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Context. Propyne (CH₃CCH), also known as methyl acetylene, has been detected in a variety of environments, from Galactic star-forming regions to extragalactic sources. These molecules are excellent tracers of the physical conditions in star-forming regions, allowing the temperature and density conditions surrounding a forming star to be determined. Aims. This study explores the emission of CH₃CCH in the low-mass protostellar binary, IRAS 16293–2422, and examines the spatial scales traced by this molecule, as well as its formation and destruction pathways. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) were used to determine the abundances and excitation temperatures of CH₃CCH towards both protostars. This data allows us to explore spatial scales from 70 to 2400 au. This data is also compared with the three-phase chemical kinetics model MAGICKAL, to explore the chemical reactions of this molecule. Results. CH₃CCH is detected towards both IRAS 16293A and IRAS 16293B, and is found the hot corino components, one around each source, in the PILS dataset. Eighteen transitions above 3σ are detected, enabling robust excitation temperatures and column densities to be determined in each source. In IRAS 16293A, an excitation temperature of 90 K and a column density of 7.8 × 10¹⁵ cm⁻² best fits the spectra. In IRAS 16293B, an excitation temperature of 100 K and 6.8 × 10¹⁵ cm⁻² best fits the spectra. The chemical modelling finds that in order to reproduce the observed abundances, both gas-phase and grain-surface reactions are needed. The gas-phase reactions are particularly sensitive to the temperature at which CH4 desorbs from the grains. Conclusions. CH₃CCH is a molecule whose brightness and abundance in many different regions can be utilised to provide a benchmark of molecular variation with the physical properties of star-forming regions. It is essential when making such comparisons, that the abundances are determined with a good understanding of the spatial scale of the emitting region, to ensure that accurate abundances are derived.

Studies of deuterated isotopologues of complex organic molecules can provide important constraints on their origin in regions of star formation. In particular, the abundances of deuterated species are very sensitive to the physical conditions in the environment where they form. Due to the low temperatures in regions of star formation, these isotopologues are enhanced to significant levels, making detections of multiply-deuterated species possible. However, for complex organic species, only the multiply-deuterated variants of methanol and methyl cyanide have been reported so far. The aim of this paper is to initiate the characterisation of multiply-deuterated variants of complex organic species with the first detection of doubly-deuterated methyl formate, CHD₂OCHO. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) of the protostellar binary IRAS 16293-2422, in the spectral range of 329.1 GHz to 362.9 GHz. We report the first detection of doubly-deuterated methyl formate CHD₂OCHO in the ISM. The D/H ratio of CHD₂OCHO is found to be 2-3 times higher than the D/H ratio of CH₂DOCHO for both sources, similar to the results for formaldehyde from the same dataset. The observations are compared to a gas-grain chemical network coupled to a dynamical physical model, tracing the evolution of a molecular cloud until the end of the Class 0 protostellar stage. The overall D/H ratio enhancements found in the observations are of the same order of magnitude as the predictions from the model for the early stages of Class 0 protostars. However, the higher D/H ratio of CHD₂OCHO compared to the D/H ratio of CH2₂DOCHO is still not predicted by the model. This suggests that a mechanism is enhancing the D/H ratio of singly- and doubly-deuterated methyl formate that is not in the model, e.g. mechanisms for H-D substitutions.

Searches for the prebiotically relevant cyanamide (NH₂CN) towards solar-type protostars have not been reported in the literature. We present here the first detection of this species in the warm gas surrounding two solar-type protostars, using data from the Atacama Large Millimeter/Submillimeter Array Protostellar Interferometric Line Survey (PILS) of IRAS 16293–2422 B and observations from the IRAM Plateau de Bure Interferometer of NGC 1333 IRAS2A. We also detected the deuterated and 13C isotopologs of NH₂CN towards IRAS 16293–2422 B. This is the first detection of NHDCN in the interstellar medium. Based on a local thermodynamic equilibrium analysis, we find that the deuteration of cyanamide (~1.7%) is similar to that of formamide (NH₂CHO), which may suggest that these two molecules share NH₂ as a common precursor. The NH₂CN/NH₂CHO abundance ratio is about 0.2 for IRAS 16293–2422 B and 0.02 for IRAS2A, which is comparable to the range of values found for Sgr B2. We explored the possible formation of NH2CN on grains through the NH₂ + CN reaction using the chemical model MAGICKAL. Grain-surface chemistry appears capable of reproducing the gas-phase abundance of NH2CN with the correct choice of physical parameters.

