Gas physical conditions and kinematic of the giant outflow Ou4

Romano L.M. Corradi (1,2), Nicolas Grosso (3), Agnes Acker(3), Robert Greimel (4), Patrick Guillout (3)

(1) Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Spain
(2) Departamento de Astrofisica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
(3) Observatoire Astronomique de Strasbourg, Universite de Strasbourg, CNRS, UMR 7550, 11 rue de l'Universite, 67000 Strasbourg, France
(4) IGAM, Institut fur Physik, Universitat Graz, Universitatsplatz 5/II, A-8010 Graz, Austria

Ou4 is a recently discovered bipolar outflow with a projected size of more than one degree in the plane of the sky. It is apparently centred on the young stellar cluster -whose most massive representative is the triple system HR8119- inside the HII region Sh 2-129. The driving source, the nature, and the distance of Ou4 are not known. Deep narrow-band imagery of the whole nebula at arcsec resolution was obtained to study its morphology. Long-slit spectroscopy of the tips of the bipolar lobes was secured to determine the gas ionization mechanism, physical conditions, and line-of-sight velocities. An estimate of the proper motions at the tip of the south lobe using archival images is attempted. The existing multi-wavelength data for Sh 2-129 and HR 8119 are also comprehensively reviewed. The morphology of Ou4, its emission-line spatial distribution, line flux ratios, and the kinematic modelling adopting a bow-shock parabolic geometry, illustrate the expansion of a shock-excited fast collimated outflow. The radial velocities and reddening are consistent with those of Sh 2-129 and HR 8119. The improved determination of the distance to HR8119 (composed of two B0 V and one B0.5 V stars) and Sh 2-129 is 712 pc. We identify in WISE images a 5 arcmin-radius (1 pc at the distance above) bubble of emission at 22 micron emitted by hot (107 K) dust, located inside the central part of Ou4 and corresponding to several [O III] emission features of Ou4. The apparent position of Ou4 and the properties studied in this work are consistent with the hypothesis that Ou4 is located inside the Sh 2-129 HII region, suggesting that it was launched some 90 000 yrs ago by HR8119. The outflow total kinetic energy is estimated to be ~4e47~ergs. However, the alternate possibility that Ou4 is a bipolar planetary nebula, or the result of an eruptive event on a massive AGB or post-AGB star not yet identified, cannot be ruled out.

Reference: Astronomy and Astrophysics, in press
Status: Manuscript has been accepted