Abstract Sulfur-bearing molecules are key constituents of the interstellar medium (ISM). Particularly, hydrogen sulfide (H 2 S) and cyano (CN) radicals are key precursors of prebiotic molecules in the ISM. However, the ultralow-temperature gas-phase reactivity remains poorly characterized yet. We report the first experimental and theoretical investigation of the CN + H 2 S reaction under conditions relevant to cold molecular clouds. Rate coefficients were determined between 11.7 and 45.5 K using the Cinétique de Réaction en Ecoulement Supersonique Uniforme technique coupled with pulsed laser photolysis–laser-induced fluorescence, yielding negligible temperature dependence values around 4.0 × 10 −10 cm 3 s −1 in excellent agreement with complementary rate coefficients calculations. AutoMeKin and coupled-cluster theory reveal that the dominant channel involves CN addition to H 2 S, followed by H elimination, forming HSCN. This pathway is energetically more favorable than the previously assumed HCN + SH channel and exhibits submerged transition states, suggesting efficient reactivity at ultracold temperatures. Astrochemical modeling indicates that inclusion of this reaction in chemical networks enhances HSCN abundances in dark clouds, with contributions comparable to those from dissociative recombination routes. Although the CN + H 2 S reaction is absent from current astrochemical databases, our results demonstrate its potential role in sulfur–nitrogen coupling and the formation of prebiotic molecules in the ISM. These findings underscore the need to update chemical models to account for this process and improve predictions of sulfur chemistry in star-forming regions.

Ab initio calculation of transition properties and infrared absorption spectrum of the NaS molecule.

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 (>8 M ⊙ ), 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.

Abstract We investigate the evolution of molecular clouds through the kinematics of their atomic hydrogen (H I ) envelopes, using 12 CO and 21 cm emission to trace the molecular and atomic gas, respectively. We measure the large-scale gradients, Ω, in the velocity fields of 22 molecular clouds and their H I envelopes, then calculate their specific angular momenta, j ∝ Ω R 2 . The molecular clouds have a median velocity gradient of 9.6 × 10 −2 km s −1 pc −1 , and a typical specific angular momentum of 2.7 × 10 24 cm 2 s −1 . The H I envelopes have smaller velocity gradients than their respective molecular clouds, with an average of Ω H I = 0.03 km s −1 pc −1 , and a median angular momentum of of j H I ≈ 5.7 × 10 24 cm 2 s −1 . For a majority of the systems, j HI > j H 2 , with an average of j HI / j H 2 = 4 . Their velocity gradient directions tend to be misaligned, indicating that angular momentum is not conserved during molecular cloud formation. Both populations exhibit a j − R scaling consistent with that expected of supersonic turbulence: j H 2 ∝ R 1.67 ± 0.22 , and j H I ∝ R 1.71±0.27 . Combining our measurements with previous observations, we demonstrate a scaling of j ∝ R 1.50±0.02 in star-forming regions spanning 5 dex in size, R ∈ (10 −3 , 10 2 ) pc. We construct a model of angular momentum transport during molecular cloud formation, and derive the angular momenta of the progenitors to the present-day systems. We calculate a typical angular momentum redistribution timescale of 13 Myr, comparable to the H I envelope free-fall times.

Context . In cores deuterium fractionation becomes highly efficient due to low temperatures and CO freeze-out. Cyclopropenylidene (c-C 3 H 2 ), a small cyclic molecule formed early in chemical evolution, and its deuterated forms serve as valuable tracers of gas-phase deuteration in these environments. Aims . In order to statistically explore the c-C 3 H 2 deuteration ratios towards starless and prestellar cores, we present observations of c-C 3 H 2 and its deuterated isotopologues on a sample of cores in the Perseus molecular cloud. Methods . Transitions of c-C 3 H 2 , c-C 3 HD, and c-C 3 D 2 were observed with the Yebes 40m, the Arizona Radio Observatory (ARO) 12m and the Institut de Radioastronomie Millimétrique (IRAM) 30 metre telescopes towards a total of 16 starless and prestellar cores in the Perseus molecular cloud. The lines were fitted with Gaussian profiles and their column densities were computed using the non-local thermodynamic equilibrium (non-LTE) software RADEX. Results . The main isotopologue c-C 3 H 2 is detected in 93% (14/15) of the targeted cores (for one of the 16 cores, none of its transitions were covered), its singly deuterated form c-C 3 HD is detected in 94% (15/16) of the targeted cores, and its doubly deuterated form c-C 3 D 2 is detected towards 56% (9/16) of the cores detected. A range of column densities towards the different cores was derived: for c-C 3 H 2 , (0.5–8.1)×10 13 cm −2 ; for c-C 3 HD, (0.2–2.1)×10 12 cm −2 ; and for c-C 3 D 2 , (0.6–1.6)×10 11 cm −2 . The ortho-to-para ratio of c-C 3 H 2 was obtained for all except one core with a median value of 3.5 ± 0.4. The D/H and D 2 /D ratios were obtained for the cores with detections, yielding a statistically corrected D/H range of 0.5–9.2% with a median value of 1.5 ± 0.2% and a statistically corrected D 2 /D range 9.0–55.2% with a median value of 25.9 ± 4.3%. Conclusions . No apparent trend is seen with the ortho-to-para ratio of c-C 3 H 2 and the evolutionary stage of the core, as traced by volume density n H 2 . The median c-C 3 H 2 D/H ratio in Perseus’ starless cores appears lower than the value for the Taurus molecular cloud and the Chamaeleon molecular cloud. The D 2 /D ratio is equivalent between the Perseus and Taurus molecular clouds within the uncertainties. There is a correlation with the D/H ratio and the n H 2 of the cores in Perseus, strengthening the idea of D/H being a tracer of core evolution. The D 2 /D ratio does not correlate with n H 2 , but positively correlates with T kin , suggesting that its formation is favoured by a slightly endothermic reaction.

