HH 666: different kinematics from H α and [Fe ii] emission provide a missing link between jets and outflows

HH 666 is an externally irradiated protostellar outflow in the Carina nebula for which we present new near-IR [Fe ii] spectra obtained with the Folded-Port Infrared Echellette spectrograph at Magellan Observatory. Earlier H α and near-IR [Fe ii] imaging revealed that the two emission lines trace substantially different morphologies in the inner ∼40 arcsec of the outflow. H α traces a broad cocoon that surrounds the collimated [Fe ii] jet that extends throughout the parent dust pillar. New spectra show that this discrepancy extends to their kinematics. Near-IR [Fe ii] emission traces steady, fast velocities of ±200 km s⁻¹ from the eastern and western limbs of the jet. We compare this to a previously published H α spectrum that reveals a Hubble-flow velocity structure near the jet-driving source. New, second-epoch Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) H α images reveal the lateral spreading of the H α outflow lobe away from the jet axis. H α proper motions also indicate a sudden increase in the mass-loss rate ∼1000 yr ago, while steady [Fe ii] emission throughout the inner jet suggest that the burst is ongoing. An accretion burst sustained for ∼1000 yr is an order of magnitude longer than expected for FU Orionis outbursts, but represents only a small fraction of the total age of the HH 666 outflow. Altogether, available data suggests that [Fe ii] traces the highly collimated protostellar jet while H α traces the entrained and irradiated outflow. HH 666 appears to be a missing link between bare jets seen in H ii regions and entrained molecular outflows seen from embedded protostars in more quiescent regions.