A Giant Herbig-Haro Flow from a Massive Young Star in G192.16−3.82

We report the discovery of a 10 pc–long Herbig-Haro (HH) flow powered by a moderately massive young star associated with the compact H II region G192.16-3.82. At a distance of 2 kpc, the luminosity of G192.16-3.82 is about 3000 L ⊙ . The HH 396/397 complex consists of a network of filamentary emission-line objects tracing a pair of collimated and limb-brightened outflow lobes that emerge from an opaque cloud core. The Hα- and [S II]-bright nebulosity has a large area covering factor, extending well beyond the mapped extent of the associated high-velocity molecular outflow. The HH 396/397 complex contains some shocks with a surface brightness comparable to HH 168 in Cepheus A, placing these among the brightest known HH objects. Furthermore, a large fraction of a 1' by 18' region surrounding G192.16-3.82 is laced with lower surface brightness emission-line features, making HH 396/397 one of the most spatially extended HH complexes studied so far. The dynamical age of this outflow is in the range 10 4 to 10 5 yr. We discuss plausible evolutionary scenarios for outflows powered by high-mass protostars and argue that the G192.16-3.82 outflow complex is relatively evolved. Despite being very elongated, this HH flow is most likely powered by a moderately collimated wind, rather than a highly collimated jet.