arXiv subjects

A. Bacmann

Publications and source records attributed to A. Bacmann.

At least 19 recordsLinked to original sources

arXiv2026

GASTON-GP: Source catalogue and millimetre variability of massive protostellar objects

The processes governing protostellar mass growth remain debated, although episodic accretion is now understood as a key feature of protostellar evolution across all masses. Luminosity bursts have been observed in both low- and high-mass protostars, but the overall statistics remain limited, especially for high-mass objects. Over the past decade, numerical simulations of high-mass core collapse have provided a theoretical framework for interpreting protostellar variability, yet additional observational constraints are required to determine the characteristics and importance of bursts. In this work, we analyse data from GASTON-GP programme, which mapped a 2.4 square degrees region of the Galactic plane (centred at l = 24 deg) at 1.15 and 2.00 mm using NIKA2 on the IRAM 30 m telescope. The survey obtained 11 epochs over four years, offering the first opportunity to study millimetre variability in a large sample of massive protostellar sources. From the combined dataset, we constructed catalogues of 2925 compact sources at 1.15 mm and 1713 at 2.00 mm. Using a dedicated relative calibration scheme, we generated millimetre light curves for around 200 high-signal-to-noise sources and identified one variable candidate. However, it is not protostellar. Consequently, we report no robust detections of variable protostellar sources in GASTON field. This is the direct consequence of observational limitations (i.e., sensitivity, resolution) combined with the lack of any 100-fold luminosity bursts during the observations, which is consistent with estimates inferred from isolated core collapse simulations. This study highlights the need for future high-resolution, high-cadence surveys to constrain the accretion histories of massive protostars.

astro-ph.GA↗

arXiv2024

Cosmic-ray induced sputtering of interstellar formaldehyde ices

H2CO is a ubiquitous molecule in the ISM and in the gas phase of prestellar cores, and is likely present in ice mantles, but its main desorption mechanism is unknown. In this paper our aim is to quantify the desorption efficiency of H2CO upon cosmic-ray impact in order to determine whether cosmic-ray induced sputtering could account for the H2CO abundance observed in prestellar cores. Using a heavy-ion beam as a cosmic-ray analogue at the GANIL accelerator, we irradiated pure H2CO ice films at 10 K under high vacuum conditions and monitored the ice film evolution with infrared spectroscopy and the composition of the sputtered species in the gas phase using mass spectrometry. We derived both the effective and intact sputtering yield of pure H2CO ices. We find that H2CO easily polymerises under heavy-ion irradiation in the ice, and is also radiolysed into CO and CO2. In the gas phase, the dominant sputtered species is CO and intact H2CO is only a minor species. We determine an intact sputtering yield for pure H2CO ices $2.5\times 10^3$ molecules ion$^{-1}$ for an electronic stopping power of $S_e\sim2830$ eV ($10^{15}$ molecules cm$^{-2}$)$^{-1}$. The corresponding cosmic-ray sputtering rate is $\Gamma_\mathrm{CRD}=1.5\times 10^{18}\zeta$ molecules cm$^{-2}$ s$^{-1}$, where $\zeta$ is the rate of cosmic-ray ionisation of molecular hydrogen in the ISM. In the frame of a simple steady-state chemical model of freeze-out and non-thermal desorption, we find that this experimental cosmic-ray sputtering rate is too low (by an order of magnitude) to account for the observed H2CO gas-phase abundance we derived in the prestellar core L1689B. We find however that this abundance can be reproduced if we assume that H2CO diluted in CO or CO2 ices co-desorbs at the same sputtering rate as pure CO or pure CO2 ices.

astro-ph.GA↗

arXiv2023

NIKA2 observations of dust grain evolution from star-forming filament to T-Tauri disk: Preliminary results from NIKA2 observations of the Taurus B211/B213 filament

