JWST Observations of Young protoStars (JOYS): HH211: Textbook case of a protostellar jet and outflow

Journal article, 2024

Context. Due to the high visual extinction and lack of sensitive mid-infrared (MIR) telescopes, the origin and properties of outflows and jets from embedded Class 0 protostars are still poorly constrained. Aims. We aim to characterise the physical, kinematic, and dynamical properties of the HH 211 jet and outflow, one of the youngest protostellar flows. Methods. We used the James Webb Space Telescope (JWST) and its Mid-InfraRed Instrument (MIRI) in the 5-28 μm range to study the embedded HH 211 flow. We mapped a 0′.95 × 0′.22 region, covering the full extent of the blueshifted lobe, the central protostellar region, and a small portion of the redshifted lobe. We extracted spectra along the jet and outflow and constructed line and excitation maps of both atomic and molecular lines. Additional JWST NIRCam H2 narrow-band images (at 2.122 and 3.235 μm) provide a visualextinction map of the whole flow, and are used to deredden our data. Results. The jet-driving source is not detected even at the longest MIR wavelengths. The overall morphology of the flow consists of a highly collimated jet, which is mostly molecular (H2, HD) with an inner atomic ([Fe I], [Fe II], [S I], [Ni II]) structure. The jet shocks the ambient medium, producing several large bow shocks (BSs) that are rich in forbidden atomic ([Fe II], [S I], [Ni II], [Cl I], [Cl II], [Ar II], [Co II], [Ne II], [S III]) and molecular lines (H2, HD, CO, OH, H2O, CO2, HCO+), and is driving an H2 molecular outflow that is mostly traced by low- J, v = 0 transitions. Moreover, H2 0-0 S(1) uncollimated emission is also detected down to 2″-3″ (~650-1000 au) from the source, tracing a cold (T=200-400 K), less dense, and poorly collimated molecular wind. Two H2 components (warm, T =300-1000 K, and hot, T =1000-3500 K) are detected along the jet and outflow. The atomic jet ([Fe II] at 26 μm) is detected down to ~130 au from the source, whereas the lack of H2 emission (at 17 μm) close to the source is likely due to the large visual extinction (AV > 80 mag). Dust-continuum emission is detected at the terminal BSs and in the blue- and redshifted jet, and is likely attributable to dust lifted from the disc. Conclusions. The jet shows an onion-like structure, with layers of different size, velocity, temperature, and chemical composition. Moreover, moving from the inner jet to the outer BSs, different physical, kinematic, and excitation conditions for both molecular and atomic gas are observed. The mass-flux rate and momentum of the jet, as well as the momentum flux of the warm H2 component, are up to one order of magnitude higher than those inferred from the atomic jet component. Our findings indicate that the warm H2 red component is the main driver of the outflow, that is to say it is the most significant dynamical component of the jet, in contrast to jets from more evolved YSOs, where the atomic component is dominant.