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Issue A&A
Volume 466, Number 2, May I 2007
Page(s) 649 - 659
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20066965



A&A 466, 649-659 (2007)
DOI: 10.1051/0004-6361:20066965

Visual/infrared interferometry of Orion Trapezium stars: preliminary dynamical orbit and aperture synthesis imaging of the $\mathsf{\theta^1}$ Orionis C system

S. Kraus1, Y. Y. Balega2, J.-P. Berger3, K.-H. Hofmann1, R. Millan-Gabet4, J. D. Monnier5, K. Ohnaka1, E. Pedretti5, Th. Preibisch1, D. Schertl1, F. P. Schloerb6, W. A. Traub7, and G. Weigelt1

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: skraus@mpifr-bonn.mpg.de
2  Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Zelenchuk region, Karachai-Cherkesia 357147, Russia
3  Laboratoire d'Astrophysique de Grenoble, UMR 5571 Université Joseph Fourier/CNRS, BP 53, 38041 Grenoble Cedex 9, France
4  Michelson Science Center, California Institute of Technology, Pasadena, CA 91125, USA
5  Astronomy Department, University of Michigan, 500 Church Street, Ann Arbor, MI 48104, USA
6  Department of Astronomy, University of Massachusetts, LGRT-B 619E, 710 North Pleasant Street, Amherst, MA 01003, USA
7  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02183, USA

(Received 19 December 2006 / Accepted 7 February 2007)

Abstract
Context.Located in the Orion Trapezium cluster, $\theta^1$Ori 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 $\theta^1$Ori 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 $\theta^1$Ori C binary system.
Results.The orbital solutions suggest a highly eccentricity ($e\approx$0.91) and short-period ($P\approx$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 $M_{\odot}$ 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 $M_{\odot}$, $T_{\rm eff}$ = 39 900 K) and O9.5 for C2 (M = 15.5 $M_{\odot}$, $T_{\rm eff}$ = 31 900 K). Using IOTA, we also obtained first interferometric observations on $\theta^1$Ori 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 (${\sim}$1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.


Key words: stars: pre-main sequence -- stars: fundamental parameters -- stars: binaries: close -- techniques: interferometric -- stars: individual: $\theta^1$ Orionis C, $\theta^1$ Orionis D



© ESO 2007

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