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Register a new userComplex organic molecules in low-mass protostars on Solar System scales -- II. Nitrogen-bearing species
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The chemical inventory of planets is determined by the physical and chemical processes that govern the early phases of star formation. The aim is to investigate N-bearing complex organic molecules towards two Class 0 protostars (B1-c and S68N) at millimetre wavelengths with ALMA. Next, the results of the detected N-bearing species are compared with those of O-bearing species for the same and other sources. ALMA observations in Band 6 ($sim$ 1 mm) and Band 5 ($sim$ 2 mm) are studied at $sim$ 0.5 resolution, complemented by Band 3 ($sim$ 3 mm) data in a $sim$ 2.5 beam. NH2CHO, C2H5CN, HNCO, HN13CO, DNCO, 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 O-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 N-bearing species have similar excitation temperatures to those of O-bearing species ($sim$ 100 $sim$ 300 K). The similarity of most abundances with respect to HNCO, including those of CH2DCN and CHD2CN, hints at a shared chemical history, especially the high D/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 two sources discussed here add to the small number of sources with such a detailed chemical analysis on Solar System scales. Future JWST data will allow a direct comparison between the ice and gas abundances of N-bearing species.
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Complex organic molecules (COMs) have been observed towards several low-mass young stellar objects (LYSOs). Small and heterogeneous samples have so far precluded conclusions on typical COM abundances, as well as the origin(s) of abundance variations between sources. We present observations towards 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30m telescope. We detect CH$_2$CO, CH$_3$CHO, CH$_3$OCH$_3$, CH$_3$OCHO, CH$_3$CN, HNCO, and HC$_3$N towards 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources respectively. Median column densities derived using survival analysis range between 6.0x10$^{10}$ cm$^{-2}$ (CH$_3$CN) and 2.4x10$^{12}$ cm$^{-2}$ (CH$_3$OCH$_3$) and median abundances range between 0.48% (CH$_3$CN) and 16% (HNCO) with respect to CH$_3$OH. Column densities for each molecule vary by about one order of magnitude across the sample. Abundances with respect to CH$_3$OH are more narrowly distributed, especially for oxygen-bearing species. We compare observed median abundances with a chemical model for low-mass protostars and find fair agreement, although some modeling work remains to bring abundances higher with respect to CH$_3$OH. Median abundances with respect to CH$_3$OH in LYSOs are also found to be generally comparable to observed abundances in hot cores, hot corinos, and massive young stellar objects. Compared with comets, our sample is comparable for all molecules except HC$_3$N and CH$_2$CO, which likely become depleted at later evolutionary stages.
We present an unbiased spectral line survey toward the Galactic Centre (GC) quiescent giant molecular cloud (QGMC), G+0.693 using the GBT and IRAM 30$,$ telescopes. Our study highlights an extremely rich organic inventory of abundant amounts of nitrogen (N)-bearing species in a source without signatures of star formation. We report the detection of 17 N-bearing species in this source, of which 8 are complex organic molecules (COMs). A comparison of the derived abundances relative to H$_2$ is made across various galactic and extragalactic environments. We conclude that the unique chemistry in this source is likely to be dominated by low-velocity shocks with X-rays/cosmic rays also playing an important role in the chemistry. Like previous findings obtained for O-bearing molecules, our results for N-bearing species suggest a more efficient hydrogenation of these species on dust grains in G+0.693 than in hot cores in the Galactic disk, as a consequence of the low dust temperatures coupled with energetic processing by X-ray/cosmic ray radiation in the GC.