Cosmic fertilization? Implantation of astrobiologically relevant cosmic rays in molecular clouds.

We investigated the properties of the interstellar medium (ISM) at giant molecular cloud (GMC) scales (∼100 pc) in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning all interacting stages along the merger sequence, i.e. from isolated systems to late-stage mergers. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage by (1) defining beam-sized (unresolved, line-of-sight) regions and (2) identifying actual gas clumps and physical structures within the galaxies. In total, we identify more than 4000 beam-sized CO-emitting regions defined on scales of ∼100 pc and more than 1000 molecular gas clumps in the sample. To map the distribution of molecular gas we used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the J = 2–1 CO transition, and to map the distribution of star formation we used the Hubble Space Telescope (HST) observations of the Pa α or Pa β hydrogen recombination lines. We derived spatially resolved Kennicutt–Schmidt (KS) relations for each LIRG in the sample. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Σ SFR and Σ H2 . However, in the remaining galaxies, the relation splits into two branches – one characterised by higher Σ SFR and Σ H2 , the other by lower value – indicating the presence of a duality in this relation. In contrast, when using physical gas clumps, the duality disappears and all galaxies show a single trend. These results provide two complementary perspectives when studying the star formation process. The first maximises the number statistics (beam-sized regions), and the second focuses on actual structures associated with gas clumps in which the measured sizes have a physical meaning. We also studied other ISM and clump properties as a function of the merger stage of the LIRG systems. We find that isolated galaxies and systems in early stages of interaction exhibit smaller amounts of gas and lower star formation rates (SFRs). As the merger progresses, however, the amount of gas in the central kiloparsecs of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in the SF efficiency of the molecular gas clumps. Clumps in late-stage mergers are predominantly located at small distances from the nucleus, confirming that most of the activity is concentrated in the central regions. Interestingly, the relation between the star formation efficiency and the boundedness parameter (which measures the effects of gravity against velocity dispersion) evolves from being roughly flat in the early stages of the merger to becoming positive in the final phases, indicating that clump self-gravity only starts to regulate the star formation process between the early and mid merger stages.

Recent observational studies on carbon-chain species with current facilities and future prospects with ALMA & JWST.

Abstract Observations of JWST have revealed that several close-in exoplanets have sulfur-rich atmospheres through SO 2 detections. Atmospheric sulfur is often thought to originate from solid accretion during planet formation, whereas recent simultaneous detections of SO 2 and NH 3 challenge this conventional scenario. In this study, we propose that ammonium salts, such as NH 4 SH, tentatively detected in comets and molecular clouds, play a significant role in producing sulfur-rich disk gases, which serve as the ingredients of giant planet atmospheres. We simulated the radial transport of dust containing volatile ices and ammonium salts, along with the dissociation, sublimation, and recondensation of these materials, thereby predicting the atmospheric chemical structures and transmission spectra of planets inheriting these compositions. Assuming that ammonium salts sequester 20% of the elemental nitrogen and sulfur budgets, our results reveal that they enhance sulfur and nitrogen abundances in disk gases to 2–10 times the solar values near the salt dissociation line. Photochemical simulations demonstrate that SO 2 , NS, H 2 S, NO, and NH 3 become the dominant N and S chemical species in the atmospheres of planets that inherited the gas compositions inside the H 2 O snowline. SO 2 features clearly appear in the infrared transmission spectra when the salt-bearing grains enhance the sulfur abundance of disk gas by pebble drift. Our model provides a novel scenario that explains the SO 2 detected in some exoplanet atmospheres solely from disk gas accretion. Volatile-element ratios, particularly N/S and C/O, would provide a key to disentangle our scenario from the conventional solid-accretion scenario.

