Searcharxiv⌕ Search

arXiv subjects

Serena Viti

Publications and source records attributed to Serena Viti.

At least 19 recordsLinked to original sources
arXiv2026

Spatially resolved molecular gas conditions in the circumnuclear disc of 3C 84

The brightest cluster galaxy NGC 1275, at the centre of the Perseus cluster, hosts the radio-loud AGN 3C 84 and a circumnuclear disc (CND) of cold molecular gas onto which large-scale CO filaments accrete, yet the physical and chemical conditions of the gas in the CND remain poorly constrained. We present a spatially resolved analysis of ALMA CO(2-1), HCN(3-2), and HCO$^+$(3-2) observations at 72 pc. The data are partitioned into beam-sized hexagonal regions and modelled with a Bayesian framework that couples a neural network emulator of time-dependent chemistry (UCLCHEM) with non-LTE radiative transfer (SpectralRadex) to infer the gas density, kinetic temperature, and cosmic ray ionisation rate in each region. All three line ratios peak in the inner disc and decline with radius. The inference shows radial gradients in density ($\log_{10} n({\rm H_2}) \approx 6.3$ to $\sim 5$), kinetic temperature (~200 K to ~160 K), and cosmic ray ionisation rate ($\log_{10}(ζ/ζ_0) \approx 4.9$ to $\sim 3$). Despite the powerful radio AGN, the observed HCN(3-2)/HCO$^+$(3-2) ratio remains $\lesssim 1$ across the disc. Our modelling attributes this to optical depth saturation of HCN(3-2) ($τ\sim 1$-3), which suppresses the intensity ratio even when the HCN abundance exceeds that of HCO$^+$ by a factor of three or more. The HCN/HCO$^+$ intensity ratio therefore cannot be used as an abundance diagnostic without accounting for optical depth, and a low ratio does not necessarily imply weak AGN influence on the chemistry. Azimuthally resolved profiles suggest a localised HCO$^+$/CO enhancement at the western disc boundary, coinciding with the filament-disc accretion interface and consistent with shock processing by velocity shear between the infalling filaments and the rotating disc. These results indicate that the CND is shaped by accretion from its filamentary environment.

astro-ph.GA↗
arXiv2026

The curious case of HCO$^+$: Extreme abundances under extreme conditions

Context. HCO$^+$ is widely observed in both Galactic and extragalactic environments and typically exhibits abundances of $10^{-9}-10^{-8}$. However, recent modeling studies suggest that in environments exposed to elevated cosmic-ray ionization rates and strong thermal or mechanical processing its abundance may increase by several orders of magnitude. Aims. To interpret these predictions, we need to understand the physical conditions that produce extreme HCO$^+$ abundances and the chemical pathways that drive these enhancements. Methods. We used UCLCHEM, a gas-grain chemical code, to model the chemistry of HCO$^+$ in dense molecular, protostellar, and shocked gas under elevated cosmic-ray ionization rates ($ζ\ge 10^{-15}\,\mathrm{s^{-1}}$). Results. Extreme HCO$^+$ enhancements leading to $X$(HCO$^+$) $\gtrsim 10^{-4}$ occur only under specific combinations of temperature, density, and cosmic-ray ionization rate, primarily in protostellar and shocked gas. Increasing density generally suppresses the peak HCO$^+$ abundance, requiring higher ionization rates to produce comparable enhancements. More importantly, the extreme enhancements seem to be very dependent on the chemical network used (in our case UMIST12 versus UMIST22, with the latter leading to extreme abundances). These differences among networks arise from the removal of the destruction pathway of HCO$^+$: C + HCO$^+$ $\rightarrow$ CO + CH$^+$, and propagate to several other species including N$_2$H$^+$, H$_2$O, and H$_3$O$^{+}$.

astro-ph.GA↗
arXiv2026

Anomalously high deuterium fractionation in a galactic translucent cloud: a challenge to chemical models

Deuterated (D-) species have long been proposed to diagnose the physical conditions and chemical evolution of cold dense molecular clouds. While deuterium fractionation has been extensively measured in dense cores, observations in diffuse and translucent clouds remain rare. We report here the detection of DCN and DNC toward a translucent cloud ($A_{\rm V} =1.2\pm0.2$ mag, $n_{\rm H_2}$ = $3.9\pm0.2\times10^2$ cm$^{-3}$) through sensitive absorption observations with the IRAM NOrthern Extended Millimeter Array (NOEMA). This detection reaches the lowest column-density and volume-density regime in which deuteration has been observed so far. Interestingly, the observed DCN/HCN and DNC/HNC abundance ratios ($3.3\pm0.6\times10^{-3}$ and $3.6\pm1.2\times10^{-3}$, respectively), which are more than two orders of magnitude higher than the element abundance [D]/[H] (1.5$\times$10$^{-5}$), suggest an unexpected enhancement of deuterium fractionation in the translucent cloud. These results represent a significant departure from established chemical models considering deuterium fractionation, which predict negligible formation of D-molecules in such environments. Although it remains unclear how D-molecules built up their abundances in translucent gas, a dispersed dense core scenario could potentially explain the observed high deuterium fraction. This interpretation is consistent with the idea proposed by Price et al. (2003) more than two decades ago: a translucent cloud may be a transient, dynamically evolving structure formed through the dissipation of a dense molecular cloud.

