Molecular Line Research Articles

Testing analytical methods to derive the cosmic-ray ionisation rate in cold regions via synthetic observations

Context. Cosmic rays (CRs) heavily impact the chemistry and physics of cold and dense star-forming regions. However, the characterisation of their ionisation rate continues to pose a challenge from the observational point of view. Aims. In the past, a few analytical formulas have been proposed to infer the cosmic-ray ionisation rate, ζ2, from molecular line observations. These have been derived from the chemical kinetics of the involved species, but they have not yet been validated using synthetic data processed with a standard observative pipeline. In this work, we aim to bridge this gap. Methods. We performed a radiative transfer on a set of three-dimensional magneto-hydrodynamical simulations of prestellar cores, exploring different initial ζ2, evolutionary stages, types of radiative transfer (for instance assuming local-thermodynamic-equilibrium conditions), and telescope responses. We then computed the column densities of the involved tracers to determine ζ2, employing a recently proposed method based on the detection of H2D+. We compared this approach with a previous method, based on more common tracers. Both approaches are commonly used. Results. Our results confirm that the equation based on the detection of H2D+ accurately retrieves the actual ζ2 within a factor of two to three in the physical conditions explored in our tests. Since we have also explored a non-local thermodynamic equilibrium (non-LTE) radiative transfer, this work indirectly offers insights into the excitation temperatures of common transitions at moderate volume densities (n ≈ 105 cm−3). We also performed a few tests using a previous methodology that is independent of H2D+, which overestimates the actual ζ2 by at least two orders of magnitude. We considered a new derivation of this method, however, we found that it still leads to high over-estimations. Conclusions. The method based on H2D+ is further validated in this work and demonstrates a reliable method for estimating ζ2 in cold and dense gas. On the contrary, the former analytical equation, as already pointed out by its authors, has no global domain of application. Thus, we find that it ought to be employed with caution.

Probing the Physics of Molecular Clouds in Spiral Galaxies NGC 5055 and NGC 3627

Although galaxies can be grouped in a few categories in terms of morphology, they have remarkably different intrinsic properties. Spiral galaxies host substantial amounts of molecular gas and have ongoing star formation activity with respect to elliptical galaxies lacking star formation. Molecular emission lines are used to probe the internal properties of molecular gas clouds where stars are born and die. Carbon monoxide (CO) is easily detectable in the interstellar medium (ISM) of galaxies. In this research, we probe the physics of the gas clouds at multiple positions in disc galaxies NGC 5055 (M63) and NGC 3627 (M66) using four CO transitions and their line ratios. 12CO(J=1-0) is the brightest across the disc of both galaxies compared to the other lines, i.e., 12CO(J=2-1, J=3-2) and 13CO(J=1-0). The CO intensities show a decrease from the center of the galaxies to the outskirts. However, NGC 3627 shows a rather irregular decrease pattern compared to NGC 5055. The CO line ratios show an increase up to a distance from the center and start to decrease. Although NGC 5055 shows a similar variation in the line ratios on each side of the disc, NGC 3627 has an opposite trend on either side. Therefore, the ISM could have different temperatures, opacity, densities, and levels of star formation in different regions of the galaxy’s disc. Our results indicate that the line ratios found at the center of both galaxies are different. The difference could be the result of the bar-driven gas accumulation in the center of NGC 3627. The line ratios in the center of NGC 5055 are within the range found for the centers of other spiral and active galaxies in the literature, but the ratios in the center of NGC 3627 are relatively lower.

An empirical view of the extended atmosphere and inner envelope of the asymptotic giant branch star R Doradus