Context. Methyl isocyanide (CH₃NC) is the isocyanide with the largest number of atoms confirmed in the interstellar medium (ISM),but it is not an abundant molecule, having only been detected towards a handful of objects. Conversely, its isomer, methyl cyanide (CH₃CN), is one of the most abundant complex organic molecules detected in the ISM, with detections in a variety of low- and high-mass sources. Aims.The aims of this work are to determine the abundances of methyl isocyanide in the solar-type protostellar binary IRAS 16293–2422 and to understand the stark abundance differences observed between methyl isocyanide and methyl cyanide in the ISM. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) to search for methyl isocyanide and compare its abundance with that of its isomer methyl cyanide. We use a new line catalogue from the Cologne Database for Molecular Spectroscopy (CDMS) to identify methyl isocyanide lines. We also model the chemistry with an updated version of the three-phase chemical kinetics model MAGICKAL, presenting the first chemical modelling of methyl isocyanide to date. Results. We detect methyl isocyanide for the first time in a solar-type protostar, IRAS 16293–2422 B, and present upper limits for itscompanion protostar, IRAS 16293–2422 A. Methyl isocyanide is found to be at least 20 times more abundant in source B compared tosource A, with a CH₃CN/CH₃NC abundance ratio of 200 in IRAS 16293–2422 B and >5517 in IRAS 16293–2422 A. We also present the results of a chemical model of methyl isocyanide chemistry in both sources, and discuss the implications for methyl isocyanide formation mechanisms and the relative evolutionary stages of both sources. The chemical modelling is unable to match the observed CH₃CN/CH₃NC abundance ratio towards the B source at densities representative of that source. The modelling, however, is consistent with the upper limits for the A source. There are many uncertainties in the formation and destruction pathways of methyl isocyanide,and it is therefore not surprising that the initial modelling attempts do not reproduce observations. In particular, it is clear that some destruction mechanism of methyl isocyanide that does not destroy methyl cyanide is needed. Furthermore, these initial model results suggest that the final density plays a key role in setting the abundance ratio. The next steps are therefore to obtain further detections of methyl isocyanide in more objects, as well as undertaking more detailed physico-chemical modelling of sources such as IRAS16293.

Organohalogens, a class of molecules that contain at least one halogen atom bonded to carbon, are abundant on the Earth where they are mainly produced through industrial and biological processes1. Consequently, they have been proposed as biomarkers in the search for life on exoplanets2. Simple halogen hydrides have been detected in interstellar sources and in comets, but the presence and possible incorporation of more complex halogen-containing molecules such as organohalogens into planet-forming regions is uncertain3,4. Here we report the interstellar detection of two isotopologues of the organohalogen CH3Cl and put some constraints on CH3F in the gas surrounding the low-mass protostar IRAS 16293–2422, using the Atacama Large Millimeter/submillimeter Array (ALMA). We also find CH3Cl in the coma of comet 67P/Churyumov–Gerasimenko (67P/C-G) by using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. The detections reveal an efficient pre-planetary formation pathway of organohalogens. Cometary impacts may deliver these species to young planets and should thus be included as a potential abiotical production source when interpreting future organohalogen detections in atmospheres of rocky planets.

