Context. The protostellar stage is known to be the richest star formation phase in emission from gaseous complex organic molecules. However, some protostellar systems show little or no millimetre (mm) line emission of such species. This can be interpreted as a low abundance of complex organic molecules. Alternatively, complex species could be present in the system, but are not seen in the gas. Aims. The goal is to investigate the second hypothesis for methanol as the most abundant complex organic molecule in protostellar systems. This work aims to determine how effective dust optical depth is in hiding methanol in the gas, and whether methanol can mainly reside in the ice due to the presence of a disk that lowers the temperatures. Hence, we attempt to answer the question whether the presence of a disk and optically thick dust reduce methanol emission even if methanol and other complex species are abundant in the ices and gas. Methods. Using the radiative transfer code RADMC-3D, we calculated methanol emission lines from an envelope-only model and from an envelope-plus-disk model. We compared the results with each other and with the observations. Methanol gas and ice abundances were parametrised inside and outside of the snow surfaces based on values from observations. Both models included either dust grains with low mm opacity or high mm opacity, and their physical parameters such as envelope mass and disk radius were varied. Results. Methanol emission from the envelope-only model is always stronger than from the envelope-plus-disk model by at least a factor ∼2 as long as the disk radius is larger than ∼30 au (for L = 8 L⊙). In most cases, this is due to lower temperatures (disk shadowing), which causes the smaller amount of warm (≳70 K) methanol inside the snow surface of the envelope-plus-disk model. The intensities drop by more than an order of magnitude for models including high mm opacity dust grains and disk radii of at least ∼50 au (for L = 8 L⊙) due to continuum over-subtraction. Conclusions. The line intensities from the envelope-only models match the observations moderately well when methanol emission is strong, but they overproduce the observations of protostars with lower methanol emission even with large dust optical depth effects. The envelope-plus-disk models can explain the bulk of the observations. However, they can only reproduce the observations of sources with high luminosities and very low methanol emission when the dust optical depth is significant in the envelope and continuum over-subtraction becomes effective in the disk (high mm opacity dust grains are used). Therefore, both the effects of disk and dust optical depth should be considered to explain the observations. In conclusion, it is important to take physical structure into account in future chemical studies of low-mass protostars: absence of gas-phase methanol emission does not imply absence of methanol molecules in either gas or ice.

Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation. Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au). Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture. Results. The protostellar envelope is well traced by C18O, DCO+, and N2D+, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH3OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H2CO. The cavity walls show tracers of UV-irradiation such as C2H, c-C3H2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H2S, SO, OCS, and H13CN, which most likely are related to ice sublimation and high-temperature chemistry. Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.

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 the James Webb Space Telescope will allow a direct comparison between the ice and gas abundances of both smaller and larger nitrogen-bearing species.

Context. Complex organic molecules (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on Solar- System scales in the Class 0/I stage is relevant. Aims. Our aim is to determine the abundance ratios of oxygen-bearing COMs in Class 0 protostellar systems on scales of ~100 AU radius. We aim to compare these abundances with one another, and to the abundances of other low-mass protostars such as IRAS 16293-2422B and HH 212. Additionally, using both cold and hot COM lines, the gas-phase abundances can be tracked from a cold to a hot component, and ultimately be compared with those in ices to be measured with the James Webb Space Telescope (JWST). The abundance of deuterated methanol allows us to probe the ambient temperature during the formation of this species. Methods. ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained for seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using local thermodynamic equilibrium models, the excitation temperature and column densities are determined for several O-bearing COMs including methanol (CH3OH), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and dimethyl ether (CH3OCH3). Abundance ratios are taken with respect to CH3OH. Results. Three out of the seven of the observed sources, B1-c, B1-bS (both Perseus), and Serpens S68N (Serpens), show COM emission. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs such as CH3OCHO, CH3OCH3, acetone (CH3COCH3), and ethylene glycol ((CH2OH)2) are remarkably similar for the three COM-rich sources; this similarity also extends to IRAS 16293-2422B and HH 212, even though collectively these sources originate from four different star-forming regions (i.e., Perseus, Serpens, Ophiuchus, and Orion). For other COMs like CH3CHO, ethanol (CH3CH2OH), and glycolaldehyde (CH2OHCHO), the abundances differ by up to an order of magnitude, indicating that local source conditions become important. B1-c hosts a cold (Tex ≈ 60 K), more extended component of COM emission with a column density of typically a few percent of the warm/hot (Tex ~ 200 K) central component. A D/H ratio of 1–3% is derived for B1-c, S68N, and B1-bS based on the CH2DOH/CH3OH ratio (taking into account statistical weighting) suggesting a temperature of ~15 K during the formation of methanol. This ratio is consistent with other low-mass protostars, but is lower than for high-mass star-forming regions. Conclusions. The abundance ratios of most O-bearing COMs are roughly fixed between different star-forming regions, and are presumably set at an earlier cold prestellar phase. For several COMs, local source properties become important. Future mid-infrared facilities such as JWST/MIRI will be essential for the direct observation of COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow ice and gas-phase abundances to be directly linked in order to constrain the routes that produce and maintain chemical complexity during the star formation process.