Context. The mass loss experienced on the asymptotic giant branch (AGB) at the end of the lives of low- and intermediate-mass stars is widely accepted to rely on radiation pressure acting on newly formed dust grains. Dust formation happens in the extended atmospheres of these stars, where the density, velocity, and temperature distributions are strongly affected by convection, stellar pulsation, and heating and cooling processes. The interaction between these processes and how that affects dust formation and growth is complex. Hence, characterising the extended atmospheres empirically is paramount to advance our understanding of the dust formation and wind-driving processes. Aims. We aim to determine the density, temperature, and velocity distributions of the gas in the extended atmosphere of the AGB star R Dor. Methods. We acquired observations using ALMA towards R Dor to study the gas through molecular line absorption and emission. We modelled the observed 12CO v = 0, J = 2 − 1, v = 1, J = 2 − 1, and 3 − 2 and 13CO v = 0, J = 3 − 2 lines using the 3D radiative transfer code LIME to determine the density, temperature, and velocity distributions up to a distance of four times the radius of the star at sub-millimetre wavelengths. Results. The high angular resolution of the sub-millimetre maps allows for even the stellar photosphere to be spatially resolved. By analysing the absorption against the star, we infer that the innermost layer in the near-side hemisphere is mostly falling towards the star, while gas in the layer above that seems to be mostly outflowing. Interestingly, the high angular resolution of the ALMA Band 7 observations reveal that the velocity field of the gas seen against the star is not homogenous across the stellar disc. The gas temperature and density distributions have to be very steep close to the star to fit the observed emission and absorption, but they become shallower for radii larger than ∼1.6 times the stellar sub-millimetre radius. While the gas mass in the innermost region is hundreds of times larger than the mass lost on average by R Dor per pulsation cycle, the gas densities just above this region are consistent with those expected based on the mass-loss rate and expansion velocity of the large-scale outflow. Our fits to the line profiles require the velocity distribution on the far side of the envelope to be mirrored, on average, with respect to that on the near side. Using a sharp absorption feature seen in the CO v = 0, J = 2 − 1 line, we constrained the standard deviation of the stochastic velocity distribution in the large-scale outflow to be ≲0.4 km s−1. We characterised two blobs detected in the CO v = 0, J = 2 − 1 line and found densities substantially larger than those of the surrounding gas. The two blobs also display expansion velocities that are high relative to that of the large-scale outflow. Monitoring the evolution of these blobs will lead to a better understanding of the role of these structures in the mass-loss process of R Dor.

Disk Wind Feedback from High-mass Protostars. III. Synthetic CO Line Emission

Abstract To test theoretical models of massive star formation it is important to compare their predictions with observed systems. To this end, we conduct CO molecular line radiative transfer post-processing of 3D magnetohydrodynamic simulations of various stages in the evolutionary sequence of a massive protostellar core, including its infall envelope and disk wind outflow. Synthetic position–position–velocity cubes of various transitions of 12CO, 13CO, and C18O emission are generated. We also carry out simulated Atacama Large Millimeter/submillimeter Array (ALMA) observations of this emission. We compare the mass, momentum, and kinetic energy estimates obtained from molecular lines to the true values, finding that the mass and momentum estimates can have uncertainties of up to a factor of 4. However, the kinetic energy estimated from molecular lines is more significantly underestimated. Additionally, we compare the mass outflow rate and momentum outflow rate obtained from the synthetic spectra with the true values. Finally, we compare the synthetic spectra with real examples of ALMA-observed protostars and determine the best-fitting protostellar masses and outflow inclination angles. We then calculate the mass outflow rate and momentum outflow rate for these sources, finding that both rates agree with theoretical protostellar evolutionary tracks.

Dark Dragon Breaks Magnetic Chain: Dynamical Substructures of IRDC G28.34 Form in Supported Environments

We have comprehensively studied the multiscale physical properties of the massive infrared dark cloud G28.34 (the Dragon cloud) with dust polarization and molecular line data from Planck, FCRAO-14 m, James Clerk Maxwell Telescope, and Atacama Large Millimeter/submillimeter Array. We find that the averaged magnetic fields of clumps tend to be either parallel with or perpendicular to the cloud-scale magnetic fields, while the cores in clump MM4 tend to have magnetic fields aligned with the clump fields. Implementing the relative orientation analysis (for magnetic fields, column density gradients, and local gravity), velocity gradient technique, and modified Davis–Chandrasekhar–Fermi analysis, we find that G28.34 is located in a trans-to-sub-Alfvénic environment; the magnetic field is effectively resisting gravitational collapse in large-scale diffuse gas, but is distorted by gravity within the cloud and affected by star formation activities in high-density regions, and the normalized mass-to-flux ratio tends to increase with increasing density and decreasing radius. Considering the thermal, magnetic, and turbulent supports, we find that the environmental gas of G28.34 is in a supervirial (supported) state, the infrared dark clumps may be in a near-equilibrium state, and core MM4-core4 is in a subvirial (gravity-dominant) state. In summary, we suggest that magnetic fields dominate gravity and turbulence in the cloud environment at large scales, resulting in relatively slow cloud formation and evolution processes. Within the cloud, gravity could overwhelm both magnetic fields and turbulence, allowing local dynamical star formation to happen.

