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Context. Symmetric molecules that contain several identical nuclei give rise to distinct isomers that are characterized by the total nuclear spin and produce separate ortho and para species that cannot be exchanged through radiative or inelastic transitions and whose relative populations can preserve information about formation conditions. In the interstellar medium, cyclopropenylidene (c-C ₃ H ₂ ) and its doubly deuterated isotopologue provide a prime laboratory for studying spin isomer ratios, which have been measured for the main species but are yet unavailable for c-C ₃ D ₂ . Aims. We present new detections of both ortho and para transitions of c-C ₃ D ₂ towards three sources in the Taurus molecular cloud from the GEMS Large program, and revisit previous detections towards four other cores from the literature in order to investigate the para-to-ortho ratio for doubly deuterated c-C ₃ D ₂ for the first time. Methods. We computed the ortho and para c-C ₃ D ₂ column densities towards all the sources using the non-local thermodynamic equilibrium (non-LTE) software RADEX thanks to the recent availability of dedicated collision rate coefficients. Results. The total derived column densities of c-C ₃ D ₂ for the new sources present a range of (2.5–5.3) × 10 ¹¹ cm ⁻² . The para-to-ortho ratio derived for B213-C16-1 and TMC1-C, 1.5 _−0.6 ^+0.7 and 1.3 _−0.7 ^+1.3 , respectively, are higher than the statistical ratio of 0.5, while for TMC1-NH3 the ratio is statistical within errors, 1.0 _−0.7 ^+1.4 . The para-to-ortho ratio for the revisited sources from the literature, L1544, TMC-1C, TMC-2, and L1517B are 0.4 _−0.1 ^+0.1 , 0.5 _−0.1 ^+0.1 , 0.6 _−0.1 ^+0.2 , and 1.4 _−0.5 ^+0.7 , respectively. In this case, the first three cores show para-to-ortho statistical values, and the latter a non-statistical one. Thus, from all the sources studied, four have statistical c-C ₃ D ₂ para-to-ortho ratios and three sources show higher non-statistical ratios. Conclusions. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule derived through a detailed radiative transfer tool. The observed larger-than-statistical c-C ₃ D ₂ para-to-ortho ratio values for some of the sources may result from p-D ₂ H ⁺ or D ₃ ⁺ driven reactions. The actual ratio is not directly related to the ortho-to-para ratios of H ₂ and c-C ₃ H ₂ . These findings provide an important reference point for future chemical models to investigate the formation pathways of c-C ₃ D ₂ .

Context . Magnetohydrodynamical simulations and interferometric observations toward young stellar objects have revealed the accretion of gas that may alter the physical properties and chemical composition of protostellar disks. Aims . Our goal is to constrain the physical history and properties of late infalling gas and assess its potential to change the physical and chemical properties of the disk. Methods . We carried out a core collapse simulation of 5 M _⊙ of gas endowed with tracer and sink particles during ∼3.6 × 10 ⁵ yr. We analyzed the properties of the rotationally supported disk and the origin and physical history of the gas from which it initially formed. We also selected the tracer particles that describe the late infall to analyze their origin and physical history and compared them to the information obtained from the disk. Results . The final mass of the sink reaches 1.02 M _⊙ , while the mass of the disk stays fairly constant around 0.02 M _⊙ . Throughout its evolution, the disk radius increased up to ∼40 au, and its density was reduced by a factor of approximately eight. Filaments that channel gas from the envelope to the disk and sink appeared as a result of the magnetic interchange instability. Tracer particles were used to identify late infalling gas, whose physical properties resemble those of observed streamers, and they are only accreted in the final snapshots of the simulation. The origin and physical history of the late infalling gas are different from those of the gas that initially forms the disk. Conclusions . Our core collapse simulation of young stellar objects predicts the appearance of filaments that connect the envelope to the disk. The late accretion of gas through these channels has properties similar to those of streamers and brings gas to the disk with a different physical history that could translate into a different chemical composition.

