arXiv subjects

R. Le Gal

Publications and source records attributed to R. Le Gal.

At least 19 recordsLinked to original sources

arXiv2026

Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya

The spatial distribution of the chemical reservoirs in protoplanetary disks is key to elucidate the composition of planets, especially habitable ones. However, the partitioning of the main elements among the refractory and volatile phases is still elusive. Key parameters such as the carbon-to-oxygen C/O elemental ratio and the ionization fraction remain poorly constrained, with the latter potentially orders of magnitude lower than in the interstellar medium. Moreover, the thermal structure of the gas is also poorly known, despite its deep influence on gas-phase chemistry. In this context, ortho-to-para ratios could provide selective and sensitive probes. Recent ALMA observations have measured the spatially resolved column density of ortho-and para-H2CO in the transition disk orbiting TW Hya and derived the radial profile of the ortho-to-para ratio. Yet, current disk models do not include the nuclear-spin-resolved chemistry required to interpret these observations. The present work aims to fill this gap, by combining a parametric disk physical model of TW Hya with the UGAN network, updated to include a comprehensive description of the nuclear-spin-resolved chemistry of formaldehyde. This new model reproduces the observed column density of H2CO to within a factor of 2, as well as the measured ortho-to-para ratio which varies from 1.5 in the outer disk to 3 inside 90au. In particular the low value of this ratio beyond 90au is well explained by our model. However, the statistical value of 3 measured below 70au cannot be reproduced, suggesting that additional processes involving ices may be involved. Our parameter space exploration shows that the abundance of H2CO is highly sensitive to the C/O elemental ratio and to the cosmic-ray ionization rate. Future observations of ortho-and para-H2CO, based on well selected rotational transitions, in a large sample of disks, appear highly desirable.

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arXiv2025

Anatomy of the Class I protostar L1489 IRS with NOEMA - II. A disk replenished by a massive streamer

(abridged) Streamers are newly identified channels that transport mass from large, molecular-cloud scales down to small, protoplanetary-disk scales. To better understand their impact on planet formation, it is essential to study their physical and chemical properties. In this framework, we aim to characterize the longest streamer identified in carbon chain emission within the Class I system L1489 IRS, connecting the nearby prestellar core L1489 to the young stellar object (YSO). We observed multiple transitions of C$_2$H, ortho-c-C$_3$H$_2$, and HC$_3$N in L1489 IRS with NOEMA and IRAM-30m at 3mm and 2mm. Using a variety of radiative transfer methods, including a hyperfine structure (HFS) fitting, rotational diagrams, and proposing a new self-consistent Markov chain Monte Carlo (MCMC) approach combined with the non-LTE RADEX code, we derived the column densities and abundances of those molecules, as well as the H$_2$ number density along the streamer. This enabled us to estimate its mass, infall rate, and its impact on the {star+disk} system's mass. We found lower limits on the streamer mass of $\geq(4.67-18.3)\times10^{-3}$ $M_\odot$ (i.e., $\geq0.65-2.57$ times the current disk mass) and an infall rate of $\geq(1.94-7.57)\times10^{-7}$ $M_\odot$ yr$^{-1}$, where the ranges correspond to the different molecular tracers. These values are consistent with those derived in similar Class I objects. This suggests that the disk could be fully replenished by streamer material. Given its mass, the streamer is likely at the origin of the external warped disk seen in this system, as predicted by numerical simulations. Moreover, the first investigations based on the C$_2$H/c-C$_3$H$_2$ and HC$_3$N/c-C$_3$H$_2$ abundance ratios suggest that the streamer chemistry may be inherited from the core. These results suggest, for the first time, that the chemical composition of a Class I object is [...]

astro-ph.SR↗

arXiv2025

PDRs4All XI. Detection of infrared CH$^+$ and CH$_3^+$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping

The methylidyne cation (CH$^+$) and the methyl cation (CH$_3^+$) are building blocks of organic molecules, yet their coupled formation and excitation mechanisms remain mainly unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Our goal is to use the first detection of CH$^+$ and CH$_3^+$ rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506, irradiated by the Trapezium cluster, to probe their formation and excitation mechanisms and constrain the physico-chemical conditions. We use spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments to study the CH$^+$ and CH$_3^+$ spatial distribution at very small scales, and compare it to excited H$_2$ emission. CH$^+$ and CH$_3^+$ emissions originate from the same region as highly excited H$_2$. Our comparison between the Bar and d203-506 reveals that both CH$^+$ and CH$_3^+$ excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH$^+$ and CH$_3^+$ rovibrational lines is around $T$ ~ 1500 K in the Bar and $T$ ~ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1 % of the total known (CH$^+$) and expected (CH$_3^+$) column densities. These results show that CH$^+$ and CH$_3^+$ level populations strongly deviate from ETL. CH$^+$ rovibrational emission can be explained by chemical formation pumping with excited H$_2$ via C$^+$ + H$_2^*$ = CH$^+$ + H. These results support a gas phase formation pathway of CH$^+$ and CH$_3^+$ via successive hydrogen abstraction reactions. However, we do not find any evidence of CH$_2^+$ emission in the JWST spectrum. Finally, observed CH$^+$ intensities coupled with chemical formation pumping model provide a diagnostic tool to trace the local density.

astro-ph.GA↗

arXiv2024

Gas phase Elemental abundances in Molecular cloudS (GEMS). X. Observational effects of turbulence on the chemistry of molecular clouds

(Abridged) We explore the chemistry of the most abundant C, O, S, and N bearing species in molecular clouds, in the context of the IRAM 30 m Large Programme Gas phase Elemental abundances in Molecular Clouds (GEMS). In this work, we aim to assess the limitations introduced in the observational works when a uniform density is assumed along the line of sight for fitting the observations, developing a very simple numerical model of a turbulent box. We perform a MHD simulation in order to reproduce the turbulent steady-state of a turbulent box with properties typical of a molecular filament before collapse. We post-process the results of the MHD simulation with a chemical code to predict molecular abundances, and then post-process this cube with a radiative transfer code to create synthetic emission maps for a series of rotational transitions observed during the GEMS project. From the chemical point of view, we find that turbulence produces variations on the predicted abundances, but they are more or less critical depending on the chosen transition and the chemical age. When compared to real observations, the results from the turbulent simulation provides a better fit than when assuming a uniform gas distribution along the line of sight. In the view of our results, we conclude that taking into account turbulence when fitting observations might significantly improve the agreement with model predictions. This is especially important for sulfur bearing species that are very sensitive to the variations of density produced by turbulence at early times (0.1 Myr). The abundance of CO is also quite sensitive to turbulence when considering the evolution beyond a few 0.1 Myr.

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arXiv2024

FAUST XVII: Super deuteration in the planet forming system IRS 63 where the streamer strikes the disk

Recent observations suggest that planets formation starts early, in protostellar disks of $\le10^5$ yrs, which are characterized by strong interactions with the environment, e.g., through accretion streamers and molecular outflows. To investigate the impact of such phenomena on disk physical and chemical properties it is key to understand what chemistry planets inherit from their natal environment. In the context of the ALMA Large Program Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST), we present observations on scales from ~1500 au to ~60 au of H$_2$CO, HDCO, and D$_2$CO towards the young planet-forming disk IRS~63. H$_2$CO probes the gas in the disk as well as in a large scale streamer (~1500 au) impacting onto the South-East (SE) disk side. We detect for the first time deuterated formaldehyde, HDCO and D$_2$CO, in a planet-forming disk, and HDCO in the streamer that is feeding it. This allows us to estimate the deuterium fractionation of H$_2$CO in the disk: [HDCO]/[H$_2$CO]$\sim0.1-0.3$ and [D$_2$CO]/[H$_2$CO]$\sim0.1$. Interestingly, while HDCO follows the H$_2$CO distribution in the disk and in the streamer, the distribution of D$_2$CO is highly asymmetric, with a peak of the emission (and [D]/[H] ratio) in the SE disk side, where the streamer crashes onto the disk. In addition, D$_2$CO is detected in two spots along the blue- and red-shifted outflow. This suggests that: (i) in the disk, HDCO formation is dominated by gas-phase reactions similarly to H$_2$CO, while (ii) D$_2$CO was mainly formed on the grain mantles during the prestellar phase and/or in the disk itself, and is at present released in the gas-phase in the shocks driven by the streamer and the outflow. These findings testify on the key role of streamers in the build-up of the disk both concerning the final mass available for planet formation and its chemical composition.

