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Abstract In the complex Titan atmospheric chemistry, photochemical reactions involving cyanodiacetylene (HC₅N) may be important as reaction pathways that could lead to the production of amines and aromatic molecules. We carried out a new comprehensive search of HC₅N in Titan’s atmosphere using Band 6 (215–275 GHz or 1.1–1.4 mm in wavelength) spectroscopic archival data from the Atacama Large Millimeter/submillimeter Array. To search for a faint spectral line feature of HC₅N, we used a spectral stacking method that integrates different transition data. Integrating six independent observation data sets with a total integration time of ∼15,000 s, we found no statistically significant detection of an HC₅N spectral feature. Using the Planetary Spectrum Generator for radiative transfer modeling and employing the previously predicted vertical distribution proposed by V. Vuitton et al., E. Lellouch et al., J. C. Loison et al., and V. A. Krasnopolsky, we derived the upper limit column density of HC₅N, corresponding to 3σnoise level, to be 0.3–10 × 10¹³molecules cm^–2above ∼60 km altitude (depending on the assumed vertical profile). The corresponding upper limit of [HC₅N]/[HC₃N] ratio value derived from three of four models seem to be lower than that measured in high-mass protostellar objects, possibly due to the lower abundance of C₄H₂. 

Abstract Most stars form in multiple systems, with profound implications in numerous astronomical phenomena intrinsically linked to multiplicity. However, our knowledge about the process of how multiple stellar systems form is incomplete and biased toward nearby molecular clouds forming only low-mass stars, which are unrepresentative of the stellar population in the Galaxy. Most stars form within dense cores in clusters alongside high-mass stars (>8M_⊙), as the Sun likely did. Here we report deep Atacama Large Millimeter/submillimeter Array (ALMA) 1.33 mm dust continuum observations at ∼160 au spatial resolution, revealing 72 low-mass multiple systems embedded in 23 high-mass cluster-forming regions, as part of the Digging into the Interior of Hot Cores with ALMA survey. We find that the companion separation distribution presents a distinct peak at ∼1200 au, in contrast to the one at ∼4000 au observed in nearby low-mass regions. The shorter fragmentation scale can be explained by considering the higher pressure exerted by the surrounding medium, which is higher than the one in low-mass regions, due to the larger turbulence and densities involved. Because the peak of the companion separation distribution occurs at much larger scales than the expected disk sizes, we argue that the observed fragmentation is produced by turbulent core fragmentation. Contrary to predictions, the multiplicity fraction remains constant as the stellar density increases. We propose that in the extremely dense environments where high-mass stars form, dynamical interactions play an important role in disrupting weakly bound systems. 

Context. Hydrogen recombination lines (HRLs) are valuable diagnostics of the physical conditions in ionized regions surrounding high-mass stars. Understanding these lines, including broadening mechanisms and intensity trends, can provide insights into HII region densities, temperatures, and kinematics. Aims. This study aims to investigate the physical properties of ionized gas around massive protostars by analysing the HRLs (Hαand Hβ) in the Q band. Methods. We carried out observations using the Yebes 40m radio telescope in the Q band (30.5–50 GHz) towards six high-mass protostars selected from the SOMA Survey (G45.12+0.13, G45.47+0.05, G28.20−0.05, G35.20−0.74, G19.08−0.29, and G31.28+0.06). The observed line profiles were analysed to assess broadening mechanisms, and electron densities and temperatures were derived. The results were compared with available Q-band data from the TianMa 65-m Radio Telescope (TMRT) that have been reported in the literature, and ALMA Band 1 (35–50 GHz) Science Verification observations towards Orion KL, analysed in this study. Results. A total of eight Hα(n = 51 to 58) and ten Hβ(n = 64 to 73) lines were detected towards G45.12+0.13, G45.47+0.05, and G28.20−0.05; there were no detections in other sources. We derived electron densities of ~1−5 × 10⁶cm⁻³and temperatures of 8000–10 000 K for the sources. However, for Orion KL, we obtained an electron density one order of magnitude lower, while its temperature was found to be more similar. Interestingly, G45.12 and G28.20 show an increasing intensity trend with frequency for both Hαand Hβtransitions, contrary to the decreasing trend observed in Orion KL. Conclusions. The line widths of the detected HRLs indicate contributions from both thermal and dynamical broadening, suggesting the presence of high-temperature ionized gas that is likely kinematically broadened (e.g. due to turbulence, outflows, rapid rotation, or stellar winds). Pressure broadening caused by electron density may also have a minor effect. We discuss different scenarios to explain the measured line widths of the HRLs. The contrasting intensity trends between the sources may reflect variations in local physical conditions or radiative transfer effects, highlighting the need for further investigation through higher-resolution observations and detailed modelling. 

