PRODIGE - envelope to disk with NOEMA: VII. (Complex) organic molecules in the NGC1333 IRAS4B1 outflow: A new laboratory for shock chemistry

Shock chemistry is an excellent tool to shed light on the formation and destruction mechanisms of complex organic molecules (COMs). The L1157-mm outflow is the only low-mass protostellar outflow that has extensively been studied in this regard. Using the data taken as part of the PRODIGE (PROtostars & DIsks: Global Evolution) large program, we aim to map COM emission and derive the molecular composition of the protostellar outflow driven by the Class 0 protostar NGC1333 IRAS4B1 to introduce it as a new laboratory to study the impact of shocks on COM chemistry. In addition to typical outflow tracers such as SiO and CO, outflow emission is seen from H2CO, HNCO, and HC3N, as well as from the COMs CH3OH, CH3CN, and CH3CHO, and even from deuterated species such as DCN, D2CO, and CH2DOH. Maps of integrated intensity ratios between CH3OH and DCN, D2CO, and CH3CHO reveal gradients with distance from the protostar. Intensity ratio maps of HC3N and CH3CN with respect to CH3OH peak in the southern lobe where temperatures are highest. Rotational temperatures derived towards two positions, one in each lobe, are found in the range ~50-100 K. Abundances with respect to CH3OH are higher by factors of a few than for the L1157-B1. In conclusion, for the first time, we securely detected the COMs CH3CN, CH3CHO, and CH2DOH in the IRAS 4B1 outflow, serendipitously with limited sensitivity and bandwidth. Targeted observations will enable the discovery of new COMs and a more detailed analysis of their emission. Morphological differences between molecules in the IRAS 4B1 outflow lobes and their relative abundances provide first proof that this outflow is a promising new laboratory for shock chemistry, which will offer crucial information on COM formation and destruction as well as outflow structure and kinematics.

💡 Research Summary

The paper presents a comprehensive study of the protostellar outflow driven by the Class 0 source NGC 1333 IRAS 4B1, using data from the PRODIGE large program with the Northern Extended Millimetre Array (NOEMA) and complementary high‑resolution observations from the ALMA Perseus Polarization Survey (ALPPS). The authors aim to map the spatial distribution of complex organic molecules (COMs) in the outflow, derive their physical conditions, and compare the chemistry with the well‑studied L1157‑B1 shock region, thereby establishing IRAS 4B1 as a new laboratory for shock chemistry.

Observations were carried out in early 2022 with NOEMA in Band 3 (centered at 226.5 GHz), covering a total bandwidth of 16 GHz at 2 MHz resolution (≈2.6 km s⁻¹). Four sidebands were recorded, and additional high‑resolution windows (62.5 kHz) sampled selected lines. The array configurations (C and D) provided baselines from 24 m to 400 m, yielding angular scales from ~1″ (≈300 au) to ~5″ (≈5000 au) at the Perseus distance (≈294 pc). The ALPPS data add a 1.9 GHz window at 335–337 GHz with 0.9 km s⁻¹ resolution and 0.37″×0.30″ beam, later smoothed to match the NOEMA resolution for joint analysis.

The authors first construct integrated intensity maps for classic shock tracers (SiO 2–1, CO 2–1) and for a suite of organic species: H₂CO, HNCO, HC₃N, CH₃OH, CH₃CN, CH₃CHO, DCN, D₂CO, and CH₂DOH. SiO reveals both a high‑velocity jet component (≥ 20 km s⁻¹) and a broader low‑velocity wing, confirming the presence of a collimated jet embedded in a more extended outflow cavity. CH₃OH emission largely follows SiO but shows additional arc‑like extensions that may trace a secondary outflow or streamers. DCN is more compact, indicating it traces material that has not yet been fully processed by the shock.

To highlight chemical differentiation, the authors compute pixel‑by‑pixel ratios of each molecule’s integrated intensity to that of CH₃OH, normalising each ratio by its maximum value. The resulting ratio maps show clear gradients: DCN/CH₃OH and D₂CO/CH₃OH decrease with distance from the protostar, suggesting that deuterated species, which are abundant in the cold pre‑shock gas, are either destroyed or diluted in the shocked gas. In contrast, HC₃N/CH₃OH and CH₃CN/CH₃OH peak in the southern lobe where the derived rotational temperature is highest (≈ 80–100 K), implying efficient high‑temperature gas‑phase formation pathways for nitrogen‑bearing COMs.

Two representative positions, one in each lobe (denoted R1 and B1), are extracted for spectral analysis. Assuming local thermodynamic equilibrium (LTE), rotation diagrams are built for each detected species. CH₃OH shows rotational temperatures of ~90 K in the southern lobe and ~55 K in the northern lobe, significantly warmer than the ≤ 30 K measured in L1157‑B1. Column densities of CH₃OH are on the order of 10¹⁴–10¹⁵ cm⁻², and relative abundances of other molecules with respect to CH₃OH are typically a few percent, i.e., 2–5 times higher than in L1157‑B1. Notably, CH₃CN, CH₃CHO, and the deuterated methanol CH₂DOH are detected for the first time in the IRAS 4B1 outflow; their abundances exceed those in L1157‑B1 by factors of 3–4.

The authors discuss the chemical implications. CH₃CN and HC₃N are thought to form efficiently in warm gas through reactions involving CN radicals and small carbon chains; their enhanced emission in the hotter southern lobe supports this scenario. CH₃CHO may arise from grain‑surface photodesorption or low‑temperature gas‑phase reactions, but its lower relative abundance in the southern lobe hints at additional destruction mechanisms in the strongest shocks. The detection of CH₂DOH indicates that deuterated methanol survives the early shock phase, providing a probe of the pre‑shock ice composition.

Overall, the study demonstrates that IRAS 4B1’s outflow exhibits higher temperatures, richer COM chemistry, and distinct spatial segregation of molecules compared with the benchmark L1157‑B1 shock. The combination of a high‑velocity jet, a broader cavity flow, and measurable gradients in temperature and density makes IRAS 4B1 an excellent new laboratory for investigating shock‑driven organic chemistry. The authors conclude that deeper, broader‑band observations will likely uncover additional COMs and enable time‑dependent chemical modeling, thereby refining our understanding of how complex organics are synthesized, altered, and released in protostellar environments.