astro-ph.EP↗

arXiv2024

FAUST XIII. Dusty cavity and molecular shock driven by IRS7B in the Corona Australis cluster

The origin of the chemical diversity observed around low-mass protostars probably resides in the earliest history of these systems. We aim to investigate the impact of protostellar feedback on the chemistry and grain growth in the circumstellar medium of multiple stellar systems. In the context of the ALMA Large Program FAUST, we present high-resolution (50 au) observations of CH$_3$OH, H$_2$CO, and SiO and continuum emission at 1.3 mm and 3 mm towards the Corona Australis star cluster. Methanol emission reveals an arc-like structure at $\sim$1800 au from the protostellar system IRS7B along the direction perpendicular to the major axis of the disc. The arc is located at the edge of two elongated continuum structures that define a cone emerging from IRS7B. The region inside the cone is probed by H$_2$CO, while the eastern wall of the arc shows bright emission in SiO, a typical shock tracer. Taking into account the association with a previously detected radio jet imaged with JVLA at 6 cm, the molecular arc reveals for the first time a bow shock driven by IRS7B and a two-sided dust cavity opened by the mass-loss process. For each cavity wall, we derive an average H$_2$ column density of $\sim$7$\times$10$^{21}$ cm$^{-2}$, a mass of $\sim$9$\times$10$^{-3}$ M$_\odot$, and a lower limit on the dust spectral index of $1.4$. These observations provide the first evidence of a shock and a conical dust cavity opened by the jet driven by IRS7B, with important implications for the chemical enrichment and grain growth in the envelope of Solar System analogues.

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arXiv2024

Anatomy of the Class I protostar L1489 IRS with NOEMA -- I. Disk, streamers, outflow(s) and bubbles at 3mm

Over the past few years, chemical studies have revealed multiple structures in the vicinity of young stellar objects (YSOs). It has become evident that specific physical conditions are associated with the emission of particular molecular lines, allowing us to use molecular probes of the YSO physics. Consequently, chemical surveys are now necessary to fully constrain the origin of the observed structures. Several surveys have been conducted to explore the chemistry of YSOs, focusing on Class 0 and Class II objects. However, our knowledge of intermediate objects, that are Class I objects, remains limited. To bridge the gap and establish the relationship between observed structures and molecular line emission at the Class I evolutionary stage, we investigate the spatial distribution of key molecular gas species in the low-mass Class I protostar L1489 IRS (IRAS 04016+2610), a source part of the ChemYSO survey. We performed a 3mm line survey at high spatial and high spectral resolution using the NOEMA interferometer and the IRAM-30m telescope. We present here the ten brightest lines of our survey, in which we identified a new ~ 3 000 au long streamer in HC3N, C2H, and c-C3H2 emission, likely associated with more localized accretion shocks probed in SO. In addition, two ~ 10 000 au bubbles are seen with the dense molecular tracers HCO+, CS, and HCN around the YSO. Additionally, potential indicators of a second outflow appear in CS and HCN emission, but its nature remains to be confirmed. The late infall identified at large scales may originate from the nearby prestellar core L1489 and is likely responsible for the formation of an external warped disk in this system. The detection of a potential second outflow could be the direct evidence of a binary system. Finally, we hypothesize that the bubbles may result from the magnetic pressure as observed in numerical simulations.

