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1. Chemical Differentiation around Five Massive Protostars Revealed by ALMA: Carbon-chain Species and Oxygen/Nitrogen-bearing Complex Organic Molecules

   https://doi.org/10.3847/1538-4365/acd110  

   Taniguchi, Kotomi; Majumdar, Liton; Caselli, Paola; Takakuwa, Shigehisa; Hsieh, Tien-Hao; Saito, Masao; Li, Zhi-Yun; Dobashi, Kazuhito; Shimoikura, Tomomi; Nakamura, Fumitaka; et al (June 2023, The Astrophysical Journal Supplement Series)

   Abstract We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC 5 N ( J = 35–34) line has been detected from three MYSOs, where nitrogen (N)-bearing COMs (CH 2 CHCN and CH 3 CH 2 CN) have been detected. The HC 5 N spatial distributions show compact features and match with a methanol (CH 3 OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC 5 N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC 5 N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs. 

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2. PRODIGE – envelope to disk with NOEMA: III. The origin of complex organic molecule emission in SVS13A

   https://doi.org/10.1051/0004-6361/202449417  

   Hsieh, T-H; Pineda, J E; Segura-Cox, D M; Caselli, P; Valdivia-Mena, M T; Gieser, C; Maureira, M J; Lopez-Sepulcre, A; Bouscasse, L; Neri, R; et al (June 2024, Astronomy & Astrophysics)

   Context.Complex organic molecules (COMs) have been found toward low-mass protostars, but the origins of the COM emission are still unclear. It can be associated with, for example, hot corinos, outflows, and/or accretion shock and disk atmospheres. Aims.We aim to disentangle the origin of the COM emission toward the chemically rich protobinary system SVS13A using six O-bearing COMs. Methods.We conducted NOrthern Extended Millimeter Array observations toward SVS13A as part of the PROtostars & DIsks: Global Evolution (PRODIGE) program. Our previous DCN observations reveal a possible infalling streamer, which may affect the chemistry of the central protobinary by inducing accretion outbursts and/or shocked gas. We further analyzed six O-bearing COMs: CH₃OH, aGg’- (CH₂OH)₂, C₂H₅OH, CH₂(OH)CHO, CH₃CHO, and CH₃OCHO. Although the COM emission is not spatially resolved, we constrained the source sizes to ≲0.3–0.4 arcsec (90–120 au) by conducting uv-domain Gaussian fitting. Interestingly, the high-spectral-resolution data reveal complex line profiles with multiple peaks; although the line emission is likely dominated by the secondary, VLA4A, atV_LSR= 7.36 km s⁻¹, the numbers of peaks (~2–5), the velocities, and the linewidths of these six O-bearing COMs are different. The local thermodynamic equilibrium (LTE) fitting unveils differences in excitation temperatures and emitting areas among these COMs. We further conducted multiple-velocity-component LTE fitting to decompose the line emission into different kinematic components. As a result, the emission of these COMs is decomposed into up to six velocity components from the LTE modeling. The physical conditions (temperature, column density, and source size) of these components from each COM are obtained, and Markov chain Monte Carlo sampling was performed to test the fitting results. Results.We find a variety in excitation temperatures (100–500 K) and source sizes (D~ 10–70 au) from these kinematic components from different COMs. The emission of each COM can trace several components, and different COMs most likely trace different regions. Conclusions.Given this complex structure, we suggest that the central region is inhomogeneous and unlikely to be heated by only protostellar radiation. We conclude that accretion shocks induced by the large-scale infalling streamer likely exist and contribute to the complexity of the COM emission. This underlines the importance of high-spectral-resolution data when analyzing COM emission in protostars and deriving relative COM abundances. 

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3. A deep search for large complex organic species toward IRAS16293-2422 B at 3 mm with ALMA

   https://doi.org/10.1051/0004-6361/202347832  

   Nazari, P; Cheung, J_S Y; Asensio, J Ferrer; Murillo, N M; van_Dishoeck, E F; Jørgensen, J K; Bourke, T L; Chuang, K-J; Drozdovskaya, M N; Fedoseev, G; et al (June 2024, Astronomy & Astrophysics)