Molecular Gas Tracers in Young and Old Protoplanetary Disks

Molecular emission is used to investigate both the physical and chemical properties of protoplanetary disks. Therefore, to derive disk properties accurately, we need a thorough understanding of the behavior of the molecular probes upon which we rely. Here we investigate how the molecular line emission of N2H+, HCO+, HCN, and C18O compare to other measured quantities in a set of 20 protoplanetary disks. Overall, we find positive correlations between multiple line fluxes and the disk dust mass and radius. We also generally find strong positive correlations between the line fluxes of different molecular species. However, some disks do show noticeable differences in the relative fluxes of N2H+, HCO+, HCN, and C18O. These differences occur even within a single star-forming region. This results in a potentially large range of different disk masses and chemical compositions for systems of similar age and birth environment. While we make preliminary comparisons of molecular fluxes across different star-forming regions, more complete and uniform samples are needed in the future to search for trends with birth environment or age.

Early Planet Formation in Embedded Disks (eDisk). XIII. Aligned Disks with Nonsettled Dust around the Newly Resolved Class 0 Protobinary R CrA IRAS 32

Young protostellar binary systems, with expected ages less than ∼105 yr, are little modified since birth, providing key clues to binary formation and evolution. We present a first look at the young, Class 0 binary protostellar system R CrA IRAS 32 from the Early Planet Formation in Embedded Disks ALMA large program, which observed the system in the 1.3 mm continuum emission, 12CO (2−1), 13CO (2−1), C18O (2−1), SO (65−54), and nine other molecular lines that trace disks, envelopes, shocks, and outflows. With a continuum resolution of ∼0.″03 (∼5 au, at a distance of 150 pc), we characterize the newly discovered binary system with a separation of 207 au, their circumstellar disks, and a circumbinary disklike structure. The circumstellar disk radii are 26.9 ± 0.3 and 22.8 ± 0.3 au for sources A and B, respectively, and their circumstellar disk dust masses are estimated as 22.5 ± 1.1 M ⊕ and 12.4 ± 0.6 M ⊕, respectively. The circumstellar disks and the circumbinary structure have well-aligned position angles and inclinations, indicating formation in a smooth, ordered process such as disk fragmentation. In addition, the circumstellar disks have a near/far-side asymmetry in the continuum emission, suggesting that the dust has yet to settle into a thin layer near the midplane. Spectral analysis of CO isotopologues reveals outflows that originate from both of the sources and possibly from the circumbinary disklike structure. Furthermore, we detect Keplerian rotation in the 13CO isotopologues toward both circumstellar disks and likely Keplerian rotation in the circumbinary structure; the latter suggests that it is probably a circumbinary disk.

ExoMol line lists – LVIII. High-temperature molecular line list of carbonyl sulphide (OCS)

ABSTRACT A new molecular line list covering wavelengths λ > 1 μm (the 0–10 000 cm−1 range) for the main isotopologue of carbonyl sulphide 16O12C32S is presented. The OCS line list, named OYT8, contains almost 2.5 billion transitions between 2.4 million rotation-vibration energy levels with the total angular momentum up to J = 223. It is suitable for high-temperature environments up to T = 2000 K. Line list calculations were performed with the variational nuclear motion code trove in conjunction with a highly accurate, empirically refined potential energy surface and a newly computed ab initio dipole moment surface of OCS. The OYT8 line list is adapted for high-resolution applications by replacing computed energy levels with empirically derived values of OCS where available. Comparisons of the OYT8 line list with other OCS line lists and spectra yields excellent agreement for both strong and weak spectroscopic bands. The increased coverage of the OYT8 line list and the many new spectral features that are available will greatly facilitate the future observation of OCS on exoplanets. Carbonyl sulphide joins a growing number of sulphur-bearing molecules available from the ExoMol database. The OYT8 line list along with the associated temperature- and pressure-dependent molecular opacities can be downloaded from www.exomol.com and the CDS astronomical database.

