Milli-arcsecond resolution Very Long Baseline Array ( VLBA ) observations of the archetype WR+O star colliding-wind binary ( CWB ) system WR 140 are presented for 23 epochs between orbital phases 0.74 and 0.97 .
At 8.4 GHz , the emission in the wind-collision region ( WCR ) is clearly resolved as a bow-shaped arc that rotates as the orbit progresses .
We interpret this rotation as due to the O star moving from SE to approximately E of the WR star , which leads to solutions for the orbit inclination of 122 ^ { \circ } \pm 5 ^ { \circ } , the longitude of the ascending node of 353 ^ { \circ } \pm 3 ^ { \circ } , and an orbit semi-major axis of 9.0 \pm 0.5 mas .
The distance to WR 140 is determined to be 1.85 \pm 0.16 kpc , which requires the O star to be a supergiant .
The inclination implies the mass of the WR and O star to be 20 \pm 4 M _ { \odot } and 54 \pm 10 M _ { \odot } respectively .
We determine a wind-momentum ratio of 0.22 , with an expected half-opening angle for the WCR of 63 ^ { \circ } , consistent with 65 ^ { \circ } \pm 10 ^ { \circ } derived from the VLBA observations .
Total flux measurements from Very Large Array ( VLA ) observations show the radio emission from WR 140 is very closely the same from one orbit to the next , pointing strongly toward emission , absorption and cooling mechanism ( s ) that are controlled largely by the orbital motion .
The synchrotron spectra evolve dramatically through the orbital phases observed , exhibiting both optically thin and optically thick emission .
We discuss a number of absorption and cooling mechanisms that may determine the evolution of the synchrotron spectrum with orbital phase .