astro-ph.SR↗

arXiv2024

Grain growth and its chemical impact in the first hydrostatic core phase

The first hydrostatic core (FHSC) phase is a brief stage in the protostellar evolution that is difficult to detect. Our goal is to characterize the chemical evolution of gas and dust during the formation of the FHSC. Moreover, we are interested in analyzing, for the first time with 3D magnetohydrodynamic (MHD) simulations, the role of grain growth in its chemistry. We postprocessed $2\times10^{5}$ tracer particles from a $\texttt{RAMSES}$ non-ideal MHD simulation using the codes $\texttt{NAUTILUS}$ and $\texttt{SHARK}$ to follow the chemistry and grain growth throughout the simulation. A great chemical inheritance is seen, as gas-phase abundances of most of the C, O, N, and S reservoirs in the hot corino at the end of the simulation match the ice-phase abundances from the prestellar phase. Additionally, interstellar complex organic molecules (iCOMs) such as methyl formate, acetaldehyde, and formamide are formed during the warm-up process. The typical grain size in the hot corino $(n_{\rm H}>10^{11}\ {\rm cm^{-3}})$ increases forty-fold during the last 30 kyr, with negligible effects on its chemical composition. At moderate densities $(10^{10}<n_{\rm H}<10^{11}\ {\rm cm^{-3}})$ and cool temperatures $15<T<50$ K, increasing grain sizes delay molecular depletion. Finally, at low densities $(n_{\rm H}\sim10^{7}\ {\rm cm^{-3}})$, grains do not grow significantly. We also compared our results with a two-step model that reproduces well the abundances of C and O reservoirs, but not the N and S reservoirs. We conclude that the chemical composition of the FHSC is heavily determined by that of the parent prestellar core, chemo-MHD computations are needed for an accurate prediction of the abundances of the main N and S elemental reservoirs, and that the impact of grain growth in moderately dense areas delaying depletion permits the use of abundance ratios as grain growth proxies.

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arXiv2024

Protonated acetylene in the z=0.89 molecular absorber toward PKS1830-211

We report the first interstellar identification of protonated acetylene, C2H3+, a fundamental hydrocarbon, in the z=0.89 molecular absorber toward the gravitationally lensed quasar PKS1830-211. The molecular species is identified from clear absorption features corresponding to the 2_12-1_01 (rest frequency 494.034 GHz) and 1_11-0_00 (431.316 GHz) ground-state transitions of ortho and para forms of C2H3+, respectively, in ALMA spectra toward the southwestern image of PKS1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C2H3+ column density of 2 x 10^12 cm^-2 and an abundance of 10^-10 compared to H_2. However, formation pumping could affect the population of metastable states, yielding a C2H3+ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C2H3+, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C2H3+ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.

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arXiv2022

Hot methanol in the [BHB2007] 11 protobinary system: hot corino versus shock origin? : FAUST V

Methanol is a ubiquitous species commonly found in the molecular interstellar medium. It is also a crucial seed species for the building-up of the chemical complexity in star forming regions. Thus, understanding how its abundance evolves during the star formation process and whether it enriches the emerging planetary system is of paramount importance. We used new data from the ALMA Large Program FAUST (Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars) to study the methanol line emission towards the [BHB2007] 11 protobinary system (sources A and B), where a complex structure of filaments connecting the two sources with a larger circumbinary disk has been previously detected. Twelve methanol lines have been detected with upper energies in the range [45-537] K along with one 13CH3OH transition. The methanol emission is compact and encompasses both protostars, separated by only 28 au and presents three velocity components, not spatially resolved by our observations, associated with three different spatial regions, with two of them close to 11B and the third one associated with 11A. A non-LTE radiative transfer analysis of the methanol lines concludes that the gas is hot and dense and highly enriched in methanol with an abundance as high as 1e-5. Using previous continuum data, we show that dust opacity can potentially completely absorb the methanol line emission from the two binary objects. Although we cannot firmly exclude other possibilities, we suggest that the detected hot methanol is resulting from the shocked gas from the incoming filaments streaming towards [BHB2007] 11 A and B, respectively. Higher spatial resolution observations are necessary to confirm this hypothesis.

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arXiv2021

H$_2$S observations in young stellar disks in Taurus

Context. Studying gas chemistry in protoplanetary disks is key to understanding the process of planet formation. Sulfur chemistry in particular is poorly understood in interstellar environments, and the location of the main reservoirs remains unknown. Protoplanetary disks in Taurus are ideal targets for studying the evolution of the composition of planet forming systems. Aims. We aim to elucidate the chemical origin of sulfur-bearing molecular emission in protoplanetary disks, with a special focus on H$_2$S emission, and to identify candidate species that could become the main molecular sulfur reservoirs in protoplanetary systems. Methods. We used IRAM 30m observations of nine gas-rich young stellar objects (YSOs) in Taurus to perform a survey of sulfur-bearing and oxygen-bearing molecular species. In this paper we present our results for the CS 3-2 ($ν_0$ = 146.969 GHz), H$_2$CO 2$_{11}$-1$_{10}$ ($ν_0$ = 150.498 GHz), and H$_2$S 1$_{10}$-1$_{01}$ ($ν_0$ = 168,763 GHz) emission lines. Results. We detected H$_2$S emission in four sources out of the nine observed, significantly increasing the number of detections toward YSOs. We also detected H$_2$CO and CS in six out of the nine. We identify a tentative correlation between H$_2$S 1$_{10}$-1$_{01}$ and H$_2$CO 2$_{11}$-1$_{10}$ as well as a tentative correlation between H$_2$S 1$_{10}$-1$_{01}$ and H$_2$O 8$_{18}$-7$_{07}$. By assuming local thermodynamical equilibrium, we computed column densities for the sources in the sample, with N(o-H$_2$S) values ranging between $2.6\times10^{12}$ cm$^{-2}$ and $1.5\times10^{13}$ cm$^{-2}$.

