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Ou4 is a recently discovered bipolar outflow with a projected size of morethan one degree in the plane of the sky. It is apparently centred on the youngstellar cluster -whose most massive representative is the triple system HR8119-inside the HII region Sh 2-129. The driving source, the nature, and thedistance of Ou4 are not known. Deep narrow-band imagery of the whole nebula atarcsec resolution was obtained to study its morphology. Long-slit spectroscopyof the tips of the bipolar lobes was secured to determine the gas ionizationmechanism, physical conditions, and line-of-sight velocities. An estimate ofthe proper motions at the tip of the south lobe using archival images isattempted. The existing multi-wavelength data for Sh 2-129 and HR 8119 are alsocomprehensively reviewed. The morphology of Ou4, its emission-line spatialdistribution, line flux ratios, and the kinematic modelling adopting abow-shock parabolic geometry, illustrate the expansion of a shock-excited fastcollimated outflow. The radial velocities and reddening are consistent withthose of Sh 2-129 and HR 8119. The improved determination of the distance toHR8119 (composed of two B0 V and one B0.5 V stars) and Sh 2-129 is 712 pc. Weidentify in WISE images a 5 arcmin-radius (1 pc at the distance above) bubbleof emission at 22 micron emitted by hot (107 K) dust, located inside thecentral part of Ou4 and corresponding to several [O III] features of Ou4. Theapparent position and the properties studied in this work are consistent withthe hypothesis that Ou4 is located inside the Sh 2-129 HII region, suggestingthat it was launched some 90 000 yrs ago by HR8119. The outflow total kineticenergy is estimated to be ~4e47~ergs. However, the alternate possibility thatOu4 is a bipolar planetary nebula, or the result of an eruptive event on amassive AGB or post-AGB star not yet identified, cannot be ruled out.

Gas physical conditions and kinematics of the giant outflow Ou4