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Astron. Astrophys. 348, 950-971 (1999)


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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 Heidelberg-Königstuhl, D-69117 Heidelberg, Germany
2 Institut für Astronomie und Astrophysik der Universität Tübingen, Waldhäuserstrasse 64, D-72076 Tübingen, Germany
3 Institut für Astronomie, ETH Zentrum, CH-8092 Zürich, Switzerland
4 Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France

Received 24 August 1998 / Accepted 16 April 1999

Abstract

We present ORFEUS spectra of the O VI [FORMULA]1032,1038 emission lines in the symbiotic stars AG Dra, V1016 Cyg, RR Tel, CD[FORMULA], AG Peg and Z And. The O VI emission lines can convert into broad and highly polarized emission lines at [FORMULA]6825 and [FORMULA]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 [FORMULA] km s-1. 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 [FORMULA]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 [FORMULA]. The efficiencies derived for RR Tel, V1016 Cyg and Z And indicate that about 30% of the released O VI [FORMULA]1032 photons interact with the neutral scattering region. The efficiencies for AG Dra and CD[FORMULA] are much higher, which may suggest that the O VI nebulosity is embedded in a H0-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 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 S-type systems AG Dra, CD[FORMULA] and Z And the line profile differences between the direct and the Raman scattered O VI emissions are less pronounced. This may suggest that the O VI profiles depend less on the emission direction than in the D-type system RR Tel.

For AG Peg we detect for the first time the Raman scattered emission at [FORMULA]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: stars: binaries: symbiotic – stars: circumstellar matter – stars: mass-loss – polarization – scattering – ultraviolet: stars

* Based on observations taken during the orfeus-spas i and orfeus-spas ii space shuttle missions, and ground based data collected at the ESO 2.2m and 3.6m telescopes at La Silla, Chile, and the 4.2m William Herschel Telescope at La Palma, Canary Islands. ESO observations were granted for the programs 52.7-040 and 58.D-0866.

Send offprint requests to: H.M. Schmid, Heidelberg (hschmid@lsw.uni-heidelberg.de)

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199

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