arXiv subjects

Mathilde Bouvier

Publications and source records attributed to Mathilde Bouvier.

At least 19 recordsLinked to original sources

arXiv2026

FAUST XXXII. Chemically layered structures and ionisation in the disk and envelope of NGC 1333 IRAS 4C

Recent molecular line observations with high spatial resolutions have revealed vertical chemical stratification in Class I/II disks (i.e. chemical composition changing with height in the vertical direction of the disk), as well as a radial variation of ionisation rates. However, such chemical layered structures in Class 0 disk and envelope systems have been investigated only for limited objects, such as L1527. In the context of the ALMA Large Program FAUST, we report the results of molecular line observations toward the disk and envelope around the low-mass Class 0 protostar NGC 1333 IRAS 4C in the Perseus Molecular Cloud. Previous observations have shown that the disk rotation axis of this object is largely tilted to the line of sight, making it suitable for studying the molecular abundance distribution as a function of disk height. We found that the molecules known as gas density tracers such as C18O and H2CO are distributed around the disk midplane. In addition, molecules known as tracers of photochemical reactions such as CCH and c-C3H2 are distributed in the low-density upper layers of the disk, unlike the distribution in L1527. Moreover, the column density ratio of HCO+/CO derived from the H13CO+ and C18O line emission suggests that the ionisation degree and the ionisation rate vary with the disk radius and they are larger in the outer region of the disk and envelope.

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arXiv2026

Tracking the flow of interstellar gas across scales: from galaxies to star clusters

While the baryon cycle of the interstellar medium - and its ability to form stars - plays a key role in galaxy evolution, unravelling the physical processes driving its evolution remains a critical challenge, given that present-day observing facilities are unable to map entire galactic ecosystems down to the scales of individual star-forming clumps. Here, we outline the technical requirements that would enable, for the first time, to connect the small (pc) scale physics of star formation, to the large (tens-of-kpc) scales of galaxies, enabling an effective tracking of the flows of matter and energy across a vast range of environments, and answering the long-standing question of what regulates star formation across cosmic time.

astro-ph.IM↗

arXiv2026

Molecular atlas of nearby galaxies through sensitive high-resolution multi-band molecular line surveys

Star formation and associated feedback processes play a key role in galaxy evolution. Astrochemistry is a powerful tool for characterising the physical conditions of dense star-forming gas and understanding the reciprocal relation between stellar feedback and the interstellar medium (ISM): from the way feedback influences the ISM to how the ISM conditions regulate the formation of new stars. The advent of large spectral surveys of a few individual galaxy centres have demonstrated the potential of molecular gas inventories to reveal the physical conditions of star-forming gas in great detail. A next-generation (sub)mm facility in the 2040s would extend this science to the local star-forming galaxy population, providing for the first time a comprehensive view of the physical and chemical conditions of the molecular ISM in the local universe.

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arXiv2026

From Scaling Relations to Physical Laws: Constraining the Physics of Star Formation Across Local Galaxies

The conversion of cold dense gas, traced by molecular line emission, into stars remains one of the fundamental unsolved problems in galaxy evolution. Observations over the past decades have established empirical relations between gas and star formation, yet their physical origin remains uncertain. Gravity, turbulence, and stellar feedback as main drivers provide fundamentally different pictures of how star formation is regulated, while reproducing remarkably similar galaxy-scale scaling relations. To disentangle the individual importance of each process, not only its mass and distribution, but also its temperature, density, excitation, and chemistry must be probed. However, current or planned (sub)millimetre facilities cannot combine statistical breadth, cloud-scale resolution, and multi-line diagnostics across large galaxy samples. A new, flagship interferometer with increased line sensitivity and bandwidth enables obtaining multi-transition, multi-isotopologue observations of molecular gas at GMC-resolved scales across nearby galaxies, enabling millions of independent measurements of its physical state across a wide range of extragalactic environments. This will transform molecular gas from an empirical tracer into a quantitative probe of the physical processes governing the relation between the dense gas and star formation. With the 2040s landscape of optical and radio facilities, ALMA2040 will facilitate a predictive theory of the conversion of gas into stars.

