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

8. Discussion

Normally, only the cool symbiotic component can be observed with optical or near IR spectroscopy and consequently it is only for the cool component that a radial velocity curve can be established. A full description of the binary orbit requires, however, the radial velocities of both stars. Up to now there was only one symbiotic system, AX Per, for which also the hot components' curve was measured. The measurement were possible because during outburst the hot component became accessible to optical observations (Mikolajewska & Kenyon 1992). BX Mon is the second system in which radial velocities of both objects are determined from photospheric absorptions. The spectrum of the hot component in BX Mon is hard to disentangle from the strong and very rich line spectrum of the M giant. We nevertheless succeeded to clearly detect absorption features of the hot component on two occasions near phase . At that phase, the hot component is located on the observers side and far out in the red giant's wind region (see Fig. 5). Also the maxima in the photographic and visual light curves occur around (see Fig. 1c,d) when the contribution of the hot component is at its highest. Thus it is not surprising that we observed the spectrum of the hot component at this phase.

The photographic and visual light curves of BX Mon are far from the typical regular light curve of a binary system. Obscuration of the hot component's light by the outer wind region of the cool giant seems to be particularly important in this system. Thus, the revolving motion of the hot star passing behind the obscuring red giant's wind could explain the shape and the large scatter in the phased light curves and the changing strength of the maxima which are not strictly periodic.

The light curve is completely different in the far UV ( Å), where the hot component strongly dominates the emission. There we found a deep eclipse by the cool giant in agreement with our radial velocity curve and the improved orbital period of  days. The duration of the eclipse is also in agreement with the red giant's radius of , derived from the spectral type, the apparent magnitude, and the distance. Unfortunately we have no UV observations between phase and 0.60, where the maxima in the visual region tend to occur. The presence of long eclipses is an important finding, because it restricts the orbital inclination to .

From our radial velocity measurements we determined the masses of the stellar components and the orbital configuration. The binary mass-ratio is , which is somewhat higher than the typical symbiotic value of 3-4 according to Mikolajewska (1997) and Schmid (1998). This is due to the relatively high mass of our red giant (). For the hot component we find . With , BX Mon has the highest orbital eccentricity measured in a symbiotic system. The separation between the two components varies between 2.0 and 5.9 AU. At periastron the red giant radius is about 0.6 of the critical Roche radius.

The mass of is an important parameter for clarifying the nature of the hot component in BX Mon. It excludes a main sequence or giant A-F star. The A-F spectrum could be produced by an accretion disk around a low mass main sequence star. However, to power the observed luminosity of the hot component () such a model requires for a detached system according to Viotti et al. (1986) an uncomfortably high mass accretion rate of . The alternative explanation for the hot component is a degenerate (white) dwarf with a hydrogen burning shell. During weak shell flashes, or in a steady state regime where the accreted material is immediately consumed, such a star can reach quite a large radius and a low surface temperature, mimicking an A-F spectrum. Yet, the expected "plateau" luminosity of such an object is (Iben & Tutukov 1996), and thus significantly higher than in BX Mon. This, however, is not a fundamental problem, as often in edge-on interacting binaries a luminous compact component is partly or fully hidden by an obscuring material in the orbital plane. We have to admit however, that this argumentation is a easy way out of the hot component's luminosity problem. Further information is needed to establish the exact nature of the hot star in BX Mon.

Advocating that the hot component is a degenerate dwarf, implies that it was initially the primary in the system and thus more massive than . It therefore appears that even the more massive progenitors among the known symbiotic systems produce white dwarfs with masses around the canonical value of . It is unlikely that objects of such low mass can accrete sufficient material from their present red giant companion to reach the Chandrasekhar limit. This disqualifies symbiotic systems in general as candidates for being progenitors of supernova Type Ia.

BX Mon has a high eccentricity of and a long orbital period of  days. All other symbiotic systems with well established radial velocity curves also have low eccentricities (). These systems have, with the exception of CH Cyg and CD, shorter orbital periods of  days. The eccentricity of the orbit for BX Mon is consistent with the circularization time scales of Verbunt & Phinney (1995). Maximum tidal force was excerted while the primary was on the AGB, which would have circularised orbits with periods shorter than 1200 days. This value is a maximum and applies to a primary with initially . It would be smaller for more massive stars. Thus theory predicts that the binary separation in BX Mon is large enough to escape circularization.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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