Context. As material from an infalling protostellar envelope hits the forming disk, an accretion shock may develop which could (partially) alter the envelope material entering the disk. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that emission originating from warm SO and SO2 might be good tracers of such accretion shocks. Aims. The goal of this work is to test under what shock conditions the abundances of gas-phase SO and SO2 increase in an accretion shock at the disk–envelope interface. Methods. Detailed shock models including gas dynamics were computed using the Paris-Durham shock code for nonmagnetized J-type accretion shocks in typical inner envelope conditions. The effect of the preshock density, shock velocity, and strength of the ultraviolet (UV) radiation field on the abundance of warm SO and SO2 is explored. Compared with outflows, these shocks involve higher densities (~107 cm−3), lower shock velocities (~few km s−1), and large dust grains (~0.2 μm) and thus probe a different parameter space. Results. Warm gas-phase chemistry is efficient in forming SO under most J-type shock conditions considered. In lower-velocity (~3 km s−1) shocks, the abundance of SO is increased through subsequent reactions starting from thermally desorbed CH4 toward H2CO and finally SO. In higher velocity (≳4 km s−1) shocks, both SO and SO2 are formed through reactions of OH and atomic S. The strength of the UV radiation field is crucial for SO and in particular SO2 formation through the photodissociation of H2O. Thermal desorption of SO and SO2 ice is only relevant in high-velocity (≳5 km s−1) shocks at high densities (≳107 cm−3). Both the composition in the gas phase, in particular the abundances of atomic S and O, and in ices such as H2S, CH4, SO, and SO2 play a key role in the abundances of SO and SO2 that are reached in the shock. Conclusions. Warm emission from SO and SO2 is a possible tracer of accretion shocks at the disk–envelope interface as long as a local UV field is present. Observations with ALMA at high-angular resolution could provide further constraints given that other key species for the gas-phase formation of SO and SO2, such as H2S and H2CO, are also covered. Moreover, the James Webb Space Telescope will give access to other possible slow, dense shock tracers such as H2, H2O, and [S I] 25 μm.