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arXiv2026

Interstellar disparity: N- vs O-bearing molecules: A multidisciplinary perspective across interstellar environments

Observations of O- and N-bearing molecules reveal systematic chemical differentiation across a wide range of interstellar environments. This pattern is peculiar and its origin likely reflects a combination of physical conditions, chemical pathways, and observational effects rather than a single mechanism. In this paper, we provide a comprehensive overview of the current understanding of the N-bearing and O-bearing species and their relative behaviour in the interstellar medium, integrating results from observations, laboratory experiments, chemical modelling, and theoretical studies. While inspired by discussions at the Lorentz Center workshop held in Leiden in August 2025, this work goes beyond a summary of the meeting and instead synthesizes the state of the field, highlights key open questions, and outlines future directions. We identify several key observational, experimental, and theoretical challenges that must be addressed to determine the origin of this chemical differentiation.

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arXiv2026

Chemical Complexity in the Early Stages of Star Formation in the SKAO Era

About 350 molecules have been identified in the interstellar medium (ISM), including complex molecules relevant to prebiotic chemistry. A remarkable level of molecular diversity has been observed from the earliest stages of star formation, providing the initial chemical inventory inherited by planetary systems. Radio observations have played a pivotal role in these discoveries, starting with the identification of the first polyatomic molecule, $\text{NH}_3$ (Cheung et al. 1968). (Sub-)millimeter observations have revealed complex organic molecules of prebiotic relevance, including formamide ($\text{NH}_2\text{CHO}$), glycolaldehyde ($\text{CH}_2\text{OHCHO}$), and even urea ($(\text{NH}_2)_2\text{CO}$), and hydroxylamine ($\text{NH}_2\text{OH}$), which are possible precursors of RNA nucleotides (Ceccarelli et al. 2023; Jiménez-Serra et al. 2020). However, in dense protostellar regions, dust opacity hampers the detection of molecular emission. Additionally, large molecules and those containing heavy atoms, which have rotational transitions at lower frequencies, often remain inaccessible to current instruments. The Square Kilometre Array Observatory (SKAO) will provide an unprecedented combination of sensitivity and angular resolution at radio wavelengths. This will allow for the detection of prebiotic species and offer new insights into the chemical pathways that shape emerging planetary systems (Jiménez-Serra et al. 2022). This chapter details the scientific questions and advancements that the SKAO, and more specifically, SKA-Mid equipped with the Band 5 receivers, will pursue in the field of astrochemistry, focusing on the chemical complexity in both high-mass and solar-type star-forming regions.

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arXiv2026

Testing the Physical Parameter Constraining Power of HCN and HNC with Neural Networks

We quantify the utility of HCN and HNC to characterize gas conditions in the nearby starburst galaxy NGC 253. We use measurements from the Atacama Large Millimeter/Submillimeter Array (ALMA) Large Program ALCHEMI: the ALMA Comprehensive High-resolution Molecular Inventory. Using different subsets of the eight total HCN and HNC transitions measured by ALCHEMI, we test the number and combinations of transitions necessary for constraining the temperature, H$_2$ volume and column densities, cosmic-ray ionization rate, and beam-filling factor in three representative regions within NGC 253. We use these combinations of HCN and HNC transitions to constrain chemical and radiative transfer models and infer the gas conditions using a Bayesian nested sampling algorithm combined with neural network models for increased efficiency. By comparing the shapes of the resulting posterior distributions, as well as the medians and uncertainties for each gas parameter, from each test case to what we obtain with the full set of eight transitions (the control), we quantify how well each test reproduces the control. We find that multiple transitions each of both molecules are required to obtain a median parameter value within a factor of 2 of the control with an uncertainty less than 2-3 times that of the control. We also find that transition combinations that feature a range of upper-state energies are most effective. We show that single transitions, such as HCN J = 1-0 or 3-2, are among the worst-performing combinations and result in parameter values up to an order of magnitude different than the control.

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arXiv2025

The molecular diversity of the ISM in galaxies across cosmic time

Submillimetre molecular lines (e.g., CO, HCN, SiO) provide a uniquely powerful view of the physical and chemical processes that govern star formation (SF) and galaxy evolution. Yet, our current picture of the molecular universe beyond the Milky Way remains strikingly incomplete: broad chemical inventories exist for only a handful of galaxies, typically more extreme than the Milky Way, constrained by sensitivity limits and narrow survey strategies. In the 2040s, surveying galaxies with multi-species, multi-transitions observations across diverse galactic environments will be crucial to establish effective chemical diagnostics of the various ISM processes from the early universe to $z=0$. Extragalactic astrochemistry provides a uniquely sensitive probe of the physical processes shaping galaxies, allowing us to understand, species by species, how gas responds to its local environment and how galaxies grow, transform, and recycle matter over cosmic time.

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arXiv2025

A vision for ground-based astronomy beyond the 2030s: How to build ESO's next big telescope sustainably

Astronomy is the study of the Universe and all the objects that it comprises. Our attention is therefore usually focused beyond Earth, home to the only form of life known today. However, how can we continue to explore the secrets of the Universe, if we stand by and watch our only home burn? We know that there is no Planet B. It is therefore urgent that, as astronomers, we collectively work to protect the Earth, allowing future generations the opportunity to continue to uncover the secrets of the cosmos. As astronomical facilities account for the majority of our community's carbon footprint, we propose guidelines that we hold crucial for the European Southern Observatory (ESO) to consider in the context of the Expanding Horizons programme as it plans a next-generation, transformational facility.

astro-ph.IM↗

arXiv2025

FAUST XXIX. OCS line emission: a new method for measuring the luminosity of embedded protostars in binary systems

The luminosity of embedded protostars is commonly measured via observations of the dust continuum spectral energy distribution from millimetre to infrared wavelengths. However, this method cannot be applied to embedded protostars in binary or multiple systems, where their components are usually unresolved over this extended wavelength range. We propose a new method, based on the idea that a molecule formed (mainly) on the grain surfaces only emits lines in the region where it thermally sublimates from the grain mantles, heated by the photons emitted by the embedded source. In this respect, carbonyl sulfide (OCS) is an optimal molecule, because of its low binding energy and rotational lines in the millimetre. We apply the method to the protobinary system NGC1333 IRAS4A, using ALMA high-spatial resolution ($\sim$50 au) observations of the OCS(19-18) line as part of the ALMA Large Program FAUST. We also present new quantum mechanics calculations of the OCS binding energy distribution, essential for the application of the method. We found that the two binary components, A1 and A2, have a comparable luminosity within the error bars, 7.5$\pm$2.5 and 7$\pm$1 L$_\odot$, respectively. We discuss the reliability of the estimated luminosities and the potential of this new method for measuring the luminosity of embedded protostars in binary and multiple systems.

astro-ph.SR↗

arXiv2024

Neural Network Constraints on the Cosmic-Ray Ionization Rate and Other Physical Conditions in NGC 253 with ALCHEMI Measurements of HCN and HNC

We use a neural network model and ALMA observations of HCN and HNC to constrain the physical conditions, most notably the cosmic-ray ionization rate (CRIR, zeta), in the Central Molecular Zone (CMZ) of the starburst galaxy NGC 253. Using output from the chemical code UCLCHEM, we train a neural network model to emulate UCLCHEM and derive HCN and HNC molecular abundances from a given set of physical conditions. We combine the neural network with radiative transfer modeling to generate modeled integrated intensities, which we compare to measurements of HCN and HNC from the ALMA Large Program ALCHEMI. Using a Bayesian nested sampling framework, we constrain the CRIR, molecular gas volume and column densities, kinetic temperature, and beam-filling factor across NGC 253's CMZ. The neural network model successfully recovers UCLCHEM molecular abundances with about 3 percent error and, when used with our Bayesian inference algorithm, increases the parameter inference speed tenfold. We create images of these physical parameters across NGC 253's CMZ at 50 pc resolution and find that the CRIR, in addition to the other gas parameters, is spatially variable with zeta a few times 10^{14} s^{-1} at greater than 100 pc from the nucleus, increasing to zeta greater than 10^{-13} s^{-1} at its center. These inferred CRIRs are consistent within 1 dex with theoretical predictions based on non-thermal emission. Additionally, the high CRIRs estimated in NGC 253's CMZ can be explained by the large number of cosmic-ray-producing sources as well as a potential suppression of cosmic-ray diffusion near their injection sites.

astro-ph.GA↗

arXiv2024

FAUST XIX. D$_2$CO in the outflow cavities of NGC\,1333 IRAS\,4A: recovering the physical structure of its original prestellar core

Molecular deuteration is a powerful diagnostic tool for probing the physical conditions and chemical processes in astrophysical environments. In this work, we focus on formaldehyde deuteration in the protobinary system NGC\,1333 IRAS\,4A, located in the Perseus molecular cloud. Using high-resolution ($\sim$\,100\,au) ALMA observations, we investigate the [D$_2$CO]/[HDCO] ratio along the cavity walls of the outflows emanating from IRAS\,4A1. Our analysis reveals a consistent decrease in the deuteration ratio (from $\sim$\,60-20\% to $\sim$\,10\%) with increasing distance from the protostar (from $\sim$\,2000\,au to $\sim$\,4000\,au). Given the large measured [D$_2$CO]/[HDCO], both HDCO and D$_2$CO are likely injected by the shocks along the cavity walls into the gas-phase from the dust mantles, formed in the previous prestellar phase. We propose that the observed [D$_2$CO]/[HDCO] decrease is due to the density profile of the prestellar core from which NGC\,1333 IRAS\,4A was born. When considering the chemical processes at the base of formaldehyde deuteration, the IRAS\,4A's prestellar precursor had a predominantly flat density profile within 3000\,au and a decrease of density beyond this radius.

astro-ph.GA↗

arXiv2024

Multiple chemical tracers finally unveil the intricate NGC\,1333 IRAS\,4A outflow system. FAUST XVI

The exploration of outflows in protobinary systems presents a challenging yet crucial endeavour, offering valuable insights into the dynamic interplay between protostars and their evolution. In this study, we examine the morphology and dynamics of jets and outflows within the IRAS\,4A protobinary system. This analysis is based on ALMA observations of SiO(5--4), H$_2$CO(3$_{0,3}$--2$_{0,3}$), and HDCO(4$_{1,4}$--3$_{1,3}$) with a spatial resolution of $\sim$150\,au. Leveraging an astrochemical approach involving the use of diverse tracers beyond traditional ones has enabled the identification of novel features and a comprehensive understanding of the broader outflow dynamics. Our analysis reveals the presence of two jets in the redshifted emission, emanating from IRAS\,4A1 and IRAS\,4A2, respectively. Furthermore, we identify four distinct outflows in the region for the first time, with each protostar, 4A1 and 4A2, contributing to two of them. We characterise the morphology and orientation of each outflow, challenging previous suggestions of bends in their trajectories. The outflow cavities of IRAS\,4A1 exhibit extensions of 10$''$ and 13$''$ with position angles (PA) of 0$^{\circ}$ and -12$^{\circ}$, respectively, while those of IRAS\,4A2 are more extended, spanning 18$''$ and 25$''$ with PAs of 29$^{\circ}$ and 26$^{\circ}$. We propose that the misalignment of the cavities is due to a jet precession in each protostar, a notion supported by the observation that the more extended cavities of the same source exhibit lower velocities, indicating they may stem from older ejection events.

astro-ph.GA↗

arXiv2024

Physical Properties of the Southwest Outflow Streamer in the Starburst Galaxy NGC 253 with ALCHEMI

The physical properties of galactic molecular outflows are important as they could constrain outflow formation mechanisms. We study the properties of the southwest (SW) outflow streamer including gas kinematics, optical depth, dense gas fraction, and shock strength in the central molecular zone of the starburst galaxy NGC 253. We image the molecular emission at a spatial resolution of $\sim$27 pc based on data from the ALCHEMI program. We trace the kinematics of molecular gas with CO(1-0) line. We constrain the optical depth of CO emission with CO/$^{13}$CO(1-0) ratio, the dense gas fraction with HCN/CO(1-0) ratio, as well as the shock strength with SiO(2-1)/$^{13}$CO(1-0) ratio. The CO/$^{13}$CO(1-0) integrated intensity ratio is $\sim$21 in the SW streamer region, which approximates the C/$^{13}$C isotopic abundance ratio. The higher integrated intensity ratio compared to the disk can be attributed to the optically thinner environment for CO(1-0) emission inside the SW streamer. The HCN/CO(1-0) and SiO(2-1)/$^{13}$CO(1-0) integrated intensity ratios both approach $\sim$0.2 in three giant molecular clouds (GMCs) at the base of the outflow streamers, which implies the higher dense gas fraction and enhanced strength of fast shocks in those GMCs than in the disk. The contours of those two integrated intensity ratios are extended towards the directions of outflow streamers, which connects the enhanced dense gas fraction and shock strength with molecular outflow. Moreover, the molecular gas with enhanced dense gas fraction and shock strength located at the base of the SW streamer shares the same velocity with the outflow. These phenomena suggest that the star formation inside the GMCs can trigger the shocks and further drive the molecular outflow.

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arXiv2024

The ALCHEMI atlas: principal component analysis reveals starburst evolution in NGC 253

Molecular lines are powerful diagnostics of the physical and chemical properties of the interstellar medium (ISM). These ISM properties, which affect future star formation, are expected to differ in starburst galaxies from those of more quiescent galaxies. We investigate the ISM properties in the central molecular zone of the nearby starburst galaxy NGC 253 using the ultra-wide millimeter spectral scan survey from the ALMA Large Program ALCHEMI. We present an atlas of velocity-integrated images at a 1".6 resolution of 148 unblended transitions from 44 species, including the first extragalactic detection of HCNH$^+$ and the first interferometric images of C$_3$H$^+$, NO, HCS$^+$. We conduct a principal component analysis (PCA) on these images to extract correlated chemical species and to identify key groups of diagnostic transitions. To the best of our knowledge, our dataset is currently the largest astronomical set of molecular lines to which PCA has been applied. The PCA can categorize transitions coming from different physical components in NGC 253 such as i) young starburst tracers characterized by high-excitation transitions of HC$_3$N and complex organic molecules (COMs) versus tracers of on-going star formation (radio recombination lines) and high-excitation transitions of CCH and CN tracing PDRs, ii) tracers of cloud-collision-induced shocks (low-excitation transitions of CH$_3$OH, HNCO, HOCO$^+$, and OCS) versus shocks from star-formation-induced outflows (high-excitation transitions of SiO), as well as iii) outflows showing emission from HOC$^+$, CCH, H$_3$O$^+$, CO isotopologues, HCN, HCO$^+$, CS, and CN. Our findings show these intensities vary with galactic dynamics, star formation activities, and stellar feedback.

astro-ph.GA↗

arXiv2023

Volume density structure of the NGC253 CMZ through ALCHEMI excitation analysis

We present a spatially-resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the ALMA Large program ALCHEMI, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way's Galactic Center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3-7 transitions from nine molecular species to delineate the position-position-velocity distributions of column density ($N_\mathrm{H_2}$), volume density ($n_\mathrm{H_2}$), and temperature ($T_\mathrm{kin}$) at 27 pc resolution. Two distinct components are detected: a low-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim(10^{3.3}\ \mathrm{cm}^{-3}, 85 K)$ and a high-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim (10^{4.4}\ \mathrm{cm}^{-3}, 110\ \mathrm{K})$, separated at $n_\mathrm{H_2}\sim10^{3.8}\ \mathrm{cm}^{-3}$. NGC 253 has $\sim10$ times the high-density gas mass and $\sim3$ times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of $\sim30$ difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The $n_\mathrm{H_2}$ histogram toward NGC 253 exhibits a shallow declining slope up to $n_\mathrm{H_2}\sim10^6\ \mathrm{cm}^{-3}$, while that of the GC steeply drops in $n_\mathrm{H_2}\gtrsim10^{4.5}\ \mathrm{cm}^{-3}$ and vanishes at $10^5\ \mathrm{cm}^{-3}$. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold $n_\mathrm{H_2}$ for the dense gas is taken at $\sim 10^{4.2\mbox{-}4.6}\ \mathrm{cm}^{-3}$. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies.

astro-ph.GA↗

arXiv2023

FAUST X: Formaldehyde in the Protobinary System [BHB2007] 11: Small Scale Deuteration

Context. Deuterium in H-bearing species is enhanced during the early stages of star formation, however, only a small number of high spatial resolution deuteration studies exist towards protostellar objects, leaving the small-scale structures unrevealed and understudied. Aims. We aim to constrain the deuterium fractionation ratios in a Class 0/I protostellar object in formaldehyde (H2CO), which has abundant deuterated isotopologues in this environment. Methods. We observed the Class 0/I protobinary system [BHB2007] 11, whose emission components are embedded in circumstellar disks that have radii of 2-3 au, using ALMA within the context of the Large Program FAUST. The system is surrounded by a complex filamentary structure connecting to the larger circumbinary disk. In this work we present the first study of formaldehyde D-fractionation towards this source with detections of H2CO 3(0,3)-2(0,2), combined with HDCO 4(2,2)-3(2,1), HDCO 4(1,4)-3(1,3) and D2CO 4(0,4)-3(0,3). These observations enable multiple velocity components associated with the methanol hotspots also uncovered by FAUST data, as well as the external envelope, to be resolved. In addition, based on the kinematics seen in the observations of the H2CO emission, we propose the presence of a second large scale outflow. Results. HDCO and D2CO are only found in the central regions of the core while H2CO is found more ubiquitously. From radiative transfer modelling, the column densities ranges found for H2CO, HDCO and D2CO are (3-8)x10$^{14}$ cm$^{-2}$, (0.8-2.9)x10$^{13}$ cm$^{-2}$ and (2.6-4.3)x10$^{12}$ cm$^{-2}$, respectively, yielding an average D/H ratio of 0.01-0.04. Following the results of kinematic modelling, the second large scale feature is inconsistent with a streamer-like nature and we thus tentatively conclude that the feature is an asymmetric molecular outflow launched by a wide-angle disk wind.

astro-ph.GA↗

arXiv2023

Streamers feeding the SVS13-A protobinary system: astrochemistry reveals accretion shocks?

We report ALMA high-angular resolution (~ 50 au) observations of the binary system SVS13-A. More specifically, we analyse deuterated water (HDO) and sulfur dioxide (SO2) emission. The molecular emission is associated with both the components of the binary system, VLA4A and VLA4B. The spatial distribution is compared to that of formamide (NH2CHO), previously analysed in the system. Deuterated water reveals an additional emitting component spatially coincident with the dust accretion streamer, at a distance larger than 120 au from the protostars, and at blue-shifted velocities (> 3 km/s from the systemic velocities). We investigate the origin of the molecular emission in the streamer, in light of thermal sublimation temperatures calculated using updated binding energies (BE) distributions. We propose that the observed emission is produced by an accretion shock at the interface between the accretion streamer and the disk of VLA4A. Thermal desorption is not completely excluded in case the source is actively experiencing an accretion burst.

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