High resolution spectroscopy of symbiotic stars. IV. BX Monocerotis: orbital and stellar parameters

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 1401 days. 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 ~ 7 which is among the highest yet found for symbiotic systems. The orbit of BX Mon is eccentric with an ellipticity of e=0.49. The binary mass function is 0.0076 M_sun. We determine the mass of the red giant as M_r=3.7 M_sun and the mass of the hot component as M_h=0.55 M_sun. This low M_h suggests that even relatively high mass symbiotics are unlikely to be supernova Type I progenitors. The distance to BX Mon of 3 kpc is determined with the Na I lambda lambda5890 ,5896 interstellar absorption lines and the interstellar extinction feature at 2200 Angstroms. For the luminosity of the cool component we find L_r=3400 L_sun and a stellar radius R_r=160 R_sun. 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. 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