To understand the evolution of dust properties in molecular clouds in the course of the star formation process, we constrain the changes in the dust emissivity index from star-forming filaments to prestellar and protostellar cores to T Tauri stars. Using the NIKA2 continuum camera on the IRAM 30~m telescope, we observed the Taurus B211/B213 filament at 1.2\,mm and 2\,mm with unprecedented sensitivity and used the resulting maps to derive the dust emissivity index $\beta$. Our sample of 105 objects detected in the $\beta$ map of the B211/B213 filament indicates that, overall, $\beta$ decreases from filament and prestellar cores ($\beta \sim 2\pm0.5$) to protostellar cores ($\beta \sim 1.2 \pm 0.2$) to T-Tauri protoplanetary disk ($\beta < 1$). The averaged dust emissivity index $\beta$ across the B211/B213 filament exhibits a flat ($\beta \sim 2\pm0.3$) profile. This may imply that dust grain sizes are rather homogeneous in the filament, start to grow significantly in size only after the onset of the gravitational contraction/collapse of prestellar cores to protostars, reaching big sizes in T Tauri protoplanetary disks. This evolution from the parent filament to T-Tauri disks happens on a timescale of about 1-2~Myr.

astro-ph.SR↗

arXiv2021

Galactic star formation with NIKA2 (GASTON): Filament convergence and its link to star formation

In the past decade filaments have been recognised as a major structural element of the interstellar medium, the densest of these filaments hosting the formation of most stars. In some star-forming molecular clouds converging networks of filaments, also known as hub filament systems, can be found. These hubs are believed to be preferentially associated to massive star formation. As of today, there are no metrics that allow the systematic quantification of a filament network convergence. Here, we used the IRAM 30m NIKA2 observations of the Galactic plane from the GASTON large programme to systematically identify filaments and produce a filament convergence parameter map. We use such a map to show that: i. hub filaments represent a small fraction of the global filament population; ii. hubs host, in proportion, more massive and more luminous compact sources that non-hubs; iii. hub-hosting clumps are more evolved that non-hubs; iv. no discontinuities are observed in the properties of compact sources as a function of convergence parameter. We propose that the rapid global collapse of clumps is responsible for (re)organising filament networks into hubs and, in parallel, enhancing the mass growth of compact sources.

astro-ph.GA↗

arXiv2021

Galactic Star Formation with NIKA2 (GASTON): Evidence of mass accretion onto dense clumps

High-mass stars ($m_* \gtrsim 8 \, M_\odot$) play a crucial role in the evolution of galaxies, and so it is imperative that we understand how they are formed. We have used the New IRAM KIDs Array 2 (NIKA2) camera on the Institut de Radio Astronomie Millim\'{e}trique (IRAM) 30-m telescope to conduct high-sensitivity continuum mapping of $\sim2$ deg$^2$ of the Galactic plane (GP) as part of the Galactic Star Formation with NIKA2 (GASTON) large program. We have identified a total of 1467 clumps within our deep 1.15 mm continuum maps and, by using overlapping continuum, molecular line, and maser parallax data, we have determined their distances and physical properties. By placing them upon an approximate evolutionary sequence based upon 8 $\mu$m $\textit{Spitzer}$ imaging, we find evidence that the most massive dense clumps accrete material from their surrounding environment during their early evolution, before dispersing as star formation advances, supporting clump-fed models of high-mass star formation.

astro-ph.GA↗

arXiv2021

GASTON: Galactic Star Formation with NIKA2. Evidence for the mass growth of star-forming clumps

Determining the mechanism by which high-mass stars are formed is essential for our understanding of the energy budget and chemical evolution of galaxies. By using the New IRAM KIDs Array 2 (NIKA2) camera on the Institut de Radio Astronomie Millim\'etrique (IRAM) 30-m telescope, we have conducted high-sensitivity and large-scale mapping of a fraction of the Galactic plane in order to search for signatures of the transition between the high- and low-mass star-forming modes. Here, we present the first results from the Galactic Star Formation with NIKA2 (GASTON) project, a Large Programme at the IRAM 30-m telescope which is mapping $\approx$2 deg$^2$ of the inner Galactic plane (GP), centred on $\ell$=23.9$^\circ$, $b$=0.05$^\circ$, as well as targets in Taurus and Ophiuchus in 1.15 and 2.00 mm continuum wavebands. In this paper we present the first of the GASTON GP data taken, and present initial science results. We conduct an extraction of structures from the 1.15 mm maps using a dendrogram analysis and, by comparison to the compact source catalogues from Herschel survey data, we identify a population of 321 previously-undetected clumps. Approximately 80 per cent of these new clumps are 70 $\mu$m-quiet, and may be considered as starless candidates. We find that this new population of clumps are less massive and cooler, on average, than clumps that have already been identified. Further, by classifying the full sample of clumps based upon their infrared-bright fraction - an indicator of evolutionary stage - we find evidence for clump mass growth, supporting models of clump-fed high-mass star formation.

