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Astron. Astrophys. 355, 261-270 (2000)


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Spectropolarimetry of the symbiotic nova HM Sge *

H.M. Schmid 1,2, R. Corradi 3, J. Krautter 1 and H. Schild 4

1 Landessternwarte Heidelberg-Königstuhl, 69117 Heidelberg, Germany
2 Max-Planck-Institut für Astronomie, Königstuhl, 69117 Heidelberg, Germany
3 Instituto de Astrofisica de Canarias, c. Via Lactea S/N, 38200 La Laguna, Tenerife, Spain
4 Institut für Astronomie, ETH Zentrum, 8092 Zürich, Switzerland

Received 14 October 1999 / Accepted 21 December 1999

Abstract

We present the first spectropolarimetric observations of the symbiotic nova HM Sge. We first note that marked spectral changes occurred: in the near IR the spectrum of the Mira in the system has clearly appeared. The nebular emission line spectrum remains largely unchanged but we detect for the first time the Raman line at [FORMULA]6825. These changes indicate that the dust content in HM Sge is rapidly decreasing and that the dust obscuration period which began in 1985 has come to an end.

In terms of polarization we find that the red giant spectrum is intrinsically polarized at the 0.5 % level and the polarization signal is structured across the TiO absorption bands. The Raman scattered O VI line at [FORMULA]6825 shows a polarization of 3.4 % at an angle of 33 o. The behaviour of the polarization angle across the Raman line strongly indicates that an axisymmetric O VI -H I scattering geometry prevails. The distribution of the neutral hydrogen in the inner binary system does not seem to be strongly perturbed by binary motion. We conclude that the present binary axis is orientated at a position angle of [FORMULA] 123 o.

The deduced orientation of the binary axis of HM Sge is parallel or perpendicular to distinct geometric features of the surrounding nebulosity. This indicates that the orientation of the binary system is of prime importance for the understanding of the nebular structure.

Key words: stars: binaries: symbiotic – stars: circumstellar matter – stars: mass-loss – polarization – scattering

* Based on observations taken with the 4.2m William Herschel Telescope at La Palma and the 2.3m ANU Telescope at Siding Spring.

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

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000

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