Irradiated Herbig‐Haro Jets in the Orion Nebula and near NGC 1333

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Abstract
We report the discovery of a dozen Herbig-Haro jets illuminated by the Lyman continuum (λ < 912 Å) and/or softer far-ultraviolet (912 < λ < 2000 Å) radiation fields of nearby high-mass stars. Five irradiated outflows lie in the outer parts of the Orion Nebula (HH 502-506), and seven lie near the reflection nebula NGC 1333 in the Perseus molecular cloud (HH 333-336 and HH 497-499). These stellar outflows are powered by optically visible low-mass young stars that suffer relatively low extinction 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 formation on a timescale short compared to the ages of these star-forming regions. Many of the irradiated jets exhibit unusual C-shaped symmetry. In the outskirts of the Orion Nebula, most irradiated jets appear to bend away from the core of the nebula. On the other hand, in NGC 1333, the C-shaped jets tend to bend back toward 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 photoionized, 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 photoionization of these outflows by external radiation fields and discuss possible mechanisms for producing the observed asymmetries. In particular, we demonstrate that the UV radiation field may alter the amount of cloud material entrained by the jet. Radiation-induced variations in mass loading and beam heating can produce differences in the beam velocities and spreading rates, which in turn determine the surface brightness 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 beams spread orthogonal to the jet propagation direction with the same speed (e.g., both beams have the same internal sound speed or shocks with similar physical conditions), the faster beam will appear brighter at the same distance from the source. Thus, depending on the parameters, either the faster 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 further evidence for a large-scale mass flow originating near the nebular core.
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