Context. The formation of planets is expected to be enhanced around snowlines in protoplanetary disks, in particular around the water snowline. Moreover, freeze-out of abundant volatile species in disks alters the chemical composition of the planet-forming material. However, the close proximity of the water snowline to the host star combined with the difficulty of observing water from Earth makes a direct detection of the water snowline in protoplanetary disks challenging. HCO+ is a promising alternative tracer of the water snowline. The destruction of HCO+ is dominated by gas-phase water, leading to an enhancement in the HCO+ abundance once water is frozen out. Aims. Following earlier observed correlations between water and H13CO+ emission in a protostellar envelope, the aim of this research is to investigate the validity of HCO+ and the optically thin isotopologue H13CO+ as tracers of the water snowline in protoplanetary disks and the required sensitivity and resolution to observationally confirm this. Methods. A typical Herbig Ae disk structure is assumed, and its temperature structure is modelled with the thermochemical code DALI. Two small chemical networks are then used and compared to predict the HCO+ abundance in the disk: one without water and one including water. Subsequently, the corresponding emission profiles are modelled for the J = 2−1 transition of H13CO+ and HCO+, which provides the best balance between brightness and the optical depth effects of the continuum emission and is less affected by blending with complex molecules. Models are then compared with archival ALMA data. Results. The HCO+ abundance jumps by two orders of magnitude over a radial range of 2 AU outside the water snowline, which in our model is located at 4.5 AU. We find that the emission of H13CO+ and HCO+ is ring-shaped due to three effects: destruction of HCO+ by gas-phase water, continuum optical depth, and molecular excitation effects. Comparing the radial emission profiles for J = 2−1 convolved with a 0′′.05 beam reveals that the presence of gas-phase water causes an additional drop of only ~13 and 24% in the centre of the disk for H13CO+ and HCO+, respectively. For the much more luminous outbursting source V883 Ori, our models predict that the effects of dust and molecular excitation do not limit HCO+ as a snowline tracer if the snowline is located at radii larger than ~40 AU. Our analysis of recent archival ALMA band 6 observations of the J = 3−2 transition of HCO+ is consistent with the water snowline being located around 100 AU, further out than was previously estimated from an intensity break in the continuum emission. Conclusions. The HCO+ abundance drops steeply around the water snowline, when water desorbs in the inner disk, but continuum optical depth and molecular excitation effects conceal the drop in HCO+ emission due to the water snowline. Therefore, locating the water snowline with HCO+ observations in disks around Herbig Ae stars is very difficult, but it is possible for disks around outbursting stars such as V883 Ori, where the snowline has moved outwards.

Context. Complex organic species are known to be abundant toward low-and high-mass protostars. No statistical study of these species toward a large sample of high-mass protostars with the Atacama Large Millimeter/submillimeter Array (ALMA) has been carried out so far. Aims. We aim to study six N-bearing species: methyl cyanide (CH 3 CN), isocyanic acid (HNCO), formamide (NH 2 CHO), ethyl cyanide (C 2 H 5 CN), vinyl cyanide (C 2 H 3 CN) and methylamine (CH 3 NH 2 ) in a large sample of line-rich high-mass protostars. Methods. From the ALMA Evolutionary study of High Mass Protocluster Formation in the Galaxy survey, 37 of the most line-rich hot molecular cores with ∼1 angular resolution are selected. Next, we fit their spectra and find column densities and excitation temperatures of the N-bearing species mentioned above, in addition to methanol (CH 3 OH) to be used as a reference species. Finally, we compare our column densities with those in other low-and high-mass protostars. Results. CH 3 OH, CH 3 CN and HNCO are detected in all sources in our sample, whereas C 2 H 3 CN and CH 3 NH 2 are (tentatively) detected in ∼78% and ∼32% of the sources. We find three groups of species when comparing their excitation temperatures: hot (NH 2 CHO; T ex 250 K), warm (C 2 H 3 CN, HN 13 CO and CH 13 3 CN; 100 K T ex 250 K) and cold species (CH 3 OH and CH 3 NH 2 ; T ex 100 K). This temperature segregation reflects the trend seen in the sublimation temperature of these molecules and validates the idea that complex organic emission shows an onion-like structure around protostars. Moreover, the molecules studied here show constant column density ratios across low-and high-mass protostars with scatter less than a factor ∼3 around the mean. Conclusions. The constant column density ratios point to a common formation environment of complex organics or their precursors, most likely in the pre-stellar ices. The scatter around the mean of the ratios, although small, varies depending on the species considered. This spread can either have a physical origin (source structure, line or dust optical depth) or a chemical one. Formamide is most prone to the physical effects as it is tracing the closest regions to the protostars, whereas such effects are small for other species. Assuming that all molecules form in the pre-stellar ices, the scatter variations could be explained by differences in lifetimes or physical conditions of the pre-stellar clouds. If the pre-stellar lifetimes are the main factor, they should be similar for low-and high-mass protostars (within factors ∼2 − 3).

The process of massive star (M ≥ 8 M ) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud (LMC). We took advantage of the spectral resolution and wavelength coverage of X-shooter (300-2500 nm), mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect Hα, β, [O i] 630.0 nm, Ca ii infrared triplet, [Fe ii] 1643.5 nm, fluorescent Fe ii 1687.8 nm, H 2 2121.8 nm, Brγ, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of 10 candidates as bona fide MYSOs. We compare our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We compute lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Brγ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features such as fluorescent Fe ii and CO bandheads which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.