Circumnuclear Multiphase Gas in the Circinus Galaxy. VI. Detectability of Molecular Inflow and Atomic Outflow

Recent submillimeter observations have revealed signs of parsec-scale molecular inflow and atomic outflow in the nearest Seyfert 2 galaxy, the Circinus galaxy. To verify the gas kinematics suggested by these observations, we performed molecular and atomic line transfer calculations based on a physics-based 3D radiation-hydrodynamic model, which has been compared with multiwavelength observations in this paper series. The major-axis position–velocity diagram (PVD) of CO(3–2) reproduces the observed faint emission at the systemic velocity, and our calculations confirm that this component originates from failed winds falling back to the disk plane. The minor-axis PVD of [C i](3 P 1–3 P 0), when created using only the gas with positive radial velocities, presents a sign of blueshifted and redshifted offset peaks similar to those in the observation, suggesting that the observed peaks indeed originate from the outflow, but that the model may lack outflows as strong as those in the Circinus galaxy. Similar to the observed HCN(3–2), the similar dense-gas tracer HCO+(3–2) can exhibit nuclear spectra with inverse P-Cygni profiles with ∼0.5 pc beams, but the line shape is azimuthally dependent. The corresponding continuum absorbers are inflowing clumps at 5–10 pc from the center. To detect significant absorption with a high probability, the inclination must be fairly edge-on (≳85°), and the beam size must be small (≲1 pc). These results suggest that HCN or HCO+ and [C i] lines are effective for observing parsec-scale inflows and outflows, respectively.

Astrochemistry of the Molecular Gas in Dusty Star-Forming Galaxies at the Cosmic Noon

Far-infrared and submillimeter observations have established the fundamental role of dust-obscured star formation in the assembly of stellar mass over the past ∼12 billion years. At z = 2–4, the so-called “cosmic noon”, the bulk of star formation is enshrouded in dust, and dusty star-forming galaxies (DSFGs) contain ∼50% of the total stellar mass density. Star formation occurs in dense molecular clouds, and is regulated by a complex interplay between all the ISM components that contribute to the energy budget of a galaxy: gas, dust, cosmic rays, interstellar electromagnetic fields, gravitational field, and dark matter. Molecular gas is the actual link between star-forming gas and its complex environment: much of what we know about star formation comes from observations of molecular line emissions. They provide by far the richest information about the star formation process. However, their interpretation requires complex modeling of the astrochemical networks which regulate molecular formation and establish molecular abundances in a cloud, and a modeling of the physical conditions of the gas in which molecular energy levels become populated. This paper critically reviews the main astrochemical parameters needed to obtain predictions about molecular signals in DSFGs. Molecular lines can be very bright compared to the continuum emission, but radiative transfer models are required to properly interpret the observed brightness. We review the current knowledge and the open questions about the interstellar medium of DSFGs, outlining the key role of molecular gas as a tracer and shaper of the star formation process.

The C[formula omitted] state of potassium dimer revisited: An extensive study by polarisation labelling spectroscopy method

The polarisation labelling spectroscopy technique was applied to study the C1Πu← X1Σg+ band system in potassium dimer. About 1100 new rotationally resolved molecular lines were measured in the 22100–24100 cm−1 spectral range. Perturbations of the lowest vibrational levels of the C state were localised and their origin discussed. A set of Dunham coefficients was deduced to fit the unperturbed levels of the C1Πu state with 0≤v≤38 and 18≤J≤101 and the potential energy curve of the state was constructed.

