Position-velocity (PV) diagrams of the six lowest NH₃ lines as observed with the VLA cutting along the axis of symmetry of OH 231.8+4.2. Vertical axes represent offsets (arcsec) in the north-east direction at the position angle of 21° and horizontal axes represent the LSR velocity ( km s−1). In each PV diagram, the global velocity gradient of ∇V = 6.5 km s−1 arcsec−1 is indicated by the straight line passing through the continuum centre (i.e. zero offset) and the systemic LSR velocity (34 km s −1) of OH 231.8+4.2.