Milliarcsecond resolution Very Long Baseline Array ( VLBA ) observations of the archetype WR+O star colliding-wind binary ( CWB ) system WR 140 have been obtained at 23 epochs between orbital phases 0.74 to 0.97 .
The emission in the wind-collision region ( WCR ) is resolved as a bow-shaped arc which rotates as the orbital phase progresses .
This rotation provides for the first time the inclination of the orbit ( 122 \hbox { $ { } ^ { \circ } $ } \pm 5 \hbox { $ { } ^ { \circ } $ } ) , the longitude of the ascending node ( 353 \hbox { $ { } ^ { \circ } $ } \pm 3 \hbox { $ { } ^ { \circ } $ } ) , and the orbit semi-major axis ( 9.0 \pm 0.5 mas ) .
The implied distance is 1.85 \pm 0.16 kpc , which requires the O star to be a supergiant , and leads to a wind-momentum ratio of 0.22 .
Quasi-simultaneous Very Large Array ( VLA ) observations show the synchrotron spectra evolve dramatically through the orbital phases observed , exhibiting both optically thin and optically thick emission .
The optically thin emission maintains a spectral index of -0.5 , as expected from diffusive shock acceleration .