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

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1. Introduction

Stars with combination spectra are those whose spectra show simultaneously very high excitation emission lines and low temperature absorption features, but which cannot be considered certain binaries. Some or all may, however, be so. Under this heading BX Mon entered Bidelman's (1954) list of 'combination stars' - which was at that time the designation for what we call symbiotic systems. Due to the low degree of excitation in the optical emission lines BX Mon was excluded from Allen's (1979, 1982) catalogs of symbiotic stars. The presence of medium ionized species became evident with the International Ultraviolet Explorer (IUE) spectra of Michalitsianos et al. (1982). Observed emission features in the UV are C III ], C IV , N III ], and O III ]. This property was most likely the reason for the inclusion in Allen's (1984) catalog.

BX Mon is an unusual symbiotic system in the sense that its hot component is rather cool and easily detected in the long wavelength IUE and blue optical spectra. Michalitsianos et al. (1982) remarked on the strong continuum in the long wavelength range of IUE, which they described as resembling that of a late A to early F star.

Photographic monitoring of BX Mon from 1890 to 1940 revealed a periodicity of 1380 days and an amplitude of about 3 magnitudes (Mayall 1940). This was often interpreted as due to a very long period Mira variable, and it entered, under this flag, the General Catalog of Variable Stars (Kukarkin et al. 1958). Later Whitelock & Catchpole (1983) suggested that the infrared spectrum and colours are appropriate for a normal [FORMULA] M5 giant and not for a Mira variable. This is supported by the published K-magnitudes for BX Mon which give a mean value of [FORMULA] with a 1[FORMULA]-scatter of [FORMULA] (Whitelock & Catchpole 1983; Viotti et al. 1986; Kenyon 1988; Munari et al. 1992). These IR data and the extensive spectroscopic monitoring by Iijima (1985) established that the periodic variations are caused by the change of the viewing angle due to orbital motion.

In this Paper, we present radial velocity measurements which provide orbital information for the BX Mon system. We re-examine photometric data in order to establish the orbital period. We determine the binary mass-ratio and the masses of the individual components. The distance to BX Mon is estimated with the help of interstellar absorption lines as well as from interstellar extinction. This enables the calculation of stellar parameters like luminosity and radius. We then measure and discuss the rotation of the cool component and present a series of H[FORMULA] line profiles. We finally consider a model in which the variability of the H[FORMULA] line can be understood.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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