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 30 00 L-.. The I-II-I 396/397 complex consists of a network of filamentary emi ssion-line objects tracing a pair of collimated and limb-brightened outflow lobes that emerge from an opaque cloud core. The H alpha- and [S II]-brigh t nebulosity has a large area covering factor, extending well beyond the ma pped extent of the associated high-velocity molecular outflow. The HH 396/3 97 complex contains some shocks with a surface brightness comparable to III -I 168 in Cepheus A, placing these among the brightest known HH objects. Fu rthermore, a large fraction of a 1' by 18' region surrounding G192.16-3.82 is laced with lower surface brightness emission-line features, making I-II- I 396/397 one of the most spatially extended HH complexes studied so far. T he dynamical age of this outflow is in the range 10(4) to 10(5) yr. We disc uss plausible evolutionary scenarios for outflows powered by high-mass prot ostars and argue that the G192.16-3.82 outflow complex is relatively evolve d. Despite being very elongated, this HH flow is most likely powered by a m oderately collimated wind, rather than a highly collimated jet.