Irradiated Herbig-Haro Jets in the Orion Nebula and Near NGC 1333
  We report the discovery of a dozen Herbig-Haro jets illuminated by the Lyman continuum and/or softer far-ultraviolet radiation fields of nearby high mass stars. Five irradiated outflows lie in the outer parts of the Orion Nebula and seven lie near the reflection nebula NGC 1333 in the Perseus molecular cloud. We propose that the UV radiation field has eroded residual material left over from the formation of these young source stars. Many of the irradiated jets exhibit unusual C-shaped bending. In the outskirts of the Orion Nebula, most irradiated jets bend away from the core of the nebula. On the other hand, in NGC 1333, the C-shaped jets bend back towards the cluster center. Jet bending in the Orion Nebula may be dominated by either the outflow of material from the nebular core or by the rocket effect pushing on the irradiated portion of a mostly neutral jet beam. But in NGC 1333, jet bending may indicate that the source stars have been ejected from the cluster core. Many irradiated jets are asymmetric with one beam much brighter than the other. When fully photo-ionized, irradiated jets may provide unique insights into the physical conditions within outflows powered by young stars, permitting the determination of the density and location of stellar ejecta even in the absence of shocks. We present a model for the photo-ionization of these outflows by external radiation fields and discuss possible mechanisms for producing the observed asymmetries. We demonstrate that the UV radiation field may alter the amount of cloud material entrained by the jet. We also report the discovery of some large scale bow shocks which face the core of the Orion Nebula and which surround visible young stars. These wind-wind collision fronts provide further evidence for a large scale mass flow originating near the nebular core. 
John Bally, and Bo Reipurth