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.

Methyl isocyanate (CH₃NCO) belongs to a select group of interstellar molecules considered to be relevant precursors in the formation of larger organic compounds, including those with peptide bonds. The molecule has only been detected in a couple of high-mass protostars and potentially on comets. A formation route on icy grains has been postulated for this molecule but experimental evidence is lacking. Here we extend the range of environments where methyl isocyanate is found and unambiguously identify CH₃NCO through the detection of 43 unblended transitions in the ALMA Protostellar Interferometric Line Survey (PILS) of the low-mass solar-type protostellar binary IRAS 16293−2422. The molecule is detected towards both components of the binary with a ratio HNCO/CH₃NCO ∼ 4–12. The isomers CH₃CNO and CH₃OCN are not identified, resulting in upper abundance ratios of CH₃NCO/CH₃CNO > 100 and CH₃NCO/CH₃OCN > 10. The resulting abundance ratios compare well with those found for related N-containing species towards high-mass protostars. To constrain its formation, a set of cryogenic UHV experiments is performed. VUV irradiation of CH₄:HNCO mixtures at 20 K strongly indicate that methyl isocyanate can be formed in the solid state through CH₃ and (H)NCO recombinations. Combined with gas-grain models that include this reaction, the solid-state route is found to be a plausible scenario to explain the methyl isocyanate abundances found in IRAS 16293−2422.

Context.The chemical inventory of planets is determined by the physical and chemical processes that govern the early phases of star formation. Nitrogen-bearing species are of interest as many provide crucial precursors in the formation of life-related matter.Aims.The aim is to investigate nitrogen-bearing complex organic molecules towards two deeply embedded Class 0 low-mass protostars (Perseus B1-c and Serpens S68N) at millimetre wavelengths with the Atacama Large Millimeter/submillimeter Array (ALMA). Next, the results of the detected nitrogen-bearing species are compared with those of oxygen-bearing species for the same and other sources. The similarities and differences are used as further input to investigate the underlying formation pathways.Methods.ALMA observations of B1-c and S68N in Band 6 (~1 mm) and Band 5 (~2 mm) are studied at ~0.5′′ resolution, complemented by Band 3 (~3 mm) data in a ~2.5′′ beam. The spectra are analysed for nitrogen-bearing species using the CASSIS spectral analysis tool, and the column densities and excitation temperatures are determined. A toy model is developed to investigate the effect of source structure on the molecular emission.Results.Formamide (NH₂CHO), ethyl cyanide (C₂H₅CN), isocyanic acid (HNCO, HN¹³CO, DNCO), and methyl cyanide (CH₃CN, CH₂DCN, and CHD₂CN) are identified towards the investigated sources. Their abundances relative to CH₃OH and HNCO are similar for the two sources, with column densities that are typically an order of magnitude lower than those of oxygen-bearing species. The largest variations, of an order of magnitude, are seen for NH₂CHO abundance ratios with respect to HNCO and CH₃OH and do not correlate with the protostellar luminosity. In addition, within uncertainties, the nitrogen-bearing species have similar excitation temperatures to those of oxygen-bearing species (~100–300 K). The measured excitation temperatures are larger than the sublimation temperatures for the respective species.Conclusions.The similarity of most abundances with respect to HNCO for the investigated sources, including those of CH₂DCN and CHD₂CN, hints at a shared chemical history, especially the high D-to-H ratio in cold regions prior to star formation. However, some of the variations in abundances may reflect the sensitivity of the chemistry to local conditions such as temperature (e.g. NH₂CHO), while others may arise from differences in the emitting areas of the molecules linked to their different binding energies in the ice. The excitation temperatures likely reflect the mass-weighted kinetic temperature of a gas that follows a power law structure. The two sources discussed in this work add to the small number of sources that have been subjected to such a detailed chemical analysis on Solar System scales. Future data from theJames WebbSpace Telescope will allow a direct comparison between the ice and gas abundances of both smaller and larger nitrogen-bearing species.

Context. The number of identified complex organic molecules (COMs) in inter- and circumstellar gas-phase environments is steadily increasing. Recent laboratory studies show that many such species form on icy dust grains. At present only smaller molecular species have been directly identified in space in the solid state. Accurate spectroscopic laboratory data of frozen COMs, embedded in ice matrices containing ingredients related to their formation scheme, are still largely lacking.Aim. This work provides infrared reference spectra of acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), and dimethyl ether (CH₃OCH₃) recorded in a variety of ice environments and for astronomically relevant temperatures, as needed to guide or interpret astronomical observations, specifically for upcoming James Webb Space Telescope observations.Methods. Fourier transform transmission spectroscopy (500–4000 cm⁻¹/20–2.5 μm, 1.0 cm⁻¹ resolution) was used to investigate solid acetaldehyde, ethanol and dimethyl ether, pure or mixed with water, CO, methanol, or CO:methanol. These species were deposited on a cryogenically cooled infrared transmissive window at 15 K. A heating ramp was applied, during which IR spectra were recorded until all ice constituents were thermally desorbed.Results. We present a large number of reference spectra that can be compared with astronomical data. Accurate band positions and band widths are provided for the studied ice mixtures and temperatures. Special efforts have been put into those bands of each molecule that are best suited for identification. For acetaldehyde the 7.427 and 5.803 μm bands are recommended, for ethanol the 11.36 and 7.240 μm bands are good candidates, and for dimethyl ether bands at 9.141 and 8.011 μm can be used. All spectra are publicly available in the Leiden Database for Ice.

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.Complex organic molecules with three carbon atoms are found in the earliest stages of star formation. In particular, propenal (C₂H₃CHO) is a species of interest due to its implication in the formation of more complex species and even biotic molecules.Aims.This study aims to search for the presence of C₂H₃CHO and other three-carbon species such as propylene (C₃H₆) in the hot corino region of the low-mass protostellar binary IRAS 16293–2422 to understand their formation pathways.Methods.We use ALMA observations in Band 6 and 7 from various surveys to search for the presence of C₃H₆and C₂H₃CHO towards the protostar IRAS 16293–2422 B (IRAS 16293B). The identification of the species and the estimates of the column densities and excitation temperatures are carried out by modeling the observed spectrum under the assumption of local thermodynamical equilibrium.Results.We report the detection of both C₃H₆and C₂H₃CHO towards IRAS 16293B, however, no unblended lines were found towards the other component of the binary system, IRAS 16293A. We derive column density upper limits for C₃H₈, HCCCHO,n-C₃H₇OH,i-C₃H₇OH, C₃O, and cis-HC(O)CHO towards IRAS 16293B. We then use a three-phase chemical model to simulate the formation of these species in a typical prestellar environment followed by its hydrodynamical collapse until the birth of the central protostar. Different formation paths, such as successive hydrogenation and radical-radical additions on grain surfaces, are tested and compared to the observational results in a number of different simulations, to assess which are the dominant formation mechanisms in the most embedded region of the protostar.Conclusions.The simulations reproduce the abundances within one order of magnitude from those observed towards IRAS 16293B, with the best agreement found for a rate of 10⁻¹²cm³s⁻¹for the gas-phase reaction C₃+ O → C₂+ CO. Successive hydrogenations of C₃, HC(O)CHO, and CH₃OCHO on grain surfaces are a major and crucial formation route of complex organics molecules, whereas both successive hydrogenation pathways and radical-radical addition reactions contribute to the formation of C₂H₅CHO.