Abstract The formation of super star clusters (SSCs) in galaxies remains a fundamental yet unresolved problem. Among the proposed mechanisms, cloud–cloud collisions (CCCs) have been suggested as a potential trigger, although observational validation has been limited. Here we present high-resolution ( 0 . ″ 12 , ∼14 pc) Atacama Large Millimeter/submillimeter Array observations of CO ( J = 1–0) emission toward a super giant molecular cloud (SGMC) in the overlap region of the Antennae galaxies. The data resolve the SGMC into two distinct velocity components separated by ∼50 km s −1 . One component exhibits a “U”-shaped structure within a large filament likely shaped by ram pressure, while the other shows hub-filament morphology. Such a morphology is naturally interpreted as a CCC scenario. The 108 GHz continuum emission detected at the apparent collision interface is dominated by free–free radiation, with an ionizing photon rate consistent with the stellar mass and age of the optically identified SSCs. Supplementary infrared imaging with JWST reveals emission spatially coincident with the inferred collision interface, further supporting the CCC scenario. These results provide compelling, multiwavelength evidence that CCCs play a key role in triggering SSC formation in merging galaxies.

Hitherto unidentified abiotic formation pathways leading to the organosulfur molecules ethanethiol (C2H5SH), methanethiol (CH3SH) and, dimethyl sulfide (CH3SCH3) were investigated through a series of laboratory simulation experiments. Interstellar analog ices of methane (CH4) and hydrogen sulfide (H2S) were exposed to proxies of galactic cosmic rays (GCRs) in the form of energetic electrons released in the GCR track in interstellar ices simulating typical cold molecular cloud lifetimes of a few 106 to 107 years. During the temperature-programmed desorption phase, the molecules subliming fractionally from the ice mixtures were photoionized with vacuum ultraviolet (VUV) photons at energies both above and below the adiabatic ionization energies of the product molecules of interest. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labelled ice experiments, the reaction products were selectively photoionized to discriminate between isomers. Ethane (C2H6) and methanethiol (CH3SH), as first-generation irradiation products, along with second-generation dimethyl sulfide (CH3SCH3), were identified via infrared spectroscopy and PI-ReToF-MS. The formation of ethanethiol (C2H5SH) was further confirmed by matching the photoionization efficiency (PIE) curve to the experimental PI-ReToF-MS data. Our findings instigate a deeper understanding of interstellar sulfur chemistry linking interstellar and cometary ices to the gas-phase detection of sulfur bearing organics in star-forming regions.

Ionised regions around OB-type stars, formed at an early stage of their evolution, are important in the investigation of the formation processes of these objects. Thus far, however, only a few observations of their physical structure and interaction with the parental molecular cloud have been carried out. The high resolution and high sensitivity of new instruments such as ALMA and the upgraded VLA allow us to fill in this knowledge gap. We investigate the well-known core-halo ultracompact region and the surrounding molecular clump to determine the density and temperature of both the ionised and neutral gas, and to possibly obtain a 3D picture of their spacial distribution. We took advantage of the full-band frequency coverage at 3 mm obtained with ALMA for the GUAPOS project to image the emission of a plethora of hydrogen recombination lines towards the ̋II region, as well as several molecular transitions that serve as tracers of medium-density (∼10^4--10^6 ̧mc) gas. The line data are complemented by continuum measurements obtained with the VLA at 1 cm and 7 mm. By fitting these lines with a model that takes into account non-local thermal equilibrium (NLTE) effects, we were able to investigate the density and temperature structure and the velocity field of the region. Our findings, based on a model fit accounting for NLTE effects, indicate that the electron temperature of the ̋II region mostly spans a range between 5000 and 6000 K, while the density varies between 2500 and 7500 ̧mc. All in all, the distribution of these parameters, along with the corresponding velocity field hint at a cometary shaped region expanding away from the observer to the NW. The molecular gas appears to be still infalling towards the peak of the region, while its density and temperature are consistent with pressure confinement of the ionised gas to the SE.

