arXiv subjects

Paola Caselli

Publications and source records attributed to Paola Caselli.

At least 19 recordsLinked to original sources

arXiv2026

Chemical and dynamical evolution of prestellar cores, the dawn of star and planet formation

Studying prestellar cores is crucial to understanding the formation of stellar and planetary systems. To date, we lack a comprehensive survey of their density and velocity structure at small scales (100-1000 au), the gas ionisation state, their lifetimes, and how these properties are influenced by the environment. A deep study into this critical stage of star formation requires a large single-dish (sub)millimetre telescope like AtLAST.

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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

Small Collisions, Big Impact: The Sensitivity of Atmospheric Temperatures to Collision-Induced Absorption

Collision-induced absorption (CIA) data are relevant to determining atmospheric temperatures and, in turn, the habitability of a diverse set of planetary atmospheres. However, these data remain sparse across broad wavelength and temperature ranges. Nevertheless, multiple, sometimes disagreeing, sources of experimental and theoretical data can be available for the same CIA pair. In this paper, we assess the impact of these varying sources on atmospheric temperatures and the limitations that the available data coverage can introduce. In order to inform future measurements and calculations, we test additional cases with extrapolated wavelength and temperature ranges. We first introduce a measure of the importance of CIA opacities depending on pressure and temperature by calculating the Rosseland mean with and without each CIA pair's contribution. Secondly, we employ each CIA source in a radiative transfer model of idealised atmospheres which maximise the effect of the CIA pair of interest. Our findings highlight the uncertainties related to the current data. Temperature differences between CIA data sources range from $\sim$10\,K when sources overlap in wavelength and temperature coverage to >100\,K when spectral gaps or temperature limits differ. Spectral windows or narrow temperature coverage can also hinder the model's convergence of the temperature profile. This work aims to provide guidance on the selection of CIA opacity datasets for atmospheric modelling and to identify the wavelength and temperature regimes in which further laboratory measurements and theoretical calculations are most urgently needed. We publish all our findings in detail in the form of an accessible website.

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arXiv2026

Isomer-resolved ion-molecule reaction kinetics of HCO+ and HOC+

The HCO$^+$/HOC$^+$ abundance ratio has been observed to vary significantly in different astrophysical environments, making it an important diagnostic tool for exploring the interstellar medium. However, uncertainties in the reaction rates and branching ratios of key chemical reactions involving these ions affect the conclusions drawn from observations of either these species or other species that are derived on their basis. Here, we present isomer-resolved ion-molecule reaction rate coefficients of reactions involving HCO$^+$ and HOC$^+$ using a cryogenic ion trap experiment. We measured the rate coefficients of the isomerization reaction of HOC$^+$ to HCO$^+$ with H$_2$ and CO, as well as three different reactions producing both isomers: CO$^+$ + H$_2$ , H$_3^+$ + CO, and ArH$^+$ + CO. Our results show that the reaction of CO$^+$ with H$_2$ predominantly produces HCO$^+$ with a negligible yield of HOC$^+$ . This result is in agreement with recent theoretical predictions, but in major contrast to the generally accepted branching ratio based on previous experimental results. We also find that the isomerization reaction of HOC$^+$ to HCO$^+$ is efficient with both H2 and CO. We were able to assess the impact of our new measured reaction rate coefficients on the predicted HCO$^+$/HOC$^+$ abundance ratio using a state-of-the-art astrochemical model.

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arXiv2026

A Cyanopolyyne-rich but COM-poor Massive Protostar: The First Hot Carbon Chain Chemistry Source G28.28-0.36

We present molecular emission line data from the massive young stellar object (MYSO) G28.28-0.36 (G28.28) obtained with the Atacama Large Millimeter/submillimeter Array Band 3. Cyanopolyynes (HC$_3$N and HC$_5$N) and three complex organic molecules (COMs; CH$_3$OH, CH$_3$CN, and CH$_3$CHO) are detected from the MYSO G28.28. In addition, strong emission regions of cyanopolyynes are identified between G28.28 and a nearby ultracompact H II region. The HC$_5$N emission is coincident with the dust continuum peak, where an excitation temperature of 100 K is derived from CH$_3$CN. These results suggest that the Hot Carbon Chain Chemistry (HCCC) mechanism produces cyanopolyynes in the hot region around G28.28. We find that G28.28 exhibits a unique chemical feature: cyanopolyynes are abundant, but COMs are deficient, unlike the other MYSOs studied previously. These results imply that G28.28 is a counterpart of the Warm Carbon Chain Chemistry (WCCC) low-mass source L1527. G28.28 is the first HCCC source identified so far.