Recovering the gas properties of protoplanetary disks through parametric visibility modeling: MHO 6

The composition and distribution of the gas in a protoplanetary disk plays a key role in shaping the outcome of the planet formation process. Observationally, the recovery of information such as the emission height and brightness temperature from interferometric data is often limited by the imaging processes. To overcome the limitations of image-reconstruction when analyzing gas emission from interferometric observations, we have introduced a parametric model to fit the main observable properties of the gaseous disk component in the visibility plane. This approach is also known as parametric visibility modeling. We applied our parametric visibility modeling to the gas brightness distribution of the molecular line emission from 12CO J=3-2 and 13CO J=3-2 in the disk around MHO\,6, a very-low-mass star in the Taurus star-forming Region. To improve the flux fidelity of our parametric models, we combined models with different pixel resolution before the computation of their visibilities, referred to as ``nesting images.'' When we apply our parametric visibility modeling to MHO\,6, with independent fits to the emission from its CO isopotologues, the models return the same consistent results for the stellar mass, disk geometry, and central velocity. The surface height and brightness temperature distribution are also recovered. When compared to other disks, MHO\,6 surface height is among the most elevated surfaces, consistent with the predictions for disks around very-low-mass stars. This work demonstrates the feasibility of running rapidly iterable parametric visibility models in moderate resolution and sensitivity interferometric observations. More importantly, this methodology opens the analysis of disk's gas morphology to observations where image-based techniques are unable to robustly operate, as in the case of the compact disk around MHO\,6.

300 million years apart: the extreme case of macromorphological skeletal convergence between deltocyathids and a turbinoliid coral (Anthozoa, Scleractinia).

The integration of morphological and molecular lines of evidence has enabled the family Deltocyathidae to be erected to accommodate Deltocyathus species that were previously ascribed to the family Caryophylliidae. However, although displaying the same morphological characteristics as other species of Deltocyathus , molecular data suggested that D. magnificus was phylogenetically distant from Deltocyathidae, falling within the family Turbinoliidae instead. To elucidate the enigmatic evolutionary history of this species and skeletal microstructural features, the phylogenetic relationships of Deltocyathidae and Turbinoliidae were investigated using nuclear ultraconserved and exon loci and complete mitochondrial genomes. Both nuclear and mitochondrial phylogenomic reconstructions confirmed the position of D. magnificus within turbinolids. Furthermore, a novel mitochondrial gene order was uncovered for Deltocyathidae species. This gene order was not present in Turbinoliidae or in D. magnificus that both have the scleractinian canonical gene order, further indicating the taxonomic utility of mitochondrial gene order. D. magnificus is therefore formally moved to the family Turbinoliidae and accommodated in a new genus (Dennantotrochus Kitahara, Vaga & Stolarski, gen. nov.). Surprisingly, turbinolids and deltocyathids do not differ in microstructural organisation of the skeleton that consists of densely packed, individualised rapid accretion deposits and thickening deposits composed of fibres perpendicular to the skeleton surface. Therefore, although both families are clearly evolutionarily divergent, macromorphological features indicate a case of skeletal convergence while these may still share conservative biomineralisation mechanisms. ZooBank: urn:lsid:zoobank.org:pub:5F1C0E25-3CC6-4D1F-B1F0-CD9D0014678E.

MeerKAT and ALMA view of the AGAL045.804−0.356 clump

Abstract This study presents a detailed analysis of the GAL045.804−0.356 massive star-forming clump. A high-angular resolution and sensitivity observations were conducted using MeerKAT at 1.28 GHz and ALMA interferometer at 1.3 mm. Two distinct centimetre radio continuum emissions (source A and source B) were identified within the clump. A comprehensive investigation was carried out on source A, the G45.804−0.355 star-forming region (SFR) due to its association with Extended Green Object (EGO), 6.7 GHz methanol maser and the spatial coincidence with the peak of the dust continuum emission at 870 μm. The ALMA observations revealed seven dense dust condensations (MM1 to MM7) in source A. The brightest (Sν ∼ 87 mJy) and massive main dense core, MM1, was co-located with the 6.7 GHz methanol maser. Explorations into the kinematics revealed gas motions characterised by a velocity gradient across the MM1 core. Furthermore, molecular line emission showed the presence of an extended arm-like structure, with a physical size of 0.25 pc × 0.18 pc (∼ 50000 au × 30000 au) at a distance of 7.3 kpc. Amongst these arms, two arms were prominently identified in both the dust continuum and some of the molecular lines. A blue-shifted absorption P-Cygni profile was seen in the H2CO line spectrum. The findings of this study are both intriguing and new, utilising data from MeerKAT and ALMA to investigate the characteristics of the AGAL45 clump. The evidence of spiral arms, the compact nature of the EGO and < 2 km s−1 velocity gradient are all indicative of G45.804−0.355 being oriented face-on.

