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Protostellar jets and outflows are signposts of active star formation. In HII regions, molecular tracers like CO only reveal embedded portions of theoutflow. Outside the natal cloud, outflows are dissociated, ionized, andeventually completely ablated, leaving behind only the high-density jet core.Before this process is complete, there should be a phase where the outflow ispartially molecular and partially ionized. In this paper, we capture the HH 900outflow while this process is in action. New observations from theERIS/SPIFFIER near-IR integral field unit (IFU) spectrograph using the K-middlefilter ($\lambda$=2.06-2.34 $\mu$m) reveal H$_2$ emission from the dissociatingoutflow and Br-$\gamma$ tracing its ionized skin. Both lines trace thewide-angle outflow morphology but H$_2$ only extends $\sim$5000 au into the HII region while Br-$\gamma$ extends the full length of the outflow($\sim$12,650 au), indicating rapid dissociation of the molecules. H$_2$ hashigher velocities further from the driving source, consistent with a jet-drivenoutflow. Diagnostic line ratios indicate that photoexcitation, not just shocks,contributes to the excitation in the outflow. We argue that HH 900 is the firstclear example of an evaporating molecular outflow and predict that a largecolumn of neutral material that may be detectable with ALMA accompanies thedissociating molecules. Results from this study will help guide theinterpretation of near-IR images of externally irradiated jets and outflowssuch as those obtained with the James Webb Space Telescope (JWST) in high-massstar-forming regions where these conditions may be common.

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