Publications
of the
MPIfR Optical & Infrared Interferometry Group

T. Preibisch, Y.Y. Balega, D. Schertl, G. Weigelt

The massive multiple stars in the Orion trapezium: orbital motion, physical properties, and implications on star formation scenarios

In: Schielikke, R.E. (ed.): Short Contrib. Ann. Sci. Meeting Astron. Ges. Freiburg 2003. Astron. Nachr. 324, Suppl. Issue 3 (2003), p.41
The Sun and Planetary Systems - Paradigms for the Universe
Annual Scientific Meeting of the Astronomische Gesellschaft, Sep 15-20, 2003, Freiburg, Germany

Abstract

We present bispectrum speckle interferometry of the multiple Orion Trapezium stars theta1 Ori A, theta 1 Ori B, and theta 1 Ori C obtained with the SAO 6m telescope in Russia over a period of 5.5 years (epochs 1995 - 2001). Our diffraction-limited images have a resolution lambda/D of 42 mas (J-band), 57 mas (H-band) and 76 mas (K-band). We clearly detect motion of the companions relative to their primary stars in the systems theta 1 Ori A1-2 (mean separation rho ~ 220 mas, change in position angle Delta P.A. = 6 deg), theta 1 Ori B2-3 (rho ~ 205 mas, Delta P.A. = 8 deg), and theta 1 Ori C1-2 (rho ~ 37 mas, Delta P.A. = 18 deg). In our K-band image of theta 1 Ori B we resolve a fourth visual component, confirming its discovery by Simon et al. (1999). A recent spectroscopic study suggested that theta1 Ori C is a spectroscopic binary. The visual companion detected in our speckle images is a very good candidate for the spectroscopic companion, if a strongly elliptical orbit is assumed, what is consistent with our data.

We determine the J, H, and K magnitudes of the system components and estimate the stellar masses of the companions in the HR-diagram. The companions theta 1 Ori C2 and theta 1 Ori B2 show clear evidence of near-infrared excess in the color-color diagram. The companions theta 1 Ori A2 and theta 1 Ori B3 show much stronger extinction than their primary stars, providing evidence of the presence of circumstellar material around the companions. This is quite interesting, considering the environment of these objects with strong UV radiation and winds form the massive stars, most notably theta 1 Ori C. Under these conditions, the typical amount of circumstellar material around T Tauri stars should be dispersed very quickly, on timescales of at most ~ 10 000 years. One possible explanation is the hypothesis of a very recent birth for theta 1 Ori C.

We also briefly discuss implications of the high observed multiplicity of the Trapezium stars. Considering both, the visual and the spectroscopic companions of the 4 Trapezium stars, there are at least 7 companions, i.e. at least~1.75 companions per primary on average. This number is clearly higher than that found for the low-mass stars in the Orion Nebula cluster as well as in the field population. This may suggest that a different mechanism is at work in the formation of high-mass multiple systems in the dense Trapezium cluster than for low-mass stars.

We discuss the implications of this result on the formation scenarios for high-mass stars.