HIGH-RESOLUTION SPECTROSCOPY OF SYMBIOTIC STARS - IV - BX MONOCEROTIS- ORBITAL AND STELLAR PARAMETERS

We investigate the orbit and the components of the symbiotic EX Mon sy stem with new high resolution spectroscopy, IUE spectra, published pho tographic magnitudes, and brightness estimates from the RASNZ. We revi ew the available photometry and deduce a new binary period of 1401 day s. We also find evidence in the IUE data that EX Mon is an eclipsing s ystem. With our high resolution spectroscopy we determine the: radial velocity curve of the M giant from photospheric absorption features. E X Mon is unusual for a symbiotic star in that its hot component is als o observable in the optical wavelength region. From corresponding abso rption features we are able to measure the hot component's radial velo city. We determine semiamplitudes for the cool and the hot components of 4.3 km s(-1) and 29 km s(-1), respectively. The mass ratio is thus similar to 7 which is among the highest yet found for symbiotic system s. The orbit of EX Mon is eccentric with an ellipticity of e = 0.49. T he binary mass function is 0.0076 M.. We determine the mass of the red giant as M-r = 3.7 M-. and the mass of the hot component as M-h = 0.5 5 M.. This low M-h suggests that even relatively high mass symbiotics are unlikely to be supernova Type I progenitors. The distance to EX Mo n of 3 kpc is determined with the Na I lambda lambda 5890, 5896 inters tellar absorption lines and the interstellar extinction feature at 220 0 Angstrom. For the luminosity of the cool component we find L-r = 340 0 L. and a stellar radius R-r = 160 R-.. The red giant's radius remain s within the inner Lagrangian point, even at periastron. The hot compo nent is unlikely to be a main sequence star.