Context. Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. Aims. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3 µm in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. Methods. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program “PDRs4All.” Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. Results. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1–4.5 µm. The continuum at wavelengths longer than 2.7 µm shows good correlations with the 3.3 µm AIB, while the correlation of the continuum at 1.2 µm is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27 µm, which are attributed to the presence in the spectra of water ice and CO ₂ ice. Conclusions. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

The physics and chemistry of molecular gas change with the dynamical evolution of the newly formed protostars. The details of this evolution are not yet fully understood. Our goal is to identify chemical diagnostics to determine the physical conditions in protostellar envelopes and to help establish the development of matter during the formation of a low-mass star. In particular, we aim to investigate a possible variation of sulphur depletion during the star formation process at the scale of the cold envelope. We used observations obtained with the Yebes-40m and IRAM-30m telescopes to estimate column densities of CS, C_2S, C_3S, OCS, HCS+$, SO, H_2CS, SO_2, NS, NS^+, H_2S, HSCN, HNCS, CH_3SH, C^18O, C^17O, CH_3OH, H$_2CO, and their isotopologues in the Class 0 objects HH,212 and NGC,1333 IRAS,4A, using both local thermodynamic equilibrium (LTE) and non-LTE calculations. We used a neural emulator of the chemical code Nautilus to investigate the physical (density, gas temperature, cosmic-ray ionisation rate, and chemical time) and chemical (sulphur elemental abundance) conditions driving the evolution of the chemical composition of the cold envelope. A comparison of the abundances of these species with those of the Class 0 object B,335 reveals a differentiated chemistry. While sulphur-bearing species containing carbon chains (C_2S, C_3S) are between three and seven times more abundant in B,335 than in the other two objects, suggesting an earlier chemistry in this protostar, sulphur oxides and nitrogen-bearing species are approximately three times more abundant in NGC,1333 IRAS,4A. Comparing with similar studies in pre-stellar and protostellar cores, we derive an increase of about two orders of magnitude in the SO/CS and SO_2/C_2S ratios and a potential decrease in the HCS+$/CS ratio of a factor of ten during the transition from the pre-stellar phase to the Class 0 phase. Our chemical modelling shows that while the chemistry of HH,212 and NGC,1333 IRAS,4A is well reproduced assuming a gas temperature of ∼25 K, we find significant differences in their average density and cosmic-ray ionisation rate. We estimate a sulphur depletion of a factor of $∼100 in their protostellar envelopes, which is similar to that measured in starless cores. Sulphur compounds are good evolutionary tracers of the pre- to protostellar phase transition, with oxygen-bearing species being more abundant than carbon-bearing species in evolved sources. These chemical differences are the consequence of envelope heating by the forming star and the environmental conditions. We do not detect any significant change in sulphur depletion in the cold envelope during the Class 0 phase.

B[e] supergiants represent a rare class of luminous, evolved massive stars surrounded by dusty circumstellar disks. Since their intense UV fields were long thought to sterilize their surroundings, molecular detections beyond carbon monoxide have remained elusive, leaving their chemical reservoirs largely unexplored. Whether these environments can sustain a complex molecular chemistry is a fundamental question with significant astrochemical implications. Here we report the detection of chemically rich molecular gas surrounding the B[e] supergiant HD 87643, using Atacama Large Millimeter/submillimeter Array observations. Our data reveal the presence of the sulfur oxides SO and SO ₂ and other sulfur-bearing species, marking the first detection of these molecules in an early-type evolved massive star. We find a high fractional abundance of SO ₂ relative to H ₂ , which our chemical modeling can reproduce in timescales as short as ∼10 ⁴ yr in an oxygen-rich environment. These results indicate that the detected molecules trace a short-lived, rapidly evolving phase of out-of-equilibrium chemistry. Furthermore, we measure an anomalously low ³² SO/ ³³ SO, which we attribute to mass-independent fractionation driven by intense photochemistry. This mechanism mirrors processes proposed to explain the ³³ S excesses in the atmosphere of the Archaean Earth. Our findings suggest that B[e] supergiants could serve as unique laboratories for studying sulfur chemistry under extreme radiation conditions, opening potential avenues to investigate the fractionation processes that shaped the isotopic signatures found in the early geological record.