Context. Hydroxylamine (NH₂OH) and methylamine (CH₃NH₂) have both been suggested as precursors to the formation of amino acids and are therefore, of interest to prebiotic chemistry. Their presence in interstellar space and formation mechanisms, however, are not well established. Aims. We aim to detect both amines and their potential precursor molecules NO, N₂O, and CH₂NH towards the low-mass protostellar binary IRAS 16293–2422, in order to investigate their presence and constrain their interstellar formation mechanisms around a young Sun-like protostar. Methods. ALMA observations from the unbiased, high-angular resolution and sensitivity Protostellar Interferometric Line Survey (PILS) are used. Spectral transitions of the molecules under investigation are searched for with the CASSIS line analysis software. Results. CH₂NH and N₂O are detected for the first time, towards a low-mass source, the latter molecule through confirmation with the single-dish TIMASSS survey. NO is also detected. CH₃NH₂ and NH₂OH are not detected and stringent upper limit column densities are determined. Conclusions. The non-detection of CH₃NH₂ and NH₂OH limits the importance of formation routes to amino acids involving these species. The detection of CH₂NH makes amino acid formation routes starting from this molecule plausible. The low abundances of CH₂NH and CH₃NH₂ compared to Sgr B2 indicate that different physical conditions influence their formation in low- and high-mass sources.

Context. The Class 0 protostellar binary IRAS 16293–2422 is an interesting target for (sub)millimeter observations due to, both, the rich chemistry toward the two main components of the binary and its complex morphology. Its proximity to Earth allows the study of its physical and chemical structure on solar system scales using high angular resolution observations. Such data reveal a complex morphology that cannot be accounted for in traditional, spherical 1D models of the envelope. Aims. The purpose of this paper is to study the environment of the two components of the binary through 3D radiative transfer modeling and to compare with data from the Atacama Large Millimeter/submillimeter Array. Such comparisons can be used to constrain the protoplanetary disk structures, the luminosities of the two components of the binary and the chemistry of simple species. Methods. We present ¹³CO, C¹⁷O and C¹⁸O J = 3–2 observations from the ALMA Protostellar Interferometric Line Survey (PILS), together with a qualitative study of the dust and gas density distribution of IRAS 16293–2422. A 3D dust and gas model including disks and a dust filament between the two protostars is constructed which qualitatively reproduces the dust continuum and gas line emission. Results. Radiative transfer modeling in our sampled parameter space suggests that, while the disk around source A could not be constrained, the disk around source B has to be vertically extended. This puffed-up structure can be obtained with both a protoplanetary disk model with an unexpectedly high scale-height and with the density solution from an infalling, rotating collapse. Combined constraints on our 3D model, from observed dust continuum and CO isotopologue emission between the sources, corroborate that source A should be at least six times more luminous than source B. We also demonstrate that the volume of high-temperature regions where complex organic molecules arise is sensitive to whether or not the total luminosity is in a single radiation source or distributed into two sources, affecting the interpretation of earlier chemical modeling efforts of the IRAS 16293–2422 hot corino which used a single-source approximation. Conclusions. Radiative transfer modeling of source A and B, with the density solution of an infalling, rotating collapse or a protoplanetary disk model, can match the constraints for the disk-like emission around source A and B from the observed dust continuum and CO isotopologue gas emission. If a protoplanetary disk model is used around source B, it has to have an unusually high scale-height in order to reach the dust continuum peak emission value, while fulfilling the other observational constraints. Our 3D model requires source A to be much more luminous than source B; LA ~ 18 L⊙ and LB ~ 3 L⊙.

Context. The enhanced degrees of deuterium fractionation observed in envelopes around protostars demonstrate the importance of chemistry at low temperatures, relevant in pre- and protostellar cores. Formaldehyde is an important species in the formation of methanol and more complex molecules. Aims. Here, we aim to present the first study of formaldehyde deuteration on small scales around the prototypical low-mass protostar IRAS 16293–2422 using high spatial and spectral resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the excitation temperature, abundances and fractionation level of several formaldehyde isotopologues, including its deuterated forms. Methods. Excitation temperature and column densities of formaldehyde in the gas close to one of the components of the binary were constrained through modeling of optically thin lines assuming local thermodynamical equilibrium. The abundance ratios were compared to results from previous single dish observations, astrochemical models and local ISM values. Results. Numerous isotopologues of formaldehyde are detected, among them H₂C¹⁷O, and D₂¹³CO for the first time in the ISM. The large range of upper energy levels covered by the HDCO lines help constrain the excitation temperature to 106 ± 13 K. Using the derived column densities, formaldehyde shows a deuterium fractionation of HDCO/H₂CO = 6.5 ± 1%, D₂CO/HDCO = 12.8–₄.₁⁺³.³%, and D₂CO/H₂CO = 0.6(4) ± 0.1%. The isotopic ratios derived are ¹⁶O/¹⁸O = 805–₇₉⁺⁴³, ¹⁸O/¹⁷O = 3.2–₀.₃⁺⁰.²and 12C/13C = 56–₁₁⁺⁸. Conclusions. The HDCO/H₂CO ratio is lower than that found in previous studies, highlighting the uncertainties involved in interpreting single dish observations of the inner warm regions. The D₂CO/HDCO ratio is only slightly larger than the HDCO/H₂CO ratio. This is consistent with formaldehyde forming in the ice as soon as CO has frozen onto the grains, with most of the deuteration happening toward the end of the prestellar core phase. A comparison with available time-dependent chemical models indicates that the source is in the early Class 0 stage.