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arXiv2026

The complex organic molecular content in the L1517B starless core

Recent observations of the pre-stellar core L1544 and the younger starless core L1498 have revealed that complex organic molecules (COMs) are enhanced in the gas phase toward their outer and intermediate-density shells. Our goal is to determine the level of chemical complexity toward the starless core L1517B, which seems younger than L1498, and compare it with the other two previously studied cores to see if there is a chemical evolution within the cores. We have carried out 3 mm high-sensitivity observations toward two positions in the L1517B starless core: the core's centre and the position where the methanol emission peaks (at a distance of $\sim$5000 au from the core's centre). Our observations reveal that a lower number of COMs and COM precursors are detected in L1517B with respect to L1498 and L1544, and also show lower abundances. Besides methanol, we only detected CH$_3$O, H$_2$CCO, CH$_3$CHO, CH$_3$CN, CH$_3$NC, HCCCN, and HCCNC. Their measured abundances are $\sim$3 times larger toward the methanol peak than toward the core's centre, mimicking the behaviour found toward the more evolved cores L1544 and L1498. We propose that the differences in the chemical complexity observed between the three studied starless cores are a consequence of their evolution, with L1517B being the less evolved one, followed by L1498 and L1544. Chemical complexity in these cores seems to increase over time, with N-bearing molecules forming first and O-bearing COMs forming at a later stage as a result of the catastrophic depletion of CO.

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arXiv2026

Electron irradiation and temperature-programmed desorption of layered ethanolamine-water-methanol ices

Ethanolamine (EtA) is a recently detected interstellar complex organic molecule and a potential precursor to more functionalised N-bearing species. We investigate the electron-driven chemistry of layered EtA--H$_2$O--CH$_3$OH ices deposited at 20~K, combining 2~keV electron irradiation with subsequent temperature-programmed desorption (TPD) while monitoring the solid phase by mid-infrared spectroscopy. These controlled irradiation conditions represent a simplified laboratory analogue of the structural and energetic conditions within interstellar grain mantles. Volatile products released during warming were followed by quadrupole mass spectrometry, and the final residue was analysed by \textit{ex situ} electrospray ionisation mass spectrometry (ESI-MS). Irradiation produces abundant radiolysis products typical of processed methanol- and water-rich ices, including CO, CO$_2$, H$_2$CO, H$_2$O$_2$, NH$_3$, and OCN$^{-}$. Bands consistent with formic acid and polyoxymethylene-like material are also observed, indicating that carbonyl chemistry and formaldehyde-driven oligomerisation can occur alongside fragmentation in the layered system. Comparison with a choline chloride--H$_2$O reference reveals the emergence of features compatible with methylated EtA derivatives and/or choline-like species, while ESI-MS shows higher-$m/z$ mass families in the residue that are not reproduced in the substrate/background blank. However, spectral congestion and matrix effects preclude an unambiguous assignment of choline. Our results show that energetic processing of EtA-containing ices can generate a diverse inventory of O- and N-bearing products, providing laboratory constraints on radiation chemistry in cold, irradiated grain mantles under idealised conditions.

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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 $μ$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 $μ$m to 1.3 mm. The line-corrected continuum maps reveal the dust distribution across successive shocks. The dust opacity index ($β\approx1.8$--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$σ$. 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.

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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.

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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.

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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.

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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

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.

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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 $log_{10}ζ= (1.4\pm 0.70)log_{10}\mathrm{SFR} + (-10.5\pm 2.9)$. 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.

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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.

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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.

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