astro-ph.GA↗

arXiv2020

Probing episodic accretion with chemistry: CALYPSO observations of IRAM 04191+1522. Results from the CALYPSO IRAM-PdBI survey

Context. The process of mass accretion in the earliest phases of star formation is still not fully understood: Does the accretion rate smoothly decline with the age of the protostar or are there short, intermittent accretion bursts? Aims. We aim to explore whether or not the observed C$^{18}$O and N$_2$H$^+$ emission pattern towards the VeLLO IRAM 04191+1522 can be understood in the framework of a scenario where the emission is chemically tracing a past accretion burst. Methods.We used high-angular-resolution Plateau de Bure Interferometer (PdBI) observations of C$^{18}$O and N$_2$H$^+$ towards IRAM 04191+1522 that were obtained as part of the CALYPSO IRAM Large Program. We model these observations using a chemical code with a time-dependent physical structure coupled with a radiative transfer module, where we allow for variations in the source luminosity. Results. We find that the N$_2$H$^+$ line emission shows a central hole, while the C$^{18}$O emission is compact. The morphology of these two lines cannot be reproduced with a constant luminosity model based on the present-day internal luminosity (0.08 L$_{sun}$). However, the N$_2$H$^+$ peaks are consistent with a constant-luminosity model of 12 L$_{sun}$. Using a model with time-dependent temperature and density profiles, we show that the observed N$_2$H$^+$ peak emission could indeed be caused by a past accretion burst. Such a burst should have occurred a couple of hundred years ago. Conclusions. We suggest that an accretion burst occurred in IRAM 04191+1522 in the recent past. If such bursts are common and sufficiently long in VeLLOs, they could lead to higher accretion onto the central object than their luminosity suggests. For IRAM 04191 in particular, our results yield an estimated final mass of 0.2 - 0.25 M$_{sun}$ by the end of the Class 0 phase, which would make this object a low-mass star rather than a brown dwarf.

astro-ph.SR↗

arXiv2020

Deuterium fractionation of nitrogen hydrides: detections of NHD and ND$_2$

Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD and ND$_2$. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH$_2$ and ND$_2$/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND$_2$ towards the class 0 protostar IRAS16293-2422, towards which NH, NH$_2$ and ND had been previously detected. Both NHD and ND$_2$ are detected in absorption towards the source. The relative abundance ratios NH$_2$ : NHD : ND$_2$ are close to 8 : 4 : 1. These ratios can be reproduced by our gas-phase chemical model within a factor of two-three. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND$_2$ , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry.

astro-ph.GA↗

arXiv2020

Questioning the spatial origin of complex organic molecules in young protostars with the CALYPSO survey

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]

astro-ph.GA↗

arXiv2020

Non-thermal desorption of complex organic molecules: Cosmic-ray sputtering of CH3OH embedded in CO2 ice

Methanol ice is embedded in interstellar ice mantles present in dense molecular clouds. We aim to measure the sputtering efficiencies starting from different ice mantles of varying compositions experimentally, in order to evaluate their potential impact on astrochemical models. The sputtering yields of complex organic molecules is of particular interest, since few mechanisms are efficient enough to induce a significant feedback to the gas phase. We irradiated methanol and carbon dioxide ice mixtures of varying ratios with swift heavy ions in the electronic sputtering regime. We monitored the evolution of the infrared spectra and the species released to the gas phase with a mass spectrometer. Methanol and 13C-methanol isotopologue were used to remove any ambiguity on the measured irradiation products. The sputtering of methanol embedded in carbon dioxide ice is an efficient process leading to the ejection of intact methanol in the gas phase. We establish that when methanol is embedded in a carbon-dioxide-rich mantle exposed to cosmic rays, a significant fraction is sputtered as intact molecules. The sputtered fraction follows the time-dependent bulk composition of the ice mantle, the latter evolving with time due to the radiolysis-induced evolution of the bulk. If methanol is embedded in a carbon dioxide ice matrix, as the analyses of the spectral shape of the CO2 bending mode observations in some lines of sight suggest, the overall methanol sputtering yield is higher than if embedded in a water ice mantle. The sputtering is increased by a factor close to the dominant ice matrix sputtering yield, which is about six times higher for pure carbon dioxide ice when compared to water ice. These experiments are further constraining the cosmic-ray-induced ice mantle sputtering mechanisms important role in the gas-phase release of complex organic molecules from the interstellar solid phase.

