ORFEUS spectroscopy of the OVI lines in symbiotic stars and the Raman scattering process

We present ORFEUS spectra of the O VI lambda lambda 1032,1038 emission line s in the symbiotic stars AG Dra, V1016 Cyg, RR Tel, CD-43 degrees 14304, AG Peg and Z And. The OVI emission lines can convert into broad and highly po larized emission lines at lambda 6825 and lambda 7082 in a Raman scattering process by neutral hydrogen. From a comparison of direct and Raman scatter ed radiation we extract new information on the scattering geometry in symbi otic systems. The nebular O VI emission lines are in all objects redshifted by about + 40 kms(-1). This can be explained as a radiative line transfer effect in a sl owly expanding emission region. A comparable redshift is measured in the Ra man scattered O VI lines. In AG Peg the O VI emissions show beside a narrow nebular line a broad component from a fast stellar wind outflow. Many interstellar absorption lines of molecular hydrogen are detected, part icularly near the O VI lambda 1038 component. With model calculations we in vestigate their impact on the O VI lines. From the dereddened line fluxes of the direct and Raman scattered O VI line s we derive the scattering efficiency, which is defined as photon flux rati o N-Raman/N-O VI The efficiencies derived for RR Tel, V1016 Cyg and Z And i ndicate that about 30% of the released O VI lambda 1032 photons interact wi th the neutral scattering region. The efficiencies for AG Dra and CD-43 deg rees 14304 are much higher, which may suggest that the O VI nebulosity is e mbedded in a H-0-region. The D-type system RR Tel shows strong line profile differences between the direct O VI emission, which is single-peaked, and the Raman scattered emiss ion, which is double-peaked. This indicates that the neutral scattering reg ion in RR Tel "sees" different O VI line profiles, implying that the O VI n ebulosity is far from spherically symmetric. In a tentative model we sugges t for RR Tel an O VI flow pattern where material streams from the cool gian t towards the hot component, which further accelerates the gas radially. For the S-type systems AG Dra, CD-43 degrees 14304 and Z And the line profi le differences between the direct and the Raman scattered O vr emissions ar e less pronounced. This may suggest that the O vr profiles depend less on t he emission direction than in the D-type system RR Tel. For AG Peg we detect for the first time the Raman scattered emission at lam bda 6825. The Raman line shows a narrow, nebular component as the O VI line , but no equivalent emission to the broad O VI wind component, The higher c onversion efficiency for the narrow component indicates that the nebular O VI emission is significantly closer to the cool giant than the hot, mass lo sing component, and strongly supports previous colliding wind models for th is object.