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Institut für Astronomie und Astrophysik

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Preprint E/99

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

H.M. Schmid (1), J. Krautter (1), I. Appenzeller (1), J. Barnstedt (2), T. Dumm (3), A. Fromm (2), M. Gölz (2), M. Grewing (2;4), W. Gringel (2), C. Haas (2), W. Hopfensitz (2), N. Kappelmann (2), G. Krämer (2), A. Lindenberger (2), H. Mandel (1), U. Mürset (3), H. Schild (3), W. Schmutz (3), and H. Widmann (2)

(1) Landessternwarte Heidelberg­Königstuhl, D­69117 Heidelberg, Germany
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) Institut für Astronomie, ETH Zentrum, CH­8092 Zürich, Switzerland
(4) Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France

Astronomy & Astrophysics 348 (1999), 950-971

Abstract. We present ORFEUS spectra of the O VI 1032,1038 emission lines in the symbiotic stars AG Dra, V1016 Cyg, RR Tel, CD-43 14304, AG Peg and Z And. The O VI emission lines can convert into broad and highly polarized emission lines at 6825 and 7082 in a Raman scattering process by neutral hydrogen. From a comparison of direct and Raman scattered radiation we extract new information on the scattering geometry in symbiotic systems.
The nebular O VI emission lines are in all objects redshifted by about +40 km/s. This can be explained as a radiative line transfer effect in a slowly expanding emission region. A comparable redshift is measured in the Raman 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, particularly near the O VI 1038 component. With model calculations we investigate their impact on the O VI lines.
From the dereddened line fluxes of the direct and Raman scattered O VI lines we derive the scattering efficiency, which is defined as photon flux ratio NRaman/NO VI. The efficiencies derived for RR Tel, V1016 Cyg and Z And indicate that about 30% of the released O VI 1032 photons interact with the neutral scattering region. The efficiencies for AG Dra and CD-43 14304 are much higher, which may suggest that the O VI nebulosity is embedded in a H0-region.
The D­type system RR Tel shows strong line profile differences between the direct O VI emission, which is single-peaked, and the Raman scattered emission, which is double-peaked. This indicates that the neutral scattering region in RR Tel "sees" different O VI line profiles, implying that the O VI nebulosity is far from spherically symmetric. In a tentative model we suggest for RR Tel an O VI flow pattern where material streams from the cool giant towards the hot component, which further accelerates the gas radially.
For the S­type systems AG Dra, CD-43 14302 Z And the line profile differences between the direct and the Raman scattered O VI emissions are less pronounced. This may suggest the the O VI profiles depend less on the emission direction than the D-type system RR Tel.
For AG Peg we detect for the first time the Raman scattered emission at 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 conversion efficiency for the narrow component indicates that the nebular O VI emission is significantly closer to the cool giant than the hot, mass losing component, and strongly supports previous colliding wind models for this object.

Key words: binaries: symbiotic - circumstellar matter - Stars: mass loss - Scattering - Polarization - Ultraviolet: stars

Paper (279k gzip'ed Postscript including figures)

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