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Orion’s Fingers: New Clarity on an Explosive Outflow

March 12, 2015

A wide-field image showing the OMC1 outflow in H2 with proper motions of the BN object, radio source I, and radio source n superimposed. The lengths of the red-solid vectors are proportional to the motions measured by Gomez et al. (2008) with the the lengths of the vectors in arcseconds equal to the motion in km s-1 (e.g. 1000 corresponds to a motion of 10 km s-1). The ejection center as determined by radio proper motions is shown by a cross.

The Orion Nebula, probably the most well-known deep sky object in the night sky, also offers rare glimpses into catastrophic episodes in the lives of stars. Widespread, high-velocity outflow points to an explosive origin of the region known as the “Orion Fingers.” With new observations using adaptive optics imaging from Gemini South, John Bally of the University of Colorado and colleagues find over 120 high-velocity outflows in this region. Direct comparisons with earlier observations reveal the motion of these fingers. Measurements of their properties, and comparison with simulations, are evidence that an explosive event drives the outflows, which may be connected to the birth of “runaway,” massive stars.


Aims. Adaptive optics images are used to test the hypothesis that the explosive BN/KL outflow from the Orion OMC1 cloud core was powered by the dynamical decay of a non-hierarchical system of massive stars.

Methods. Narrow-band H2, [Fe II], and broad-band Ks obtained with the Gemini South multi-conjugate adaptive optics (AO) system GeMS and near-infrared imager GSAOI are presented. The images reach resolutions of 0.08 to 0.10”, close to the 0.07” diffraction limit of the 8-meter telescope at 2.12µm. Comparison with previous AO-assisted observations of sub-fields and other ground-based observations enable measurements of proper motions and the investigation of morphological changes in H2 and [Fe II] features with unprecedented precision. The images are compared with numerical simulations of compact, high-density clumps moving ~ 103 times their own diameter through a lower density medium at Mach 103.

Results. Several sub-arcsecond H2 features and many [FeII] ‘fingertips’ on the projected outskirts of the flow show proper motions of ~ 300 km/sec. High-velocity, subarcsecond H2 knots (‘bullets’) are seen as far as 140” from their suspected ejection site. If these knots propagated through the dense Orion A cloud, their survival sets a lower bound on their densities of order 107 cm3, consistent with an origin within a few au of a massive star and accelerated by a final multi-body dynamic encounter that ejected the BN object and radio source I from OMC1 about 500 years ago.

Conclusions. Over 120 high-velocity bow-shocks propagating in nearly all directions from the OMC1 cloud core provide evidence for an explosive origin for the BN/KL outflow triggered by the dynamic decay of a non-hierarchical system of massive stars. Such events may be linked to the origin of runaway, massive stars.

The research is accepted for publication in Astronomy and Astrophysics and the preprint can be accessed at