Abstract

Context. Located in the Orion Trapezium cluster,theta(1)OriC 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 theta(1)OriC 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 theta(1)OriC binary system. Results. The orbital solutions suggest a highly eccentricity (e approximate to 0.91) and short-period (P approximate to 10.9 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-circle dot and a distance of 434 pc 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 theta(1)Ori C1 (M = 34.0 M-circle dot, T-eff = 39 900 K) and O9.5 for C2 (M = 15.5 M-circle dot, T-eff = 31 900 K). Using IOTA, we also obtained first interferometric observations on.1Ori D, finding some evidence for a resolved structure, maybe by 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 (similar to 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.