   Context.Complex organic molecules (COMs) have been detected ubiquitously in protostellar systems. However, at shorter wavelengths (~0.8 mm), it is generally more difficult to detect larger molecules than at longer wavelengths (~3 mm) because of the increase in millimeter dust opacity, line confusion, and unfavorable partition function. Aims.We aim to search for large molecules (more than eight atoms) in the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 spectrum of IRAS 16293-2422 B. In particular, the goal is to quantify the usability of ALMA Band 3 for molecular line surveys in comparison to similar studies at shorter wavelengths. Methods.We used deep ALMA Band 3 observations of IRAS 16293-2422 B to search for more than 70 molecules and identified as many lines as possible in the spectrum. The spectral settings were set to specifically target three-carbon species such as i- and n-propanol and glycerol, the next step after glycolaldehyde and ethylene glycol in the hydrogenation of CO. We then derived the column densities and excitation temperatures of the detected species and compared the ratios with respect to methanol between Band 3 (~3 mm) and Band 7 (~1 mm, Protostellar Interferometric Line Survey) observations of this source to examine the effect of the dust optical depth. Results.We identified lines of 31 molecules including many oxygen-bearing COMs such as CH₃OH, CH₂OHCHO, CH₃CH₂OH, and c-C₂H₄O and a few nitrogen- and sulfur-bearing ones such as HOCH₂CN and CH₃SH. The largest detected molecules are gGg-(CH₂OH)₂and CH₃COCH₃. We did not detect glycerol or i- and n-propanol, but we do provide upper limits for them which are in line with previous laboratory and observational studies. The line density in Band 3 is only ~2.5 times lower in frequency space than in Band 7. From the detected lines in Band 3 at a ≳ 6σ level, ~25–30% of them could not be identified indicating the need for more laboratory data of rotational spectra. We find similar column densities and column density ratios of COMs (within a factor ~2) between Band 3 and Band 7. Conclusions.The effect of the dust optical depth for IRAS 16293-2422 B at an off-source location on column densities and column density ratios is minimal. Moreover, for warm protostars, long wavelength spectra (~3 mm) are not only crowded and complex, but they also take significantly longer integration times than shorter wavelength observations (~0.8 mm) to reach the same sensitivity limit. The 3 mm search has not yet resulted in the detection of larger and more complex molecules in warm sources. A full deep ALMA Band 2–3 (i.e., ~3–4 mm wavelengths) survey is needed to assess whether low frequency data have the potential to reveal more complex molecules in warm sources. 

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4. A Chemistry-first Centered Icy Chemical Inventory of Protostellar Sources with JWST

   https://doi.org/10.3847/1538-4357/ae3743  

   Turner, Andrew M; Yang, Yao-Lun; Gross, Rachel; Sakai, Nami; Kaiser, Ralf I (March 2026, The Astrophysical Journal)

   Abstract The chemical evolution in star forming regions is driven by the interplay between gas and ice mantles. Identifying the ice compositions at the early stage of star formation thus provides constraints on the chemical processes inaccessible from gas-phase characterizations. As part of the CORINOS program, spectra from the James Webb Space Telescope (JWST) Mid-Infrared Instruments Medium Resolution Spectroscopy were taken toward four Class 0 protostars: IRAS 15398-3359, Ser-emb7, L483, and B335. The spectra were processed with simultaneous fitting of a continuum and silicate absorption to produce optical depth mid-infrared spectra of the ices at 5–28μm (360–2000 cm⁻¹) toward these four sources. Simple molecules such as water (H₂O), carbon dioxide (CO₂), methanol (CH₃OH), formic acid/formate (HCOOH/HCOO⁻), ammonia/ammonium (NH₃/NH₄⁺), and formaldehyde (H₂CO) are the most abundant features in these ices, while complex organic molecules (COMs) represent a smaller contribution. Likely COMs include hydroxylamine (NH₂OH), methylamine (CH₃NH₂), and ethanol (CH₃CH₂OH). Absorption features belonging to functional groups such as –CH₃and –OH suggest that additional COMs are present, but these cannot be unambiguously assigned due to overlapping bands. Formation pathways toward these COMs utilizing radical–radical combination reactions based on laboratory simulation experiments is presented. By extension, COMs predicted by these reactions, but absent from the spectra, are discussed. The results provide insight into the chemical environment of these ices and also highlight the critical need for caution and sufficient evidence in order to confidently identify COMs in ice. 

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5. The Reservoir of the Per-emb-2 Streamer

   https://doi.org/10.3847/1538-4357/ad2fa1  

   Taniguchi, Kotomi; Pineda, Jaime E; Caselli, Paola; Shimoikura, Tomomi; Friesen, Rachel K; Segura-Cox, Dominique M; Schmiedeke, Anika (April 2024, The Astrophysical Journal)

   Abstract Streamers bring gas from outer regions to protostellar systems and could change the chemical composition around protostars and protoplanetary disks. We have carried out mapping observations of carbon-chain species (HC₃N, HC₅N, CCH, and CCS) in the 3 mm and 7 mm bands toward the streamer flowing to the Class 0 young stellar object (YSO) Per-emb-2 with the Nobeyama 45 m radio telescope. A region with a diameter of ∼0.04 pc is located to the north at a distance of ∼20,500 au from the YSO. The streamer connects to this northern region, which is its origin. The reservoir has high density and low temperature ( $n_{{\mathrm{H}}_2}\approx 1.9\times {10}^4$ cm⁻³,T_kin= 10 K), which are similar to those of early-stage starless cores. By comparison of the observed abundance ratios of CCS/HC₃N to the chemical simulations, the reservoir and streamer are found to be chemically young. The total mass available for the streamer is derived to be 24–34M_⊙. It has been estimated that, if all of the gas in the reservoir were to accrete onto the Per-emb-2 protostellar system, the lifetime of the streamer would be (1.1–3.2) × 10⁵yr, suggesting that the mass accretion via the streamer would continue until the end of the Class I stage. 

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