Visual/infrared interferometry of Orion Trapezium stars: preliminary dynamical orbit and aperture synthesis imaging of the Orionis C system

Context: Located in the Orion Trapezium cluster, ¿^1Ori C is one of the youngest and nearest high-mass stars (O5-O7) known. Besides its unique properties as a magnetic rotator, the system is also known to be a close binary. Aims: By tracing its orbital motion, we aim to determine the orbit and dynamical mass of the system, yielding a characterization of the individual components and, ultimately, also new constraints for stellar evolution models in the high-mass regime.Furthermore, a dynamical parallax can be derived from the orbit,providing an independent estimate for the distance of the Trapezium cluster. Methods: Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we traced the orbital motion of the 1Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit.Besides fitting the relative position and the flux ratio, we applied aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the Ori C binary system. Results: The orbital solutions suggest a highly eccentricity(e0.91) and short-period (P10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. Of these two, the one implying a system mass of 48 M&sun; and a distance of 434pc to the Trapezium cluster can be favored. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for ¿^1Ori C1 (M = 34.0 M&sun;, T_eff =39 900 K) and O9.5 for C2 (M = 15.5 M&sun;, T_eff = 31 900K). Using IOTA, we also obtained first interferometric observations on¿^1Ori D, finding some evidence for a resolved structure, maybeby a faint, close companion. Conclusions: We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (¿1.5 AU, next passage around 2007.5),suggesting strong wind-wind interaction between the two O stars.