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 Lyma n continuum (lambda < 912 <Angstrom>) and/or softer far-ultraviolet (912 < <lambda> < 2000 <Angstrom>) radiation fields of nearby high-mass stars. Fiv e irradiated outflows lie in the outer parts of the Orion Nebula (HH 502-50 6), and seven lie near the reflection nebula NGC 1333 in the Perseus molecu lar cloud (HH 333-336 and HH 497-499). These stellar outflows are powered b y optically visible low-mass young stars that suffer relatively low extinct ion and seem not to be embedded within opaque cloud cores. We propose that the UV radiation field has eroded residual material left over from their fo rmation on a timescale short compared to the ages of these star-forming reg ions. Many of the irradiated jets exhibit unusual C-shaped symmetry. In the outskirts of the Orion Nebula, most irradiated jets appear to bend away fr om the core of the nebula. On the other hand, in NGC 1333, the C-shaped jet s tend to bend back toward the cluster center. Jet bending in the Orion Neb ula may be dominated by either the outflow of material from the nebular cor e or by the rocket effect pushing on the irradiated portion of a mostly neu tral jet beam. But in NGC 1333, jet bending may indicate that the source st ars have been ejected from the cluster core. Many irradiated jets are asymm etric with one beam much brighter than the other. When fully photoionized, irradiated jets may provide unique insights into the physical conditions wi thin outflows powered by young stars, permitting the determination of the d ensity and location of stellar ejecta even in the absence of shocks. We pre sent a model for the photoionization of these outflows by external radiatio n fields and discuss possible mechanisms for producing the observed asymmet ries. In particular, we demonstrate that the UV radiation field may alter t he amount of cloud material entrained by the jet. Radiation-induced variati ons in mass loading and beam heating can produce differences in the beam ve locities and spreading rates, which in turn determine the surface brightnes s of the radiating plasma. In a bipolar irradiated jet in which both beams have the same mass-loss rate and opening angle, the slower beam will appear brighter at a given distance from the source. On the other hand, if both b eams spread orthogonal to the jet propagation direction with the same speed (e.g., both beams have the same internal sound speed or shocks with simila r physical conditions), the faster beam will appear brighter at the same di stance from the source. Thus, depending on the parameters, either the faste r or slower beam of a jet can be brighter. Finally, we report the discovery of some large-scale bow shocks that face the core of the Orion Nebula and surround visible young stars. These wind-wind collision fronts provide furt her evidence for a large-scale mass flow originating near the nebular core.