astro-ph.SR↗

arXiv2021

Chemical compositions of five Planck cold clumps

Aims: Interstellar molecules form early in the evolutionary sequence of interstellar material that eventually forms stars and planets. To understand this evolutionary sequence, it is important to characterize the chemical composition of its first steps. Methods: In this paper, we present the result of a 2 and 3 mm survey of five cold clumps identified by the Planck mission. We carried out a radiative transfer analysis on the detected lines in order to put some constraints on the physical conditions within the cores and on the molecular column densities. We also performed chemical models to reproduce the observed abundances in each source using the gas-grain model Nautilus. Results: Twelve molecules were detected: H2CO, CS, SO, NO, HNO, HCO+, HCN, HNC, CN, CCH, CH3OH, and CO. Here, CCH is the only carbon chain we detected in two sources. Radiative transfer analyses of HCN, SO, CS, and CO were performed to constrain the physical conditions of each cloud with limited success. The sources have a density larger than $10^4$ cm$^{-3}$ and a temperature lower than 15 K. The derived species column densities are not very sensitive to the uncertainties in the physical conditions, within a factor of 2. The different sources seem to present significant chemical differences with species abundances spreading over one order of magnitude. The chemical composition of these clumps is poorer than the one of Taurus Molecular Cloud 1 Cyanopolyyne Peak (TMC-1 CP) cold core. Our chemical model reproduces the observational abundances and upper limits for 79 to 83\% of the species in our sources. The "best" times for our sources seem to be smaller than those of TMC-1, indicating that our sources may be less evolved and explaining the smaller abundances and the numerous non-detections. Also, CS and HCN are always overestimated by our models.

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arXiv2021

Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.

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arXiv2020

Gas-phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking the CS chemistry: the CS+O reaction

CS is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. Chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. The CS+O -> CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Here we calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. We performed ab initio calculations to obtain the three lowest PES of the CS+O system. These PESs are used to study the reaction dynamics, using several methods to eventually calculate the CS+O thermal reaction rates. We compare the results of our theoretical calculations for 150-400 K with those obtained in the laboratory. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, which is consistent with the extrapolation of experimental data using the Arrhenius expression. We use the updated chemical network to model the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.

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arXiv2020

Gas kinematics of key prebiotic molecules in GV Tau N revealed with an ALMA, PdBI, and Herschel synergy

A large effort has been made to detect warm gas in the planet formation zone of circumstellar discs using space and ground-based near infrared facilities. GV Tau N, the most obscured component of the GV Tau system, is an outstanding source, being one of the first targets detected in HCN and the only one detected in CH$_4$ so far. Although near infrared observations have shed light on its chemical content, the physical structure and kinematics of the circumstellar matter remained unknown. We use interferometric images of the HCN 3-2 and $^{13}$CO 3-2 lines, and far-IR observations of $^{13}$CO, HCN, CN and H$_2$O transitions to discern the morphology, kinematics, and chemistry of the dense gas close to the star. These observations constitute the first detection of H$_2$O towards GV Tau N. Moreover, ALMA high spatial resolution (~ 7 au) images of the continuum at 1.1 mm and the HCN 3-2 line resolve different gas components towards GV Tau N, a gaseous disc with R~25 au, an ionized jet, and one (or two) molecular outflows. The asymmetric morphology of the gaseous disc shows that it has been eroded by the jet. All observations can be explained if GV Tau N is binary, and the primary component has a highly inclined individual disc relative to the circumbinary disc. We discuss the origin of the water and the other molecules emission according to this scenario. In particular, we propose that the water emission would come from the disrupted gaseous disc and the molecular outflows.