Completing and Balancing Database Excerpted Chemical Reactions with a Hybrid Mechanistic-Machine Learning Approach.

Computer-aided synthesis planning (CASP) development of reaction routes requires an understanding of complete reaction structures. However, most reactions in the current databases are missing reaction coparticipants. Although reaction prediction and atom mapping tools can predict major reaction participants and trace atom rearrangements in reactions, they fail to identify the missing molecules to complete reactions. This is because these approaches are data-driven models trained on the current reaction databases, which comprise incomplete reactions. In this work, a workflow was developed to tackle the reaction completion challenge. This includes a heuristic-based method to identify balanced reactions from reaction databases and complete some imbalanced reactions by adding candidate molecules. A machine learning masked language model (MLM) was trained to learn from simplified molecular input line entry system (SMILES) sentences of these completed reactions. The model predicted missing molecules for the incomplete reactions, a workflow analogous to predicting missing words in sentences. The model is promising for the prediction of small- and middle-sized missing molecules in incomplete reaction records. The workflow combining both the heuristic and machine learning methods completed more than half of the entire reaction space.

[C ii] Emission in a Self-regulated Interstellar Medium

The [C ii] 157.74 μm fine-structure transition is one of the brightest and most well-studied emission lines in the far-infrared, produced in the interstellar medium (ISM) of galaxies. We study its properties in subparsec-resolution hydrodynamical simulations for an ISM patch with gas surface density of Σ g = 10 M ⊙ pc−2, coupled with time-dependent chemistry, far-ultraviolet dust and gas shielding, star formation, photoionization and supernova feedback, and full line radiative transfer. We find a [C ii]-to-H2 conversion factor that scales weakly with metallicity X[CII]=6.31×1019Z′0.17cm−2(Kkms−1)−1 , where Z′ is the normalized metallicity relative to solar. The majority of [C ii] originates from atomic gas with hydrogen number density n ∼ 10 cm−3. The [C ii] line intensity positively correlates with the star formation rate (SFR), with a normalization factor that scales linearly with metallicity. We find that this is broadly consistent with z ∼ 0 observations. As such, [C ii] is a good SFR tracer even in metal-poor environments where molecular lines might be undetectable. Resolving the clumpy structure of the dense (n = 10−103 cm−3) ISM is important, as it dominates [C ii] 157.74 μm emission. We compare our full radiative transfer computation with the optically thin limit and find that the [C ii] line becomes marginally optically thick only at supersolar metallicity for our assumed gas surface density.

Geometry-Based Molecular Generation With Deep Constrained Variational Autoencoder.

Finding target molecules with specific chemical properties plays a decisive role in drug development. We proposed GEOM-CVAE, a constrained variational autoencoder based on geometric representation for molecular generation with specific properties, which is protein-context-dependent. In terms of machine learning, it includes continuous feature embedding encoder and molecular generation decoder. Our key contribution is to propose an efficient geometric embedding method, including the spatial structure representations of drug molecule (converting the 3-D coordinates into image) and the geometric graph representations of protein target (modeling the protein surface as a mesh). The 3-D geometric information is vital to successful molecular generation, which is different from previous molecular generative methods based on 1-D or 2-D. Our model framework generates specific molecules in two phases, by first generating special image with molecular 3-D information to learn latent representations and generating molecules with constrained condition based on geometric graph convolution for specific protein and then inputting the generated structural molecules into a parser network for obtaining Simplified Molecular Input Line Entry System (SMILES) strings. Our model achieves competitive performance that implies its potential effectiveness to enable the exploration of the vast chemical space for drug discovery.

