Illuminating evaporating protostellar outflows: ERIS/SPIFFIER reveals the dissociation and ionization of HH 900

Protostellar jets and outflows are signposts of active star formation. In H II regions, molecular tracers like CO only reveal embedded portions of the outflow. Outside the natal cloud, outflows are dissociated, ionized, and eventually completely ablated, leaving behind only the high-density jet core. Before this process is complete, there should be a phase where the outflow is partially molecular and partially ionized. In this paper, we capture the HH 900 outflow while this process is in action. New observations from the Enhanced Resolution Imager and Spectrograph/SPIFFIER near-infrared (IR) integral field unit spectrograph using the K-middle filter (λ = 2.06-2.34 μm) reveal H₂ emission from the dissociating outflow and Br-γ tracing its ionized skin. Both lines trace the wide-angle outflow morphology but H₂ only extends ~5000 au into the H II region while Br-γ extends the full length of the outflow (~12 650 au), indicating rapid dissociation of the molecules. H₂ has higher velocities further from the driving source, consistent with a jet-driven outflow. Diagnostic line ratios indicate that photoexcitation, not just shocks, contributes to the excitation in the outflow. We argue that HH 900 is the first clear example of an evaporating molecular outflow and predict that a large column of neutral material that may be detectable with Atacama Large Millimeter Array accompanies the dissociating molecules. Results from this study will help guide the interpretation of near-IR images of externally irradiated jets and outflows such as those obtained with the JWST in high-mass star-forming regions where these conditions may be common.