Abstract The phase spiral is a perturbation to the vertical phase-space distribution of stars in the Milky Way disk. We study the phase spiral’s properties and how they vary with spatial position, in order to constrain its origin and evolution, as well as properties of the disk itself. We produce high-resolution maps using two complementary data processing schemes: (a) we bin the Gaia proper-motion sample in a disk-parallel spatial grid, reaching distances up to 4 kpc; and (b) we bin the spatially nearby line-of-sight velocity sample in terms of disk-parallel orbital parameters. We find complex structure, most significantly with respect to Galactocentric radius and guiding radius, but also in Galactic azimuth and epicyclic action and phase. We find that spiral winding and rotation phase vary smoothly across the disk, with close-to-flat radial profiles. This uniform structure, in particular for the rotation phase, disfavors small-scale perturbations as the primary source of the phase spiral, for example, from giant molecular clouds or dark matter subhalos. Curiously, these close-to-flat profiles also imply that the winding time has a strong slope with respect to Galactocentric radius, with low values for the inner disk. A uniform perturbation time cannot be reconciled by self-gravity effects that act only in the disk’s vertical dimension, because this would delay winding but not the evolution of the rotation phase. This suggests that more complex self-gravity effects, acting also in dimensions parallel to the disk plane, are important for the spiral’s formation and evolution.

Abstract A recent study proposed that calcium–aluminum-rich inclusions (CAIs) and CM hibonites exhibit an exclusivity between 50 Ti anomalies and initial 26 Al/ 27 Al ratios, such that refractory inclusions with large 50 Ti variations necessarily formed from 26 Al-poor material, and vice versa. This pattern was attributed to two distinct populations of presolar dust inherited from the protosolar molecular cloud. Reassessment using an updated presolar grain database shows that this proposed Ti–Al exclusivity arises from the contaminated Mg isotope data for a subset of presolar graphite grains. Furthermore, the correlated 48 Ca– 50 Ti signatures among CAIs and hibonites reflect input from rare supernova sources whose dust has not been unambiguously identified. Current presolar grain data, therefore, do not support a presolar-dust origin for the proposed Ti–Al exclusivity.

Abstract We report the discovery of multiple compact molecular features exhibiting extremely broad velocity widths toward the W44 molecular cloud. Atacama Large Millimeter/submillimeter Array CO J = 3–2 data reveal eight “Petit-Bullets” surrounding the previously known “Bullet.” Each Petit-Bullet shows a distinct V-shaped structure in position–velocity space, reminiscent of the Y-shaped morphology of the Bullet, suggesting a common origin. These features are interpreted as the result of high-velocity plunges of compact gravitational objects into dense molecular gas. The spatial and kinematic properties of the Petit-Bullets suggest that the plunging material was not a single object but rather a small cluster of compact bodies. A virial mass of 1.0 × 10 5 M ⊙ inferred from their velocity dispersion is comparable to that of typical globular clusters. Momentum analysis further implies that the main Bullet likely formed by an isolated black hole. These findings provide new evidence for dynamical interactions between halo clusters and disk molecular gas.

Abstract The Serpens OB2 association (ℓ ∼ 18.○5, b ∼ 1.○9, d = 1950 ± 30 pc) is a large star-forming complex ∼65 pc above the Galactic midplane, with a clumpy, elongated structure extending ∼50 pc parallel to the plane. We analyse probable association members, including OB stars and low-to-intermediate-mass young stellar objects (YSOs) from the SPICY catalogue. We use 13CO MWISP data to trace the molecular clouds. The OB stars are concentrated toward the centre of the association, coincident with a gap in the molecular clouds, and toward the side nearest the Galactic plane. The YSOs are distributed throughout the association, but cluster around molecular-cloud clumps. Using Gaia DR3 proper motions to probe the association’s internal kinematics, we find aligned stellar velocities on length scales ≲2 pc, two-point statistics that show increasing velocity differences and predominantly divergent motions at larger separations, and distinct velocities for star clusters within the association. Finally, the association exhibits gradual but statistically significant global expansion perpendicular to the Galactic plane, with a spatial gradient of 0.10 ± 0.02 km s−1 pc−1. The clumpy stellar distribution, correlated velocities on small scales, and increasingly divergent motions on larger scales are consistent with an initial velocity field inherited from a turbulent molecular cloud modified by stellar feedback. The global vertical expansion may arise from large-scale turbulence or feedback-driven shell expansion, with the H ii region Sh 2-54 preferentially pushing the molecular gas away from the Galactic plane. Ser OB2 demonstrates that the multi-scale expansion of an OB association can begin even while star formation is still ongoing throughout the complex.