Since the start of ALMA observatory operation, new and important chemistry of infrared cold core was revealed. Molecular transitions at millimeter range are being used to identify and to characterize these sources. We have investigated the 231 GHz ALMA archive observations of the infrared dark cloud region C9, focusing on the brighter source that we called as IRDC-C9 Main. We report the existence of two sub-structures on the continuum map of this source: a compact bright spot with high chemistry diversity that we labelled as core, and a weaker and extended one, that we labelled as tail. In the core, we have identified lines of the molecules OCS(19-18), $^{13}$CS(5-4) and CH$_{3}$CH$_{2}$CN, several lines of CH$_{3}$CHO and the k-ladder emission of $^{13}$CH$_{3}$CN.We report two different temperature regions: while the rotation diagram of CH$_{3}$CHO indicates a temperature of 25 K, the rotation diagram of $^{13}$CH$_{3}$CN indicates a warmer phase at temperature of $sim450$K. In the tail, only the OCS(19-18) and $^{13}$CS(5-4) lines were detected. We used the $Nautilus$ and the textsc{Radex} codes to estimate the column densities and the abundances. The existence of hot gas in the core of IRDC-C9 Main suggests the presence of a protostar, which is not present in the tail.
Context. Several sulfur-bearing molecules are observed in the interstellar medium and in comets, in strong contrast to protoplanetary disks where only CS, H$_2$CS and SO have been detected so far. Aims. We combine observations and chemical models to constrain the sulfur abundances and their sensitivity to physical and chemical conditions in the DM Tau protoplanetary disk. Methods. We obtained $0.5^{}$ ALMA observations of DM Tau in Bands 4 and 6 in lines of CS, SO, SO$_2$, OCS, CCS, H$_2$CS and H$_2$S, achieving a $sim 5$ mJy sensitivity. Using the non-LTE radiative transfer code RADEX and the forward-modeling tool DiskFit, disk-averaged CS column densities and upper limits for the other species were derived. Results. Only CS was detected with a derived column density of $sim 2-6 times 10^{12}$ cm$^{-2}$. We report a first tentative detection of SO$_2$ in DM Tau. The upper limits range between $sim 10^{11}$ and $10^{14}$ cm$^{-2}$ for the other S-bearing species. The best-fit chemical model matching these values requires a gas-phase C/O ratio of > 1 at $r sim 50-100$ au. With chemical modeling we demonstrate that sulfur-bearing species could be robust tracers of the gas-phase C/O ratio, surface reaction rates, grain size and UV intensities. Conclusions. The lack of detections of a variety of sulfur-bearing molecules in DM Tau other than CS implies a dearth of reactive sulfur in the gas phase, either through efficient freeze-out or because most of the elemental sulfur is in other large species, as found in comets. The inferred high CS/SO and CS/SO$_2$ ratios require a non-solar C/O gas-phase ratio of > 1, consistent with the recent observations of hydrocarbon rings in DM Tau. The stronger depletion of oxygen-bearing S-species compared to CS is likely linked to the low observed abundances of gaseous water in DM Tau and points to a removal mechanism of oxygen from the gas.
Complex organic molecules (COMs) have been detected in a few Class 0 protostars but their origin is not well understood. Going beyond studies of individual objects, we want to investigate the origin of COMs in young protostars on a statistical basis. We use the CALYPSO survey performed with the IRAM PdBI to search for COMs at high angular resolution in a sample of 26 solar-type protostars, including 22 Class 0 and four Class I objects. Methanol is detected in 12 sources and tentatively in one source, which represents half of the sample. Eight sources (30%) have detections of at least three COMs. We find a strong chemical differentiation in multiple systems with five systems having one component with at least three COMs detected but the other component devoid of COM emission. The internal luminosity is found to be the source parameter impacting the most the COM chemical composition of the sources, while there is no obvious correlation between the detection of COM emission and that of a disk-like structure. A canonical hot-corino origin may explain the COM emission in four sources, an accretion-shock origin in two or possibly three sources, and an outflow origin in three sources. The CALYPSO sources with COM detections can be classified into three groups on the basis of the abundances of oxygen-bearing molecules, cyanides, and CHO-bearing molecules. These chemical groups correlate neither with the COM origin scenarii, nor with the evolutionary status of the sources if we take the ratio of envelope mass to internal luminosity as an evolutionary tracer. We find strong correlations between molecules that are a priori not related chemically (for instance methanol and methyl cyanide), implying that the existence of a correlation does not imply a chemical link. [abridged]
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