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Astron. Astrophys. 336, 637-647 (1998)

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High resolution spectroscopy of symbiotic stars *

IV. BX Monocerotis: orbital and stellar parameters

T. Dumm 1, U. Mürset 1, H. Nussbaumer 1, H. Schild 1, H.M. Schmid 2, W. Schmutz 1 and S.N. Shore 3

1 Institut für Astronomie, ETH-Zentrum, CH-8092 Zürich, Switzerland
2 Landessternwarte Heidelberg-Königstuhl, D-69117 Heidelberg, Germany
3 Department of Physics and Astronomy, Indiana University South Bend, 1700 Mishawaka Avenue, South Bend, IN 46634-7111, USA

Received 22 April 1998 / Accepted 19 May 1998


We investigate the orbit and the components of the symbiotic BX Mon system with new high resolution spectroscopy, IUE spectra, published photographic magnitudes, and brightness estimates from the RASNZ. We review the available photometry and deduce a new binary period of [FORMULA]. We also find evidence in the IUE data that BX Mon is an eclipsing system.

With our high resolution spectroscopy we determine the radial velocity curve of the M giant from photospheric absorption features. BX Mon is unusual for a symbiotic star in that its hot component is also observable in the optical wavelength region. From corresponding absorption features we are able to measure the hot component's radial velocity. We determine semi-amplitudes for the cool and the hot components of 4.3  km s-1 and 29  km s-1, respectively. The mass ratio is thus [FORMULA] 7 which is among the highest yet found for symbiotic systems.

The orbit of BX Mon is eccentric with an ellipticity of [FORMULA]. The binary mass function is [FORMULA]. We determine the mass of the red giant as [FORMULA] and the mass of the hot component as [FORMULA]. This low [FORMULA] suggests that even relatively high mass symbiotics are unlikely to be supernova Type I progenitors.

The distance to BX Mon of [FORMULA] is determined with the Na i [FORMULA] interstellar absorption lines and the interstellar extinction feature at 2200 Å. For the luminosity of the cool component we find [FORMULA] and a stellar radius [FORMULA]. The red giant's radius remains within the inner Lagrangian point, even at periastron. The hot component is unlikely to be a main sequence star.

Key words: binaries: eclipsing – binaries: symbiotic – stars: fundamental parameters – stars: individual: BX Mon

* Based on observations obtained at the European Southern Observatory, La Silla (Chile), visual brightness data from the RASNZ, and UV spectra from the IUE archive. ESO observations were granted for the programs 47.7-081, 48.7-083, 49.7-041, 50.7-129, 51.7-093, 52.7-068, 53.7-083, 54.E-061, 55.E-446, 56.E-526

Send offprint requests to: T. Dumm

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998