MERLIN measurements of 1.6-GHz OH masers associated with Orion-BN/KL are presented , and the data are compared with data on other masers , molecular lines , compact radio continuum sources and infrared sources in the region .
OH masers are detected over an area 30 arcsec in diameter , with the majority lying along an approximately E-W structure that extends for \sim 18 arcsec , encompassing the infrared sources IRc2 , IRc6 and IRc7 .
Radial velocities range from –13 to +42 km s ^ { -1 } .
The system of OH masers shows a velocity gradient together with non-circular motions .
The kinematics are modelled in terms of an expanding and rotating disc or torus .
The rotation axis is found to be in the same direction as the molecular outflow .
There is an inner cavity of radius \sim 1300 au with no OH masers .
The inner cavity , like the H _ { 2 } O ‘ shell ’ masers and SiO masers , is centred on radio source I .
Some of the OH masers occur in velocity-coherent strings or arcs that are longer than 5 arcsec ( 2250 au ) .
One such feature , Stream A , is a linear structure at position angle \sim 45° , lying between IRc2 and BN .
We suggest that these masers trace shock fronts , and have appeared , like a vapour trail , 200 yr after the passage of the runaway star BN .
The radio proper motions of BN , source I and source n project back to a region near the base of Stream A that is largely devoid of OH masers .
The 1612-MHz masers are kinematically distinct from the other OH masers .
They are also more widely distributed and appear to be associated with the outflow as traced by H _ { 2 } O masers and by the 2.12- \mu m emission from shocked H _ { 2 } .
The magnetic field traced by the OH masers ranges from 1.8 to 16.3 mG , with a possible reversal .
No OH masers were found associated with even the most prominent proplyds within 10 arcsec of \theta ^ { 1 } Ori C .