that at around 7000 angstrom the continuum polarisation in symbiotic stars is mostly interstellar. Exceptions are RR Tel, He2-38 and possibly V1016 Cyg. The polarisation integrated over the lambda6825 and lambda7082 lines is typically around 5%. We find striking variations in the degree and angle of the polarisation across the line profiles. Maximum polarisation can be as high as 15%. The polarisation structure is always very similar in both lines. We can distinguish between three types of profiles with increasing complexity: I. Profiles with constant polarisation through the line, II. Profiles with decreasing polarisation towards the red line wing and constant polarisation angle and III. Profiles with two components and a rotating or flipping polarisation angle. There are about equal numbers of symbiotic stars in each class. The last profile type is only seen in systems with late M giants or Miras. Examples are HBV 475, V1016 Cyg, RR Tel and He2-106. We present a simple model which can account for the morphological differences seen in the polarisation structure. It shows that the double-peaked polarisation profiles can only be produced in a very extended scattering region. Another interesting feature is that the polarisation vector in the blue line wing has to be perpendicular to the line connecting the two binary components.