astro-ph.SR↗

arXiv2020

Gas phase Elemental abundances in Molecular cloudS (GEMS). II. On the quest for the sulphur reservoir in molecular clouds: the $H_{2}S$ case

Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium, and the identity of the main sulphur reservoir is still an open question. Our goal is to investigate the H$_{2}$S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Using millimeter observations of CS, SO, H$_{2}$S, and their isotopologues, we determine the physical conditions and H$_{2}$S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model Nautilus is then used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H$_2$S abundance. Our model shows that chemical desorption is the main source of gas-phase H$_2$S in dark cores. The measured H$_{2}$S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when $n_{\rm H}>2\times10^{4}$. This change in the desorption rate is consistent with the formation of thick H$_2$O and CO ice mantles on grain surfaces. The observed SO and H$_2$S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of $5-10$. Along the three cores, atomic S is predicted to be the main sulphur reservoir. We conclude that the gaseous H$_2$S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H$_2$S. The behavior of the observed H$_{2}$S abundance suggests a changing desorption efficiency, which would probe the snowline in these cores. Our model, however, overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.

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arXiv2019

Chlorine-bearing molecules in molecular absorbers at intermediate redshifts

We use observations of chlorine-bearing species in molecular absorbers at intermediate redshifts to investigate chemical properties and $^{35}$Cl/$^{37}$Cl isotopic ratios in the absorbing sightlines. Chloronium (H$_2$Cl$^+$) is detected along three independent lines of sight in the z=0.89 and z=0.68 molecular absorbers located in front of the lensed quasars PKS 1830-211 and B 0218+357, respectively. Hydrogen chloride (HCl) was observed only toward PKS 1830-211, and is found to behave differently from H$_2$Cl$^+$. It is detected in one line of sight with an abundance ratio [H$_2$Cl$^+$]/[HCl] $\sim 1$, but remains undetected in the other, more diffuse, line of sight, with a ratio [H$_2$Cl$^+$]/[HCl]~$>17$. The absorption profiles of these two chlorine-bearing species are compared to other species and discussed in terms of the physical properties of the absorbing gas. Our findings are consistent with the picture emerging from chemical models where different species trace gas with different molecular hydrogen fraction. The $^{35}$Cl/$^{37}$Cl isotopic ratios are measured in the different lines of sight and are discussed in terms of stellar nucleosynthesis.

astro-ph.GA↗

arXiv2019

Abundances of sulphur molecules in the Horsehead nebula. First NS+ detection in a photodissociation region

Aims. Our goal is to complete the inventory of S-bearing molecules and their abundances in the prototypical photodissociation region (PDR) the Horsehead nebula to gain insight into sulphur chemistry in UV irradiated regions. Based on the WHISPER millimeter (mm) line survey, our goal is to provide an improved and more accurate description of sulphur species and their abundances towards the core and PDR positions in the Horsehead. Methods. The Monte Carlo Markov chain (MCMC) methodology and the molecular excitation and radiative transfer code RADEX were used to explore the parameter space and determine physical conditions and beam-averaged molecular abundances. Results. A total of 13 S-bearing species (CS, SO, SO2, OCS, H2CS - both ortho and para - HDCS, C2S, HCS+, SO+, H2S, S2H, NS and NS+) have been detected in the two targeted positions. This is the first detection of SO+ in the Horsehead and the first detection of NS+ in any PDR. We find a differentiated chemical behaviour between C-S and O-S bearing species within the nebula. The C-S bearing species C2S and o-H2CS present fractional abundances a factor grater than two higher in the core than in the PDR. In contrast, the O-S bearing molecules SO, SO2, and OCS present similar abundances towards both positions. A few molecules, SO+, NS, and NS+, are more abundant towards the PDR than towards the core, and could be considered as PDR tracers. Conclusions. This is the first complete study of S-bearing species towards a PDR. Our study shows that CS, SO, and H2S are the most abundant S-bearing molecules in the PDR with abundances of a few 1E-9. We recall that SH, SH+, S, and S+ are not observable at the wavelengths covered by the WHISPER survey. At the spatial scale of our observations, the total abundance of S atoms locked in the detected species is < 1E-8, only ~0.1% of the cosmic sulphur abundance.

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