Observations indicate that the total abundance of S-bearing species in dense clouds is orders of magnitude lower than the cosmic sulfur abundance. Addressing this “missing sulfur problem” requires a combination of astronomical observations, laboratory experiments, and theoretical models. For this study, we used the pyRate astrochemical model to simulate the vacuum-ultraviolet-photon irradiation of a CO ₂ :CS ₂ ice mixture at 10 K in the laboratory, with the goal of supporting the interpretation of the experimental results and testing our current understanding of the sulfur evolution in interstellar ices. For this purpose, the astrochemical model was adapted to the experimental conditions, and the chemical network was compiled from several sources to ensure that all known reactions involving sulfur species were included. The results indicate that non-diffusive chemistry is necessary to reproduce the formation of S-bearing species observed in the experiment. However, some discrepancies were found in the major S-bearing ice chemistry products predicted by the model and the experiment. The compounds OCS, CS, and SO are overpredicted by the model, but it falls short in accounting for SO ₂ and sulfur allotropes. These discrepancies are likely due to a combination of incomplete knowledge on the chemical reactions at play (either because of missing reactions and/or because of unconstrained reaction barriers), and uncertainties in the experimental analysis. This work represents the first effort to model the chemistry of a multicomponent ice analog with a rate-equation-based code, and highlights the complementary nature of theoretical and experimental astrochemistry to disentangle the chemical evolution of sulfur in the interstellar medium.

Context . Shock chemistry is an excellent tool for shedding 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. Aims . We mapped COM emission and derived the molecular composition of the protostellar outflow driven by the Class 0 protostar NGC 1333 IRAS 4B1 to introduce it as a new laboratory for studying the effect of shocks on COM chemistry. Methods . We used the data taken as part of the PROtostars & DIsks: Global Evolution (PRODIGE) large program to compute integrated-intensity maps of outflow emission to identify spatial differences between species. The emission spectra were then analysed towards two positions, one in each outflow lobe, by deriving synthetic spectra and population diagrams assuming conditions of local thermodynamic equilibrium. Results . In addition to typical outflow tracers such as SiO and CO, outflow emission is seen from H ₂ CO, HNCO, and HC ₃ N, as well as from the COMs CH ₃ OH, CH ₃ CN, and CH ₃ CHO, and even from deuterated species such as DCN, D ₂ CO, and CH ₂ DOH. The maps of integrated-intensity ratios between CH ₃ OH and DCN, D ₂ CO, and CH ₃ CHO reveal gradients with distance from the protostar. For DCN and D ₂ CO, this may reflect their pre-stellar abundance profile, provided the outflow is young enough, while an explanation is still required for CH ₃ CHO. Intensity ratio maps of HC ₃ N and CH ₃ CN with respect to CH ₃ OH peak in the southern lobe where the temperatures are highest. This might indicate enhanced HC ₃ N and CH ₃ CN formation at this location, potentially in the warmer gas phase. Rotational temperatures are found in the range ∼50-100K, which is warmer on average than for the L1157-B1 shock spot (≲ 30K). The abundances with respect to CH ₃ OH are higher by factors of a few than for L1157-B1. Conclusions . For the first time, we securely detected the COMs CH ₃ CN, CH ₃ CHO, and CH ₂ DOH in the IRAS 4B1 outflow, serendipitously with a limited sensitivity and bandwidth. Targeted observations will enable the discovery of new COMs and a more detailed analysis of their emission. The morphological differences between the 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 on outflow structure and kinematics.