astro-ph.GA↗

arXiv2019

GASTON: Galactic Star Formation with NIKA2: A new population of cold massive sources discovered

Understanding where and when the mass of stars is determined is one of the fundamental, mostly unsolved, questions in astronomy. Here, we present the first results of GASTON, the Galactic Star Formation with NIKA2 large programme on the IRAM 30m telescope, that aims to identify new populations of low-brightness sources to tackle the question of stellar mass determination across all masses. In this paper, we focus on the high-mass star formation part of the project, for which we map a $\sim2$ deg$^2$ region of the Galactic plane around $l=24^\circ$ in both 1.2 mm and 2.0 mm continuum. Half-way through the project, we reach a sensitivity of 3.7 mJy/beam at 1.2mm. Even though larger than our target sensitivity of 2 mJy, the current sensitivity already allows the identification of a new population of cold, compact sources that remained undetected in any (sub-)mm Galactic plane survey so far. In fact, about 25% of the $\sim 1600$ compact sources identified in the 1.2 mm GASTON image are new detections. We present a preliminary analysis of the physical properties of the GASTON sources as a function of their evolutionary stage, arguing for a potential evolution of the mass distribution of these sources with time.

astro-ph.GA↗

arXiv2018

Characterizing young protostellar disks with the CALYPSO IRAM-PdBI survey: large Class 0 disks are rare

Understanding the formation mechanisms of protoplanetary disks and multiple systems, and their pristine properties, is a key question for modern astrophysics. The properties of the youngest disks, embedded in rotating infalling protostellar envelopes, have largely remained unconstrained up to now. In the framework of the IRAM-PdBI CALYPSO survey, we have obtained sub-arcsecond observations of the dust continuum emission at 231 GHz and 94 GHz, for a sample of 16 solar-type Class 0 protostars. In an attempt to identify disk-like structures embedded at small scales in the protostellar envelopes, we model the dust continuum emission visibility profiles using both Plummer-like envelope models and envelope models including additional Gaussian disk-like components. Our analysis shows that in the CALYPSO sample, 11 of the 16 Class 0 protostars are better reproduced by models including a disk-like dust continuum component contributing to the flux at small scales, but less than 25% of these candidate protostellar disks are resolved at radii > 60 au. Including all available literature constraints on Class 0 disks at subarcsecond scales, we show that our results are representative: most (> 72% in a sample of 26 protostars) Class 0 protostellar disks are small and emerge only at radii < 60 au. Our multiplicity fraction at scales 100-5000 au is in global agreement with the multiplicity properties of Class I protostars at similar scales. We confront our observational constraints on the disk size distribution in Class 0 protostars to the typical disk properties from protostellar formation models. Because they reduce the centrifugal radius, and produce a disk size distribution peaking at radii <100 au during the main accretion phase, magnetized models of rotating protostellar collapse are favored by our observations.

astro-ph.GA↗

arXiv2017

French SKA White Book - The French Community towards the Square Kilometre Array

The "Square Kilometre Array" (SKA) is a large international radio telescope project characterised, as suggested by its name, by a total collecting area of approximately one square kilometre, and consisting of several interferometric arrays to observe at metric and centimetric wavelengths. The deployment of the SKA will take place in two sites, in South Africa and Australia, and in two successive phases. From its Phase 1, the SKA will be one of the most formidable scientific machines ever deployed by mankind, and by far the most impressive in terms of data throughput and required computing power. With the participation of almost 200 authors from forty research institutes and six private companies, the publication of this French SKA white paper illustrates the strong involvement in the SKA project of the French astronomical community and of a rapidly growing number of major scientific and technological players in the fields of Big Data, high performance computing, energy production and storage, as well as system integration.