Discovery of Thionylimide, HNSO, in Space: The first N-, S-, and O-bearing Interstellar Molecule

We present the first detection in space of thionylimide (HNSO) toward the Galactic center molecular cloud G + 0.693-0.027, thanks to the superb sensitivity of an ultradeep molecular line survey carried out with the Yebes 40 m and IRAM 30 m telescopes. This molecule is the first species detected in the interstellar medium containing, simultaneously, N, S, and O. We have identified numerous K a = 0, 1, and 2 transitions belonging to HNSO covering from J up = 2 to J up =10, including several completely unblended features. We derive a molecular column density of N = (8 ± 1)×1013 cm−2, yielding a fractional abundance relative to H2 of ∼6 × 10−10, which is about ∼37 and ∼4.8 times less abundant than SO and SO2, respectively. Although there are still many unknowns in the interstellar chemistry of NSO-bearing molecules, we propose that HNSO is likely formed through the reaction of the NSO radical and atomic H on the surface of icy grains, with alternative routes also deserving exploration. Finally, HNSO appears as a promising link between N, S, and O interstellar chemistry, and its discovery paves the route to the detection of a new family of molecules in space.

Surveys of clumps, cores, and condensations in Cygnus-X

Context.Theories and models have suggested that circumstellar disks could channel material to the central protostar, and resist star formation feedback. Our current knowledge of the picture and role of disks around massive protostars is unclear because the observational evidence of these circumstellar disks is limited.Aims.To investigate whether disk-mediated accretion is the primary mechanism in high-mass star formation, we have established a survey of a large sample of massive dense cores within a giant molecular cloud.Methods.We used high angular resolution (~1.8″) observations with SMA to study the dust emission and molecular line emission of about 50 massive dense cores in Cygnus-X. At a typical distance of 1.4 kpc for Cygnus-X, these massive dense cores are resolved into ~2000 au condensations. We combined the CO outflow emission and gas kinematics traced by several high-density tracers to search for disk candidates.Results.We extracted hundreds of dust condensations from the SMA 1.3 mm dust continuum emission. The CO data show bipolar or unipolar outflow signatures toward 49 dust condensations. Among them, only 27 sources are detected in dense gas tracers, which reveals the gas kinematics, and nine sources show evidence of rotating envelopes, suggesting the existence of embedded accretion disks. The position-velocity diagrams along the velocity gradient of all rotating condensations suggest that four condensations are possible to host Keplerian-like disks.Conclusions.A detailed investigation of the 27 sources detected in dense gas tracers suggests that the nine disk candidates are at earlier evolutionary stages compared to the remaining 18 sources. Non-detection of rotating disks in our sample may be due to several factors, including an unknown inclination angle of the rotation axis and an early evolutionary stage of the central source, and the latter could be important, considering that young and powerful outflows could confuse the observational evidence for rotation. The detection rate of disk candidates in our sample is 1/3, which confirms that disk accretion is a viable mechanism for high-mass star formation, although it may not be the only one.

Interstellar Detection of O-protonated Carbonyl Sulfide, HOCS+

We present the first detection in space of O-protonated carbonyl sulfide (HOCS+), in the midst of an ultradeep molecular line survey toward the G+0.693-0.027 molecular cloud. From the observation of all K a = 0 transitions ranging from J lo = 2 to J lo = 13 of HOCS+ covered by our survey, we derive a column density of N = (9 ± 2) × 1012 cm−2, translating into a fractional abundance relative to H2 of ∼7 × 10−11. Conversely, the S-protonated HSCO+ isomer remains undetected, and we derive an upper limit to its abundance with respect to H2 of ≤3 × 10−11, a factor of ≥2.3 less abundant than HOCS+. We obtain an HOCS+/OCS ratio of ∼2.5 × 10−3, in good agreement with the prediction of astrochemical models. These models show that one of the main chemical routes to the interstellar formation of HOCS+ is likely the protonation of OCS, which appears to be more efficient at the oxygen end. Also, we find that high values of cosmic-ray ionization rates (10−15–10−14 s−1) are needed to reproduce the observed abundance of HOCS+. In addition, we compare the O/S ratio across different interstellar environments. G+0.693-0.027 appears as the source with the lowest O/S ratio. We find an HOCO+/HOCS+ ratio of ∼31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (∼37). This fact indicates that S is not significantly depleted within this cloud due to the action of large-scale shocks, unlike in other sources where S-bearing species remain trapped on icy dust grains.