Context. Evidence that the chemical characteristics around low- and high-mass protostars are similar has been found: notably, a variety of carbon-chain species and complex organic molecules (COMs) form around both types. On the other hand, the chemical compositions around intermediate-mass (IM) protostars (2M_⊙<m_*< 8M_⊙) have not been studied with large samples. In particular, it is unclear the extent to which carbon-chain species form around them. Aims. We aim to obtain the chemical compositions of a sample of IM protostars, focusing particularly on carbon-chain species. We also aim to derive the rotational temperatures of HC₅N to confirm whether carbon-chain species are formed in the warm gas around these stars. Methods. We conducted Q-band (31.5–50 GHz) line survey observations toward 11 mainly IM protostars with the Yebes 40 m radio telescope. The target protostars were selected from a subsample of the source list of the SOFIA Massive Star Formation project. Assuming local thermodynamic equilibrium, we derived the column densities of the detected molecules and the rotational temperatures of HC₅N and CH₃OH. Results. Nine carbon-chain species (HC₃N, HC₅N, C₃H, C₄Hlinear-H₂CCC,cyclic-C₃H₂, CCS, C₃S, and CH₃CCH), three COMs (CH₃OH, CH₃CHO, and CH₃CN), H₂CCO, HNCO, and four simple sulfur-bearing species (¹³CS, C³⁴S, HCS⁺, and H₂CS) are detected. The rotational temperatures of HC₅N are derived to be ~20–30 K in three IM protostars (Cepheus E, HH288, and IRAS 20293+3952). The rotational temperatures of CH₃OH are derived in five IM sources and found to be similar to those of HC₅N. Conclusions. The rotational temperatures of HC₅N around the three IM protostars are very similar to those around low- and high-mass protostars. These results indicate that carbon-chain molecules are formed in lukewarm gas (~20–30 K) around IM protostars via the warm carbon-chain chemistry process. Thus, carbon-chain formation occurs ubiquitously in the warm gas around protostars across a wide range of stellar masses. Carbon-chain molecules and COMs coexist around most of the target IM protostars, which is similar to the situation for low- and high-mass protostars. In summary, the chemical characteristics around protostars are the same in the low-, intermediate- and high-mass regimes. 

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. 

Abstract We present ∼8–40μm SOFIA-FORCAST images of seven regions of “clustered” star formation as part of the SOFIA Massive Star Formation Survey. We identify a total of 34 protostar candidates and build their spectral energy distributions (SEDs). We fit these SEDs with a grid of radiative transfer models based on the turbulent core accretion (TCA) theory to derive key protostellar properties, including initial core mass,M_c, clump environment mass surface density, Σ_cl, and current protostellar mass,m_*. We also carry out empirical graybody (GB) estimation of Σ_cl, which allows a case of restricted SED fitting within the TCA model grid. We also release version 2.0 of the open-source Python packagesedcreator, which is designed to automate the aperture photometry and SED building and fitting process for sources in clustered environments, where flux contamination from close neighbors typically complicates the process. Using these updated methods, SED fitting yields values ofM_c∼ 30–200M_⊙, Σ_cl,SED∼ 0.1–3 g cm⁻², andm_*∼ 4–50M_⊙. The GB fitting yields smaller values of Σ_cl,GB≲ 1 g cm⁻². From these results, we do not find evidence for a critical Σ_clneeded to form massive (≳8M_⊙) stars. However, we do find tentative evidence for a dearth of the most massive (m_*≳ 30M_⊙) protostars in the clustered regions, suggesting a potential impact of environment on the stellar initial mass function. 

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. 

Abstract We present emission maps ( $1.\prime 5\times 1.\prime 5$scale, corresponding to 0.18 pc) of the DCN (J= 2 − 1) and DCO⁺(J= 2 − 1) lines in the 2 mm band toward the Orion KL region obtained with the 2 mm receiver system named B4R installed on the Large Millimeter Telescope. The DCN emission shows a peak at the Orion KL hot core position, whereas no DCO⁺emission has been detected there. The DCO⁺emission shows enhancement at the west side of the hot core, which is well shielded from the UV radiation from OB massive stars in the Trapezium cluster. We have derived the abundance ratio of DCN/DCO⁺at three representative positions where both species have been detected. The gas components withV_LSR≈ 7.5–8.7 km s⁻¹are associated with low abundance ratios of ∼4–6, whereas much higher abundance ratios (∼22–30) are derived for the gas components withV_LSR≈ 9.2–11.6 km s⁻¹. We have compared the observed abundance ratio to our chemical models and found that the observed differences in the DCN/DCO⁺abundance ratios are explained by different densities.