Abstract The density distribution within molecular clouds offers critical insights into their underlying physical processes, which are essential for understanding star formation. As a statistical measure of column density on the cloud scale, the shape and evolution of the column density probability density function (N-PDF) serve as important tools for understanding the dynamics between turbulence and gravity. Here, we investigate the N-PDFs of Cygnus-X using the column density map obtained from Herschel, supplemented by H I and young stellar objects data. We find that the N-PDFs of Cygnus-X and four subregions display log-normal + power-law shapes, indicating the combined effects of turbulence and gravity in sculpting the density structure. We find evidence that the power-law segment of the N-PDFs flattens over time, and the transitional column density can be seen as a unique and stable star formation threshold specific to each molecular cloud. These results not only clarify the physical state of Cygnus-X but also emphasize the utility of the N-PDF as a statistical diagnostic tool, as it is an accessible indicator of evolutionary stages and star formation thresholds in molecular clouds.

Apart from benzene, aromatic compounds, crucial in biological and chemical processes, were conspicuously absent from the interstellar inventory before 2017, despite extensive searches. Since then, high-resolution laboratory rotational spectroscopic studies in combination with extremely high-sensitivity spectral line surveys have led to the discovery of numerous cyclic and aromatic molecules in the starless dark cloud TMC-1, a source previously thought unsuitable for such chemical complexity. Detections include polycyclic aromatic hydrocarbons (PAHs) and their cyano derivatives with as many as seven fused rings. Discrepancies of more than four orders of magnitude between observed and predicted abundances challenge established astrochemical models. The detection of benzonitrile in other molecular clouds further suggests that aromatic chemistry is common in space. New spectroscopic studies and analysis methods hold promise to refine models of PAH formation and better constrain PAH stabilities in the diffuse gas, thereby aiding in the identification of the carriers of the diffuse interstellar and unidentified infrared emission bands, and potentially reshaping our understanding of the chemical pathways that link interstellar organic molecules to the origins of terrestrial carbon.

Abstract We characterize star-forming gas in six molecular clouds (Sgr B1-off, Sgr B2, Sgr C, the 20 and 50 km s −1 molecular clouds, and the Brick) in the Galactic central molecular zone (CMZ), and compare their star-forming activities with those in molecular clouds outside the CMZ. Using multiband continuum observations taken from Planck, Herschel, James Clerk Maxwell Telescope/SCUBA-2, and Caltech Submillimeter Observatory/SHARC2, we derived 8 . ″ 5 resolution column density maps for the CMZ clouds and evaluated the column density probability distribution functions (N-PDFs). With the archival Atacama Large Millimeter/submillimeter Array 1.3 mm dust continuum data, we further evaluated the mass of the most massive cores ( M core ma x ). We find that the N-PDFs of four of the selected CMZ clouds are well described by a piecewise log-normal+power-law function, while the N-PDFs of the remaining two can be approximated by log-normal functions. In the first four targets, the masses in the power-law component ( M gas bound ), M core max , and star formation rate are correlated. These correlations are very similar to those derived from low-mass clouds in the solar neighborhood and massive star-forming regions on the Galactic disk. These findings lead to our key hypotheses: (1) in the extreme environment of the CMZ, the power-law component in the N-PDF also represents self-gravitationally bound gas structures, and (2) evolution and star-forming activities of self-gravitationally bound gas structures may be self regulated, insensitive to the exterior environment on ≳5–10 pc scales.

Abstract Star formation occurs within dusty molecular clouds that are then disrupted by stellar feedback. However, the timing and physical mechanisms that govern the transition from deeply embedded to exposed stars remain uncertain. Using the STARFORGE simulations, we analyze the evolution of “embeddedness,” identifying what drives emergence. We find the transition from embedded to exposed is fast for individual stars, within 1.3 Myr after the star reaches its maximum mass. This rapid transition is dominated by massive stars, which accrete while remaining highly obscured until their feedback eventually balances, then overcomes, the local accretion. For these massive stars, their maximum mass is reached simultaneously with their emergence. Once these stars are revealed, their localized, pre-supernova feedback then impacts the cloud, driving gas clearance. Because massive stars dominate the luminosity, their fast, local evolution dominates the light emergence from the dust. We calculate the dependence of these processes on the mass of the cloud and find that emergence always depends on when massive stars form, which scales with the cloud’s free-fall time. We also measure the evolution of dust and H α luminosities, where for ∼2 Myr, these tracers outshine the emerging stellar continuum, reaching their peak when gas and dust remain tightly coupled to the massive stars. These results closely resemble observationally observed lifetimes, tying the observable dust and line emission directly to the same localized processes that drive stellar emergence, evidence that our simulated de-embedding physics is representative of real star-forming regions. Thus, because the initial embedding of the most luminous stars is highly local, the emergence of stars is a faster, earlier, more local event than the overall disruption of the cloud by gas expulsion.