astro-ph.IM↗

arXiv2017

Probing changes of dust properties along a chain of solar-type prestellar and protostellar cores in Taurus with NIKA

The characterization of dust properties in the interstellar medium (ISM) is key for star formation. Mass estimates are crucial to determine gravitational collapse conditions for the birth of new stellar objects in molecular clouds. However, most of these estimates rely on dust models that need further observational constraints from clouds to prestellar and protostellar cores. We present results of a study of dust emissivity changes based on mm-continuum data obtained with the NIKA camera at the IRAM-30m telescope. Observing dust emission at 1.15 mm and 2 mm allows us to constrain the dust emissivity index ($β$) in the Rayleigh-Jeans tail of the dust spectral energy distribution (SED) far from its peak emission, where the contribution of other parameters (i.e. dust temperature) is important. Focusing on the Taurus molecular cloud, a low-mass star-forming regions in the Gould Belt, we analyze the emission properties of several distinct objects in the B213 filament: three prestellar cores, two Class-0/I protostellar cores and one Class-II object. By means of the ratio of the two NIKA channel-maps, we show that in the Rayleigh-Jeans approximation the dust emissivity index varies among the objects. For one prestellar and two protostellar cores, we produce a robust study using Herschel data to constrain the dust temperature of the sources. By using the Abel transform inversion technique we get accurate radial $β$ profiles. We find systematic spatial variations of $β$ in the protostellar cores that is not observed in the prestellar core. While in the former case $β$ decreases toward the center, in the latter it remains constant. Moreover, $β$ appears anticorrelated with the dust temperature. We discuss the implication of these results in terms of dust grain evolution between pre- and protostellar cores.

astro-ph.GA↗

arXiv2016

Probing the CO and methanol snow lines in young protostars. Results from the CALYPSO IRAM-PdBI survey

Context. "Snow lines", marking regions where abundant volatiles freeze out onto the surface of dust grains, play an important role for planet growth and bulk composition in protoplanetary disks. They can already be observed in the envelopes of the much younger, low-mass Class 0 protostars that are still in their early phase of heavy accretion. Aims. We aim at using the information on the sublimation regions of different kinds of ices to understand the chemistry of the envelope, its temperature and density structure, and the history of the accretion process. Methods. As part of the CALYPSO IRAM Large Program, we have obtained observations of C$^{18}$O, N$_2$H$^+$ and CH$_3$OH towards nearby Class 0 protostars with the IRAM Plateau de Bure interferometer at sub-arcsecond resolution. For four of these sources we have modeled the emission using a chemical code coupled with a radiative transfer module. Results. We observe an anti-correlation of C$^{18}$O and N$_2$H$^+$ in NGC 1333-IRAS4A, NGC 1333-IRAS4B, L1157, and L1448C, with N$_2$H$^+$ forming a ring around the centrally peaked C$^{18}$O emission due to N$_2$H$^+$ being chemically destroyed by CO. The emission regions of models and observations match for a CO binding energy of 1200 K, which is higher than the binding energy of pure CO ices ($\sim$855 K). Furthermore, we find very low CO abundances inside the snow lines in our sources, about an order of magnitude lower than the total CO abundance observed in the gas on large scales in molecular clouds before depletion sets in. Conclusions. The high CO binding energy may hint at CO being frozen out in a polar ice environment like amorphous water ice or in non-polar CO$_2$-rich ice. The low CO abundances are comparable to values found in protoplanetary disks, which may indicate an evolutionary scenario where these low values are already established in the protostellar phase. (Abbr. Version)