The Orion Nebula Cluster (ONC) provides the closest example of ongoing star and planet formation in highly irradiated environments. In particular, it is a key region for studying how ultraviolet (UV) radiation from massive stars can drive mass loss in protoplanetary disks through photo-evaporation. Far-UV (FUV, energy 6< E PDRs4All program and combined them with those of the guaranteed time observation (GTO) program 1256. From these images, we extracted key information on ONC disks, such as the radii of the disks observed in silhouette against the bright background, the presence and positions of the dissociation fronts (DFs), the presence and positions of ionization fronts (IFs), intensities of Paschen α lines, and their near-infrared spectral energy distributions (SEDs). From this information we constructed a typology for ONC disks: Type I sources show an IF and DF nearly merged at the disk surface; Type II sources have their DFs at the disk surface and IFs at a distance of several tens of astronomical units from the disk; and Type III sources also have their DF at the disk surface, but show no IF. For all types of disks, we find that PAH emission traces the PDR. We established that the SEDs of candidate Jupiter-mass binary objects (JuMBOs) observed as part of the PDRs4All program are similar to the SEDs of Type III ONC disks, except for one of them, JuMBO24, which is of Type I or II . A detailed look at this SED shows that it is compatible with a young low-mass binary star with an unresolved ionized disk: a microproplyd binary. We observe that the disk radius of ONC disks, r _disk , increases with increasing projected distance to the ionizing source, d _proj , following a power law, r _disk ∝ d _proj ^0.30 , which is interpreted as evidence of the truncation of the disks by the photoevaporation (as reported in previous studies). The disk radii measured at infrared wavelengths appear larger than the disk radii measured at millimeter wavelengths, which is interpreted as evidence of the dust radial segregation within the disks. In agreement with theoretical models and observations of PDRs in the interstellar medium, the thermal pressure within the PDRs of ONC disk increases with the intensity of the FUV radiation field, G ₀ , but with a flatter slope.

As the James Webb Space Telescope (JWST) pursues its observing journey, several thousands of icy-grain spectra are expected to be measured and analysed. The inventory of ices in particular, via the observations of background sources, is accessible for hundreds of lines of sight (LOSs) per molecular-cloud region, opening the possibility to add strong constraints on the solid phase chemistry in a vast domain of cloud densities. SynthIceSpec is a synthetic infrared (IR) spectrum generator that has been designed as a tool to support observing proposals and to test the outcome of chemical models. It is based on laboratory measurements of pure and mixed ices, where each vibrational component is fitted by a sum of Gaussian profiles. Given an initial ice chemical composition (either set by the user or the outputs of a chemical model), a full JWST spectrum is generated, to which the contribution of silicates; continuum, stellar photospheric absorption bands; and extinction law can be added. For the continuum, stellar photospheric models for a wide range of spectral types can be selected by the program, or, Spectral Energy Distribution (SEDs). We present a few use cases of SynthIceSpec: we probed the impact of dust temperature on CO ₂ ice formation using IR data and gas-grain modelling. Next, we used SynthIceSpec to explore the detectability of the main feature of CH ₃ CN at 4.45 µm in a cold core environment with the JWST, which was previously tentatively detected in YSOs. The detection thresholds we derive are reasonably low and observable, but identification is directly impacted by the photosphere absorptions that can greatly hinder identification. For some photostellar types, it could remain feasible. Coupled with the Estimated Time Calculator of the Space Telescope Science Institute, SynthIceSpec can be used to find the optimum observational setup for new observations.

Context . Protoplanetary disks are the birthplaces of planets. As such, they set the initial chemical abundances available for planetary atmosphere formation. Thus, studying elemental abundances, molecular compositions, and abundance ratios in protoplanetary disks is key to linking planetary atmospheres to their formation sites. Aims . We aim to derive the sulfur abundance and the C/O ratio in the AB Aur disk using interferometric observations of CS and SO. Methods . New NOEMA observations of CS 3–2 toward AB Aur are presented. We used velocity-integrated intensity maps to determine the inclination and position angles. Keplerian masks were constructed for all observed species to assess the presence of non-Keplerian motions. We used the CS/SO ratio to study the C/O ratio. We compared our present and previous interferometric observations of AB Aur with a NAUTILUS disk model to gain an insight into the S elemental abundance and C/O ratio. Results . We derived an observational CS/SO ratio ranging from 1.8 to 2.6. Only NAUTILUS models with C/O≳1 can reproduce such ratios. The comparison with models points to strong sulfur depletion, with [S/H]=8 × 10 ⁻⁸ , but we note that no single model can simultaneously fit all observed species.