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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. We aim to characterise the physical, kinematic, and dynamical properties of the HH\,211 jet and outflow, one of the youngest protostellar flows. We used the James Webb Space Telescope (JWST) and its Mid-InfraRed Instrument (MIRI) in the 5--28 mu m range to study the embedded HH\,211 flow. We mapped a 0farcm 95times 0farcm 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 H$_2$ narrow-band images (at 2.122 and 3.235\,mu m) provide a visual-extinction map of the whole flow, and are used to deredden our data. 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 (H$_2$, HD) with an inner atomic ( structure. The jet shocks the ambient medium, producing several large bow shocks (BSs) that are rich in forbidden atomic ( and molecular lines (H$_2$, HD, CO, OH, H$_2$O, CO$_2$, HCO$^+$), and is driving an H$_2$ molecular outflow that is mostly traced by low-$J$, $v=0$ transitions. Moreover, H$_2$ 0-0\,S(1) uncollimated emission is also detected down to 2arcsec --3arcsec (sim 650--1000\,au) from the source, tracing a cold ($T$=200--400\,K), less dense, and poorly collimated molecular wind. Two H$_2$ components (warm, $T$=300--1000\,K, and hot, $T$=1000--3500\,K) are detected along the jet and outflow. The atomic jet ( at 26\,mu m) is detected down to sim 130\,au from the source, whereas the lack of H$_2$ emission (at 17\,mu m) close to the source is likely due to the large visual extinction ( 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. 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 H$_2$ component, are up to one order of magnitude higher than those inferred from the atomic jet component. Our findings indicate that the warm H$_2$ 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.
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. We aim to characterise the physical, kinematic, and dynamical properties of the HH\,211 jet and outflow, one of the youngest protostellar flows. We used the James Webb Space Telescope (JWST) and its Mid-InfraRed Instrument (MIRI) in the 5--28 mu m range to study the embedded HH\,211 flow. We mapped a 0farcm 95times 0farcm 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 H$_2$ narrow-band images (at 2.122 and 3.235\,mu m) provide a visual-extinction map of the whole flow, and are used to deredden our data. 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 (H$_2$, HD) with an inner atomic ( structure. The jet shocks the ambient medium, producing several large bow shocks (BSs) that are rich in forbidden atomic ( and molecular lines (H$_2$, HD, CO, OH, H$_2$O, CO$_2$, HCO$^+$), and is driving an H$_2$ molecular outflow that is mostly traced by low-$J$, $v=0$ transitions. Moreover, H$_2$ 0-0\,S(1) uncollimated emission is also detected down to 2arcsec --3arcsec (sim 650--1000\,au) from the source, tracing a cold ($T$=200--400\,K), less dense, and poorly collimated molecular wind. Two H$_2$ components (warm, $T$=300--1000\,K, and hot, $T$=1000--3500\,K) are detected along the jet and outflow. The atomic jet ( at 26\,mu m) is detected down to sim 130\,au from the source, whereas the lack of H$_2$ emission (at 17\,mu m) close to the source is likely due to the large visual extinction ( 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. 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 H$_2$ component, are up to one order of magnitude higher than those inferred from the atomic jet component. Our findings indicate that the warm H$_2$ 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.
The Mid-InfraRed Instrument (MIRI) on board the James Webb Space Telescope (JWST) allows one to probe the molecular gas composition at mid-infrared (mid-IR) wavelengths with unprecedented resolution and sensitivity. It is important to study these features in low-mass embedded protostellar systems, since the formation of planets is thought to start in this phase. Previous studies were sensitive primarily to high-mass protostars. The aim of this paper is to derive the physical conditions of all gas-phase molecules detected toward a sample of 18 low-mass protostars as part of the JWST Observations of Young protoStars (JOYS) program and to determine the origin of the molecular emission and absorption features. This includes molecules such as CO$_2$, C$_2$H$_2$, and CH$_4$ that cannot be studied at millimeter wavelengths. We present JWST/MIRI data taken with the Medium Resolution Spectrometer (MRS) of 18 low-mass protostellar systems, focusing on gas-phase molecular lines in spectra extracted from the central protostellar positions. The column densities and excitation temperatures were derived for each molecule using local thermodynamic equilibrium (LTE) slab models. Ratios of the column densities (absorption) or total number of molecules (emission) were taken with respect to H$_2$O in order to compare these to ratios derived in interstellar ices. Continuum emission is detected across the full MIRI-MRS wavelength toward 16/18 sources; the other two sources (NGC 1333 IRAS 4B and Ser-S68N-S) are too embedded to be detected. The MIRI-MRS spectra show a remarkable richness in molecular features across the full wavelength range, in particular toward B1-c (absorption) and L1448-mm (emission). Besides H$_2$, which is not considered here, water is the most commonly detected molecule (12/16) toward the central continuum positions followed by CO$_2$ (11/16), CO (8/16), and OH (7/16). Other molecules such as 13CO$_2$, C$_2$H$_2$, 13CCH$_2$, HCN, C$_4$H$_2$, CH$_4$, and SO$_2$ are detected only toward at most three of the sources, particularly toward B1-c and L1448-mm. The JOYS data also yield the surprising detection of SiO gas toward two sources (BHR71-IRS1, L1448-mm) and for the first time CS and NH$_3$ at mid-IR wavelengths toward a low-mass protostar (B1-c). The temperatures derived for the majority of the molecules are 100--300 K, much lower than what is typically derived toward more evolved Class II sources ($ K). Toward three sources (e.g., TMC1-W), hot ($ K) H$_2$O is detected, indicative of the presence of hot molecular gas in the embedded disks, but such warm emission from other molecules is absent. The agreement in abundance ratios with respect to H$_2$O between ice and gas points toward ice sublimation in a hot core for a few sources (e.g., B1-c), whereas their disagreement and velocity offsets hint at high-temperature (shocked) conditions toward other sources (e.g., L1448-mm, BHR71-IRS1). Molecular emission and absorption features trace various warm components in young protostellar systems, from the hot core regions to shocks in the outflows and disk winds. The typical temperatures of the gas-phase molecules of $100-300$ K are consistent with both ice sublimation in hot cores as well as high-temperature gas phase chemistry. Molecular features originating from the inner embedded disks are not commonly detected, likely because they are too extincted even at mid-IR wavelengths by small, unsettled dust grains in upper layers of the disk.
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