astro-ph.GA↗

arXiv2016

N$_2$H$^+$ and N$^{15}$NH$^+$ towards the prestellar core 16293E in L1689N

Understanding the processes that could lead to enrichment of molecules in $^{15}$N atoms is of particular interest in order to shed light on the relatively large variations observed in the $^{14}$N/$^{15}$N ratio in various solar system environments. Currently, the sample of molecular clouds where $^{14}$N/$^{15}$N ratios have been measured is small and has to be enlarged in order to allow statistically significant studies. In particular, the N$_2$H$^+$ molecule currently shows the largest spread of $^{14}$N/$^{15}$N ratios in high-mass star forming regions. However, the $^{14}$N/$^{15}$N ratio in N$_2$H$^+$ was obtained in only two low-mass star forming regions (L1544 and B1b). The current work extends this sample to a third dark cloud. We targeted the 16293E prestellar core, where the N$^{15}$NH$^+$ $J$=1-0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a non-local radiative transfer code. For that purpose, we computed specific collisional rate coefficients for the N$^{15}$NH$^+$-H$_2$ collisional system. As a first step of the analysis, the N$_2$H$^+$ abundance profile was constrained by reproducing the N$_2$H$^+$ $J$=1-0 and 3-2 maps. A scaling factor was then applied to this profile to match the N$^{15}$NH$^+$ $J$=1-0 spectrum. We derive a column density ratio N$_2$H$^+$ / N$^{15}$NH$^+$ = $330^{+170}_{-100}$. The current estimate $\sim$330 agrees with the value typical of the elemental isotopic ratio in the local ISM. It is however lower than in some other cores, where values as high as 1300 have been reported.

astro-ph.GA↗

arXiv2016

Detection of protonated formaldehyde in the prestellar core L1689B

Complex organic molecules (COMs) are detected in many regions of the interstellar medium, including prestellar cores. However, their formation mechanisms in cold (~10 K) cores remain to this date poorly understood. The formyl radical HCO is an important candidate precursor for several O-bearing terrestrial COMs in cores, as an abundant building block of many of these molecules. Several chemical routes have been proposed to account for its formation, both on grain surfaces, as an incompletely hydrogenated product of H addition to frozen-out CO molecules, or in the gas phase, either the product of the reaction between H2CO and a radical, or as a product of dissociative recombination of protonated formaldehyde H2COH+. The detection and abundance determination of H2COH+, if present, could provide clues as to whether this latter scenario might apply. We searched for protonated formaldehyde H2COH+ in the prestellar core L1689B using the IRAM 30m telescope. The H2COH+ ion is unambiguously detected, for the first time in a cold (~10 K) source. The derived abundance agrees with a scenario in which the formation of H2COH+ results from the protonation of formaldehyde. We use this abundance value to constrain the branching ratio of the dissociative recombination of H2COH+ towards the HCO channel to ~10-30%. This value could however be smaller if HCO can be efficiently formed from gas-phase neutral-neutral reactions, and we stress the need for laboratory measurements of the rate constants of these reactions at 10 K. Given the experimental difficulties in measuring branching ratios experimentally, observations can bring valuable constraints on these values, and provide a useful input for chemical networks.

astro-ph.GA↗

arXiv2016

Collisional excitation of doubly and triply deuterated ammonia ND$_2$H and ND$_3$ by H$_2$

The availability of collisional rate coefficients is a prerequisite for an accurate interpretation of astrophysical observations, since the observed media often harbour densities where molecules are populated under non--LTE conditions. In the current study, we present calculations of rate coefficients suitable to describe the various spin isomers of multiply deuterated ammonia, namely the ND$_2$H and ND$_3$ isotopologues. These calculations are based on the most accurate NH$_3$--H$_2$ potential energy surface available, which has been modified to describe the geometrical changes induced by the nuclear substitutions. The dynamical calculations are performed within the close--coupling formalism and are carried out in order to provide rate coefficients up to a temperature of $T$ = 50K. For the various isotopologues/symmetries, we provide rate coefficients for the energy levels below $\sim$ 100 cm$^{-1}$. Subsequently, these new rate coefficients are used in astrophysical models aimed at reproducing the NH$_2$D, ND$_2$H and ND$_3$ observations previously reported towards the prestellar cores B1b and 16293E. We thus update the estimates of the corresponding column densities and find a reasonable agreement with the previous models. In particular, the ortho--to--para ratios of NH$_2$D and NHD$_2$ are found to be consistent with the statistical ratios.

astro-ph.GA↗