Nitrogen oxides are thought to play a significant role as a nitrogen reservoir and to potentially participate in the formation of more complex species. Until now, only NO, N₂O, and HNO have been detected in the interstellar medium. We report the first interstellar detection of nitrous acid (HONO). Twelve lines were identified towards component B of the low-mass protostellar binary IRAS 16293–2422 with the Atacama Large Millimeter/submillimeter Array, at the position where NO and N₂O have previously been seen. A local thermodynamic equilibrium model was used to derive the column density (∼9 × 10¹⁴ cm⁻² in a 0 .″5 beam) and excitation temperature (∼100 K) of this molecule. HNO, NO₂, NO+, and HNO3 were also searched for in the data, but not detected. We simulated the HONO formation using an updated version of the chemical code Nautilus and compared the results with the observations. The chemical model is able to reproduce satisfactorily the HONO, N₂O, and NO₂ abundances, but not the NO, HNO, and NH2OH abundances. This could be due to some thermal desorption mechanisms being destructive and therefore limiting the amount of HNO and NH2OH present in the gas phase. Other options are UV photodestruction of these species in ices or missing reactions potentially relevant at protostellar temperatures.

Context. Complex organic molecules are detected in many sources in the warm inner regions of envelopes surrounding deeply embedded protostars. Exactly how these species form remains an open question. Aims. This study aims to constrain the formation of complex organic molecules through comparisons of their abundances towards the Class 0 protostellar binary IRAS 16293–2422. Methods. We utilised observations from the ALMA Protostellar Interferometric Line Survey of IRAS 16293–2422. The species identification and the rotational temperature and column density estimation were derived by fitting the extracted spectra towards IRAS 16293–2422 A and IRAS 16293–2422 B with synthetic spectra. The majority of the work in this paper pertains to the analysis of IRAS 16293–2422 A for a comparison with the results from the other binary component, which have already been published. Results. We detect 15 different complex species, as well as 16 isotopologues towards the most luminous companion protostar IRAS 16293–2422 A. Tentative detections of an additional 11 isotopologues are reported. We also searched for and report on the first detections of methoxymethanol (CH₃OCH₂OH) and trans-ethyl methyl ether (t-C₂H₅OCH₃) towards IRAS 16293–2422 B and the follow-up detection of deuterated isotopologues of acetaldehyde (CH₂DCHO and CH₃CDO). Twenty-four lines of doubly-deuterated methanol (CHD₂OH) are also identified. Conclusions. The comparison between the two protostars of the binary system shows significant differences in abundance for some of the species, which are partially correlated to their spatial distribution. The spatial distribution is consistent with the sublimation temperature of the species; those with higher expected sublimation temperatures are located in the most compact region of the hot corino towards IRAS 16293–2422 A. This spatial differentiation is not resolved in IRAS 16293–2422 B and will require observations at a higher angular resolution. In parallel, the list of identified CHD₂OH lines shows the need of accurate spectroscopic data including their line strength.

Context. Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. Aims. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a –C≡N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293–2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Methods. Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients – Velocity-corrected INtegrated emission (VINE) maps. Results. We detect methyl cyanide (CH₃CN) as well as five of its isotopologues, including CHD₂CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C₂H₅CN), vinyl cyanide (C₂H₃CN), and cyanoacetylene (HC₃N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material. Conclusions. With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions.