arXiv subjects

Paola Caselli

Publications and source records attributed to Paola Caselli.

At least 19 recordsLinked to original sources

arXiv2026

Spatially resolved thermal dust emission in the L1157 outflow reveals grain-driven molecular enrichment

Protostellar outflow shocks reshape local dust properties and molecular chemistry. The L1157 outflow is an archetypal chemically rich shocked region, but the thermal dust associated with its successive shocks has remained unresolved because molecular-line contamination obscures the broadband continuum. We obtained new James Clerk Maxwell Telescope (825--906 $\mu$m) spectral-line observations and Submillimeter Array (1.1--1.4 mm) continuum observations toward L1157 B0-B1-B2, probing spatial scales from 0.4 pc to 1200 au. After removing molecular-line contamination on a pixel-by-pixel basis, we derived the dust temperature, column density, and dust opacity index from continuum data spanning 70 $\mu$m to 1.3 mm. The line-corrected continuum maps reveal the dust distribution across successive shocks. The dust opacity index ($\beta\approx1.8$--2.3) indicates that grains have not grown to millimeter sizes throughout the shocked regions. Combined with previous $\rm NH_3$ observations, we find that the dust emission resolves into compact clumps along the precessing jet, whereas gaseous $\rm NH_3$ peaks at the shock fronts, reaching abundances of $\sim10^{-5}$ relative to $\rm H_2$, even where the 0.85 and 1.3 mm dust emission is detected at only 3--5$\sigma$. Our newly developed physicochemical shock model shows that $\rm NH_3$ forms predominantly on grain surfaces and is released by shock-induced sputtering, with the highest abundances occurring where post-shock re-adsorption remains inefficient. These results establish spatially resolved dust continuum imaging as a direct observational probe of grain evolution and provide new observational constraints on dust-gas interactions in protostellar shocks.

astro-ph.GA↗

arXiv2026

A tale of two isotopes: Spatial variation in HCN fractionation toward young cores

Context. Isotopic fractionation can serve as a powerful tracer of the chemical evolution during star and planet formation. To accurately interpret observations, it is crucial to identify the dominant pathways of nitrogen and carbon fractionation at different evolutionary stages. Aims. We aim to study nitrogen and carbon fractionation in a sample of young cores at the onset of star formation. Methods. We map H$^{13}$CN and HC$^{15}$N around one starless and three pre-stellar cores. We compute the $N$(H$^{13}$CN)/$N$(HC$^{15}$N) column density ratio across the cores and compare the distribution with $N$(H$_2$) maps from $\textit{Herschel}$/SPIRE. In addition, we calculate $^{14}$N/$^{15}$N maps using the double isotope method for comparison with earlier studies. The results are compared with astrochemical modeling of carbon and nitrogen fractionation for a one-dimensional pre-stellar core model. Results. The computed $N$(H$^{13}$CN)/$N$(HC$^{15}$N) ratio exhibit clear spatial variation across the maps. This variation is correlated with $N$(H$_2$) in three out of four cores. Conclusions. Our analysis reveals a correlation between the H$^{13}$CN/HC$^{15}$N ratios and the $N$(H$_2$) maps. According to the astrochemical model, the correlation is mainly due to variations in the $^{12}$C/$^{13}$C ratio. Consequently, the results caution against applying the double-isotope method to derive $^{14}$N/$^{15}$N ratios without independently assessing possible spatial variations in the $^{12}$C/$^{13}$C ratio. Furthermore, the leading cause of the isotopic variation in the model is not isotope-selective photodissociation, but rather more efficient fractionation through exchange reactions at lower temperatures in the denser regions of the cores.

astro-ph.SR↗

arXiv2026

Chemistry of Dark Molecular Clouds

Recent molecular line surveys, particularly toward the starless core TMC-1 CP, have greatly expanded the inventory of interstellar molecules, revealing numerous isomers and even aromatic species. Their diverse formation pathways---from ion-molecule reactions to the possible fragmentation of carbonaceous grains---remain under debate, linking chemistry to the life cycle of the interstellar medium. Simple tracers such as carbon chains and deuterated ions are used to probe the physical conditions and evolutionary state of nearby filaments and cores, as well as in massive infrared dark clouds. Ice chemistry has also entered a new era with JWST: spatial distributions of ices indicate a connection between catastrophic freeze-out, established in the prestellar core L1544, and formation of complex organic molecules. Overall, TMC-1 CP and L1544 are not outliers but representative laboratories of molecular cloud physics and chemistry. Extending these findings across diverse environments, from the Central Molecular Zone to low-metallicity galaxies, is essential for a unified picture of how interstellar chemistry regulates the path from clouds to stars and planets.

astro-ph.GA↗

arXiv2026

Atmospheric retrieval evidence for water isotopologue HDO on exoplanet WASP-39b

Hydrogen-isotopologues are commonly used to trace the chemical processing and origin of hydrogen-bearing species throughout the Universe, however, their abundance remains unconstrained for extrasolar planets. Here, we report atmospheric retrieval evidence for the deuterated water molecule HDO in an exoplanet atmosphere, retrieved from James Webb Space Telescope transmission spectra of the hot Jupiter WASP-39 b, resulting in a deuterium-to-hydrogen ratio in water of $4.0^{+1.3}_{-1.1} \times 10^{-3}$. The inferred value is substantially higher than those measured for the Solar System gas giants and overlaps numerically with values reported for some protostellar and inner Solar System environments. This enrichment may reflect either inherited water-rich material accreted beyond the snow line or isotopic processing in the observable atmosphere through transport, photochemistry, and subsequent escape.

astro-ph.EP↗

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; Jim\'enez-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 (Jim\'enez-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.

astro-ph.SR↗

arXiv2026

Unveiling Complex Chemistry in Planet-forming Disks with the SKAO

The chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. In recent years, we have made huge progress in characterizing disk chemistry. (Sub-)millimeter interferometers, such as ALMA, allowed us to detect emission lines from simple to complex organic molecules and to probe their radial and vertical distribution in disks. On the other hand, JWST has started to unveil the composition of disk ices, and line emission from the innermost disk regions. The advent of SKA will open new domains in the field, by observing emission lines from heavier molecules including heavy carbon chains and rings, and prebiotic molecules with peak emission in the cm range. Moreover, SKA will probe molecular emission from regions which are obscured by dust opacity at mm wavelengths, hence from the disk midplane, and often from the inner 30 au region. These observations will constrain the initial conditions for disk evolution and planet formation, allowing us to predict the chemical composition of the forming planets and their atmospheres. Comparison with forthcoming results on exoplanet atmospheres and on the chemistry of pristine bodies in the Solar System will provide new hints on the origin and evolution of planetary systems including our own.

astro-ph.SR↗

arXiv2026

Jets and Outflows in Young Stellar Objects with the SKAO

Jets and outflows are ubiquitous phenomena associated with the formation of young stellar objects (YSOs). They play a crucial role in removing angular momentum from the accreting system and in regulating star-formation efficiency. Theoretical studies and observations with ALMA and VLA have shown that jets and winds may have a crucial role in promoting dust growth in the envelope-disc system and in shaping the physical and chemical properties of the surrounding environment. Despite these significant advances, many fundamental questions remain unanswered regarding the acceleration, collimation, and chemical impact of jets and outflows from YSOs. The SKA-project will overcome the limitations of current mm/cm-facilities by enabling high-angular resolution and high-sensitivity cm-observations, crucial for probing jets/outflows near YSOs. Radio recombination lines, combined with proper motions, offer a unique opportunity to study the 3D-kinematics of jets. Non-thermal linearly polarised synchrotron emission will allow measuring magnetic field strength and morphology at unprecedented scales of a few au. Observations of dust emission in outflow cavities will allow studying how dust grows and is eventually transported from the disc to the envelope and back. Finally, the SKA-project will allow exploring the dust composition and chemical enrichment in shocks, where sputtering/shattering of grains cause the release of their mantles and refractory cores in the gas-phase. Complementary to ALMA's detection of simple and complex organic molecules, the SKAO will probe, for the first time, long carbon chains/rings, several Cl-, Al-, Mg-, and other metal-bearing species (missed by current sub-mm facilities).

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arXiv2026

Oxygen in the protostellar clump OMC-2 FIR4

Atomic oxygen (OI), OH, H2O, and CO are the main carriers of oxygen in dense interstellar gas and important coolants of shocks associated with protostellar outflows. We determine the relative abundances of these species in the warm inner parts of the protostellar clump OMC-2 FIR4 in Orion A. The clump contains several young stellar objects. The upGREAT receiver including the High Frequency Array (HFA, operating at 4.74 THz, 63 micron) onboard the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) was used to observe OMC-2 FIR4 in the lines of OI, OH, OD, HDO, and CO. Additional HDO lines were observed with the Atacama Pathfinder Experiment (APEX). Archival H2O and CO spectra observed by the Herschel satellite were included in the analysis. The observed lines were reasonably well reproduced by an expanding spherical shell model. The OI spectrum at 63 micron towards OMC-2 FIR4 is dominated by a broad line component, on top of which medium-wide and narrow line components can be discerned. The same components are present in the OH, H2O, and high-J CO spectra towards this source. We find that OI is more abundant than H2O in the shocked gas. In the broad line component, the following abundance ratios are derived: OI/H2O ~ 700, OI/OH ~ 300, OI/CO ~ 4. The high relative abundance of atomic oxygen there suggests an origin in dissociative J-shocks that are associated with strong ultraviolet radiation. The OI/CO ratio decreases below unity in the components with a smaller velocity dispersion, and these components also have higher abundances of H2O than the broad line component, although remaining below that of CO. The HDO/H2O ratio in the low-velocity components corresponds to the average ratio in the icy mantles of dust grains, and the presence of water there could also be understood in terms of sublimation without invoking high-temperature chemistry.

astro-ph.GA↗

arXiv2026

Star Formation Drives Production of Low Energy Cosmic Rays

For over a century, the origin of low-energy cosmic rays (LECRs), the dominant heaters and ionizers of dense interstellar gas, remains elusive owing to solar modulation and uncertain transport processes. In this study, we introduce a new astrophysical approach based on HI Narrow Self-Absorption (HINSA) to obtain spatially resolved measurements of LECR ionization rates using high-fidelity HI observations toward the Orion region from the FAST telescope. The LECR ionization rate is found to scale with local star formation rate (SFR) as $log_{10}\zeta = (1.4\pm 0.70)log_{10}\mathrm{SFR} + (-10.5\pm 2.9)$. Moreover, it increases with visual extinction, and is found to exceed, toward active star-forming regions, the value predicted for diffuse regions based on \textit{Voyager} measurements and an external propagation model. These findings demonstrate that LECRs are generated in situ by star-forming activities rather than penetrating from the broader Galactic cosmic-ray population. This is further supported by \textit{Fermi}-LAT gamma-ray observations toward the Orion region. Together, these results resolve a key uncertainty in cosmic-ray origin and establish a new avenue for quantifying the energetic feedback that regulates the interstellar medium.

astro-ph.GA↗

arXiv2026

Non-detection of HC(S)SH: Estimating Upper Limits and Constraining Chemistry

The search for dithioformic acid (t-HC(S)SH) in star-forming regions is crucial for understanding interstellar sulfur chemistry and addressing the 'missing sulfur' problem. Motivated by a recent claim of t-HC(S)SH detection in NGC 1333 IRAS 4A2, we independently reanalyzed the same ALMA dataset using comprehensive spectral and chemical modeling. We find no credible evidence for t-HC(S)SH: all reported transitions are fully accounted for by known, abundant molecules, with no unblended features unique to t-HC(S)SH. We critically reassess the reported detection, deriving stringent upper limits on the column density (N_t-HC(S)SH <= 4 x 10^14 cm^-2) and the fractional abundance (<= 1 x 10^-10 relative to H2). Our astrochemical models place these limits in context, showing the claimed detection likely results from spectral blending and inconsistent modeling assumptions. The non-detection aligns with chemical expectations given the rarity of complex and doubly sulfur-substituted molecules in hot corino environments. Furthermore, our analysis establishes a rigorous framework to guide future searches for sulfur-bearing species and highlights the critical importance of thorough line identification and modeling practices in astrochemistry.

astro-ph.GA↗

arXiv2026

Probing the ion-neutral drift velocity towards the L1544 prestellar core: Detection of ambipolar diffusion using N$_2$D$^+$ and para-NH$_2$D

The dynamical role of the magnetic field in the star formation process is tightly linked to the coupling between matter and the field. This coupling is due to the interaction between ions and neutrals in the partially ionized interstellar medium. When the ionization degree drops in the dense environment of prestellar cores, the magnetic field and the matter may decouple, leading to differences in the infalling velocities of ions and neutrals known as ambipolar diffusion. The onset of gravitational collapse resulting from ion-neutral decoupling has never been observed. The aim of this work is to search for signatures of ambipolar diffusion within a prestellar core. We observed the deuterated N$_2$D$^+$ ion and the neutral para-NH$_2$D species towards the prototypical prestellar core L1544. These two species are ideal tracers of prestellar cores sampling the same high densities in the core interior. We compared the velocity centroid and linewidth maps of the ion-neutral pair. We find a mean ion-neutral velocity difference of $\sim$0.05 km/s towards the core. By comparing with predictions from self-consistent calculations of the ambipolar resistivity including dust grain growth, we interpret the observed ion-neutral velocity difference in L1544 as a signature of ambipolar diffusion. We do not detect a significant ion-neutral linewidth difference that may be attributed to the subsonic infall motions of the gas in L1544 and geometrical effects in the presence of inclination. These results emphasize the role of dust grain growth at the prestellar core stage in setting the ambipolar resistivity and regulating the dynamical evolution of dense cores towards their collapse into protostars. We propose that measurements of ion-neutral drift velocities provide new constraints on the total magnetic field strength and the dust size distribution within prestellar cores.

astro-ph.GA↗

arXiv2026

Complex organic molecules and cosmic ray ionisation rate towards the massive protostar Cepheus A HW2

Cosmic rays (CRs) are important drivers for molecular chemistry in star-forming regions, and laboratory experiments have shown that CRs can stimulate the release of complex organic molecules (COMs) such as methanol. Observationally, this has primarily been tested in cold, low-mass cores, so studying how CRs affect COM formation in a high-mass star-forming environment is of great interest. We performed a high-sensitivity wide-band spectral line survey with the Onsala 20 m telescope towards the high-mass protostar Cepheus A HW2, which is known to host an ionised jet. Consistent with previous studies, two primary velocity components ($-11$ km s$^{-1}$ and $-5$ km s$^{-1}$) were identified. Column densities and relative abundances of the detected ions and COMs were estimated from rotational diagrams, single transitions and RADEX grid searches (CH$_3$OH: $1.6\times10^{-9}$, CH$_3$CN: $5.9\times10^{-11}$, t-HCOOH: $7.9\times10^{-11}$, H$_2$CCO: $1.7\times10^{-11}$, CH$_3$CHO: $1.9\times10^{-11}$, CH$_3$OCHO: $7.6\times10^{-10}$ at $-11$ km s$^{-1}$). Deuterium fractions were also estimated (in range $0.002-0.3$ at $-11$ km s$^{-1}$), and the volume density of molecular hydrogen ($2.6\times10^5$ cm$^{-3}$ at $-11$ km s$^{-1}$) was constrained from the RADEX grid searches. Electron fractions and CR ionisation rates (CRIR, $6.8\times10^{-17}$ s$^{-1}$ at $-11$ km s$^{-1}$, $\leq9.2\times10^{-19}$ s$^{-1}$ at $-5$ km s$^{-1}$) were estimated through analytic chemistry using different ions as probes. The gas-grain chemical code Nautilus reproduced the observed abundances of CH$_3$OH, CH$_3$CN, HCO$^+$, N$_2$H$^+$ at the observed density, temperature and CRIR within the uncertainty of the model. The results indicate that the CR ionisation rate of the kinematic component associated with most of the COMs' emission in the region is locally enhanced.

astro-ph.GA↗

arXiv2026

PRODIGE -- envelope to disk with NOEMA VIII. Sulfur oxides trace a shock caused by a streamer in the inner envelope of a protostar

(Abridged) Recently, streamers have been observed causing shocks at the outer edge of protoplanetary disks. The study of sulfur-bearing species can help us to understand the physical and chemical changes caused by infalling streamers toward their landing positions. We study the physical properties traced by SO$_2$ and SO toward the Class I protostar Per-emb 50, which is possibly related to the streamer infalling toward its disk. We present new NOEMA A-array observations as part of the large program "Protostars and Disks: Global Evolution" (PRODIGE). We analyzed the morphology of SO$_2$ and SO, and complement our interpretations with additional H_$2$CO and CO data from the same program. We compared the SO$_2$ and SO morphology with an infalling-rotating model. We applied Bayesian model selection to the brightest SO$_2$ line to disentangle the different kinematic components traced by this molecule. We used Local Thermodynamic Equilibrium (LTE) and non-LTE analyses to determine the temperature and density of the SO$_2$ emission. There are two separate peaks of SO$_2$ emission offset toward the southwest of Per-emb 50, one brighter (peak 1) at about 180 au from the protostar, and a weaker one (peak 2) at about 400 au. Peak 2 is blueshifted with respect to an infalling-rotating envelope. We propose that this peak is caused by the shock between the inner envelope and the streamer. Peak 1 is consistent with the expected envelope motion, and could thus be caused by shocks at the disk-envelope interface, but potential streamer influence cannot be neglected. Both peaks show abundance ratios consistent with a low velocity shock ($\sim 3-4$ \kms) when compared with shock models. Streamers can affect the physical and chemical structure of both disks and envelopes, suggesting that streamers can play an important role in shaping both structures in the embedded stages of star formation.

astro-ph.SR↗

arXiv2026

ALMA Central Molecular Zone Exploration Survey (ACES) I: Overview

The mass flows and energy cycles within the inner regions of galaxies exert a powerful influence on the evolution of the galaxy population. The centre of the Milky Way is the only galactic nucleus for which it is possible to resolve the physical mechanisms that drive these cycles, namely star formation and feedback, while also tracing global (>100 pc) processes which determine where and when star formation and feedback occur. We present an overview of ACES, the 'Atacama Large Millimeter/submillimeter Array (ALMA) CMZ Exploration Survey', a ~1.5" angular resolution, 0.2-3 km/s spectral resolution ALMA Band 3 (85-102 GHz), survey of the 'Central Molecular Zone' (CMZ) -- the inner-100 pc of the Galaxy (l = 359.4 deg to 0.8 deg). ACES spectral setup is tuned to observe optimal tracers of the physical, chemical, and kinematic conditions in over 70 spectral features (e.g. HCO+, HNCO, SiO, H40alpha, complex molecules) of the gas in the CMZ, to derive the properties of all potentially star-forming Galactic Centre gas, from global scales (100 pc) to dense ~0.05 pc structures that are expected to host individual star-forming cores, down to sub-sonic (<0.4 km/s) velocity resolution. In this overview paper, we provide the scientific justification for the ACES survey, explain the choice of observational setup, and describe the data legacy products. Finally, we show some of the initial ACES data which highlight the power of ACES' combination of high angular resolution, unprecedented spatial dynamic range, sensitivity, spectral resolution and spectral bandwidth as an illustration of how ACES aims to understand how global processes set the location, intensity, and timescales for star formation and feedback in the CMZ.

astro-ph.GA↗

arXiv2026

Carbon chain diversity in L1544 and IRAS 16293-2422: an astrochemical pathfinder study for the SKAO

Astrochemical observations have revealed a surprisingly high level of chemical complexity, including long carbon chains, in the earliest stages of Sun-like star formation. The origin of these species and whether they undergo further growth, possibly contributing to the molecular complexity of planetary systems, remain open questions. We present recent observations performed using the 100-m Green Bank Telescope of the prestellar core L1544 and the protostellar system IRAS 16293-2422. In L1544, we detected several complex carbon-bearing species, including $\mathrm{C_2S}$, $\mathrm{C_3S}$, $\mathrm{C_3N}$, $\mathrm{c\text{-}C_3H}$, $\mathrm{C_4H}$, and $\mathrm{C_6H}$, complementing previously reported emission of cyanopolyynes. In IRAS 16293-2422, we detected $\mathrm{c\text{-}C_3H}$ and, for the first time, $\mathrm{HC_7N}$. Thanks to the high spectral resolution, we refine the rest frequencies of several $\mathrm{c\text{-}C_3H}$ and $\mathrm{C_6H}$ transitions. We perform radiative transfer analysis, highlighting a chemical difference between the two sources: IRAS 16293-2422 shows column densities 10-100 times lower than L1544. We perform astrochemical modeling, employing an up-to-date chemical network with revised reaction rates. The models reproduce the general trends, with cyanopolyyne and polyynyl radical abundances decreasing as molecular size increases, but they underestimate the abundances of cyanopolyynes longer than $\mathrm{HC_5N}$ by up to two orders of magnitude. Current models, which include the dominant neutral-neutral formation routes, cannot account for this discrepancy, suggesting that the chemical network is incomplete. We propose that additional ion-molecule reactions are crucial for the formation of these species. Developing a more comprehensive chemical network for long carbon chains is essential for accurately interpreting present and future observations.

astro-ph.GA↗

arXiv2026

Using astrochemical models to simulate reactivity experiments on cold surfaces

The development of molecular complexity during stellar and planetary formation owes much to the interaction of gas and dust. When the first astrochemical models including solid-state chemistry were developed more than forty years ago, data from dedicated laboratory experiments were limited. Since then, many groups have developed specific experimental setups to address this issue, but astrochemical models have rarely been directly confronted with these new results. We want to demonstrate whether it is possible to use rate-equation-type astrochemical models developed in the context of the Interstellar Medium to compare them with laboratory astrophysics experiments. In this work, we use the case of low-temperature hydrogenation of CO, which is known to lead to methanol, among other molecules. We carried out 9 experiments, varying the experimental parameters such as temperature and dose. We give quantitative results and take care of detailing the vocabulary used in the experiments. We use astrochemical codes, NAUTILUS, pyRate and MONACO, to reproduce our experimental conditions, which requires good control of the change of vocabulary and scales, especially for fluxes and time scales. This work demonstrates that it is possible to use different astrochemical codes to compare modelling results directly with the output of experiments. There are discrepancies between models and experiments, as well as between models, but a fair agreement is achieved. We discuss the possible origin of the differences, which could originate from the chemical network or the difference in the description of physical processes.

astro-ph.GA↗

arXiv2026

Habitability of Tidally Heated H$_2$-Dominated Exomoons around Free-Floating Planets

Exomoons around free-floating planets (FFPs) can survive their host planet's ejection. Such ejections can increase their orbital eccentricity, providing significant tidal heating in the absence of any stellar energy source. Previous studies suggested that liquid water could exist on such moons under thick CO$_2$-dominated atmospheres, but these models faced challenges with CO$_2$ condensation and atmospheric collapse, particularly in the high-pressure regimes that favoured long-term habitability. To address this, we employ a self-consistent model, including radiative transfer and equilibrium chemistry with condensation, to simulate a more stable hydrogen-dominated atmosphere for a range of initial chemical compositions, including C, O, and N. We find that such atmospheres can effectively trap heat via collision-induced absorption of H$_2$, maintaining surface temperatures suitable for liquid water for time-scales of up to 4.3 Gyr, depending on the surface pressure, while not prone to condensation-induced collapse. Wet-dry cycling caused by the strong tides together with the alkalinity of dissolved NH$_3$ could create favourable conditions for RNA polymerisation and thus support the emergence of life.

astro-ph.EP↗

arXiv2026

Broadband infrared spectroscopy of methanol isotopologues in pure, H2O-rich, and CO-rich ice analogues

Deuterium fractionation is highly efficient during the early stages of star formation, particularly in starless and prestellar cores where temperatures are low (<10 K) and molecular freeze-out onto dust grains is significant. Methanol forms early in these environments following CO freeze-out via successive hydrogenation reactions on grain surfaces, while the production of deuterated methanol requires elevated gas-phase D/H ratios generated through dissociative recombination of deuterated H3+. Consequently, large abundances of deuterated methanol are observed toward young stellar objects where prestellar ices have recently sublimated. Here, we present laboratory infrared spectra of methanol and its deuterated isotopologues in astrophysical ice analogues, complemented by anharmonic vibrational calculations used to guide band assignments. Experiments were performed at the CASICE laboratory using a Bruker Vertex 70v spectrometer coupled to a closed-cycle helium cryostat, with isotopologue ices deposited at 10 K under high-vacuum conditions. Infrared transmission spectra were recorded over 6000-30 cm-1 (1.67-333 um) and compared with spectra of pure isotopologue ices. Distinctive mid-infrared band patterns are identified for each deuterated species. In particular, CH2DOH exhibits a characteristic doublet at 1293 and 1326 cm-1 (7.73 and 7.54 um), while CHD2OH shows a similar doublet at 1301 and 1329 cm-1 (7.69 and 7.52 um), both remaining largely invariant across all studied ice mixtures. These robust spectral signatures provide reliable tracers for identifying deuterated methanol in JWST observations and for constraining astrochemical gas-grain models of deuterium enrichment prior to star and planet formation.

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