astro-ph.GA↗
arXiv2026

Physical and Chemical Conditions of Molecular Gas in NGC 1068: The nuclear feedback in the circumnuclear disk and starburst ring

Molecular gas in galaxies is shaped by both star formation and active galactic nuclei. In NGC 1068, the circumnuclear disk and the starburst ring offer a nearby case to study these effects with many molecular tracers. Earlier work has shown strong outflow activity and complex chemistry, which motivates the use of methods that combine radiative transfer with time-dependent chemistry. Our aim is to map the physical conditions across the circumnuclear disk and the starburst ring of NGC 1068 and to test whether the nuclear outflow influences the molecular gas in the ring. We also examine whether the heating or the quiescent cloud scenario better matches the observations. We use archival ALMA observations obtained in Bands 3, 4, and 5, covering molecular species including HCN, HCO+, HNC, CS, CN and C2H. All data cubes are convolved to a common resolution of 0.8" and are sampled into 56 pc hexagons with a signal-to-noise threshold of three. We perform hierarchical Bayesian inference that links a non-LTE radiative transfer module SpectralRadex with chemical modelling. To make the analysis efficient, we replace direct UCLCHEM calculations with a neural network emulator trained on a large model grid. Sampling is done with Nautilus. We also compare our results with previous studies that used RADEX and UCLCHEM for selected regions. The emulator reproduces the UCLCHEM abundances with low error and allows inference at modest computational cost. We find clear radial and azimuthal variations in gas density, temperature, column density, and cosmic-ray ionization rate.

astro-ph.GA↗
arXiv2026

UCLCHEM 4.0: An open source gas-grain astrochemistry simulation framework

Astrochemical modeling is a key tool for the understanding of the formation and destruction of molecules in the dense gas of the interstellar medium, as observed by modern day observational facilities. UCLCHEM is a comprehensive astrochemical modeling framework that can model the interstellar medium ranging from extra-galactic to protoplanetary disks scales. The framework consists of a core routine that solves chemical reaction networks as a function of time. The chemistry includes a description of gas and ice grain chemistry and the interactions between the two. The physical modeling includes parametrizations for modelling cloud collapse, protostellar cores and shocks as well as the ability to provide user defined inputs. This manuscript provides an overview of the physics and chemistry included in UCLCHEM, as well as the inner workings of the solver routine and the programming interface.

astro-ph.GA↗
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

Radiation magnetohydrodynamics simulations of Population III star formation during the Epoch of Reionization

Cosmological simulations find that pockets of star-forming gas could remain pristine up until the Epoch of Reionization (EoR) due to the inhomogeneous nature of metal mixing and enrichment in the early Universe. Such pristine clouds could have formed Population III stars, which could have distinct properties compared to their very high redshift ($z \geq 20$) counterparts. We investigate how Population III stars form and grow during the EoR, and whether the resulting mass distribution varies with environment or across cosmic time. We perform high-resolution ($7.5\,\rm{au}$) radiation-magnetohydrodynamics simulations of identical primordial clouds exposed to the CMB appropriate for $z=30$ and $z=6$, respectively, as part of the POPSICLE project. We also run a simulation at $z=6$ with a strong external Lyman-Werner (LW) background, to span across radiative environments which could host metal-free clumps during the EoR. In the limit of no external LW radiation, we find that while the evolution of the most massive star ($M_{\star} \approx 70\,\rm{M_{\odot}}$) is almost identical between $z=30$ and $z=6$, the latter exhibits less fragmentation, leading to a smaller cluster of stars with a higher median stellar mass. In the limit of high external LW radiation, we see vigorous accretion and high star formation efficiencies, leading to the formation of very massive ($M_{\star} > 100\,\rm{M_{\odot}}$) stars. Our results suggest that Population III IMF could vary with redshift simply due to the CMB, independent of the environment. We find that less massive and more compact Pop III star clusters could form during the EoR as compared to $z \geq 20$, with the formation of very massive and supermassive stars likely in strongly irradiated environments.

astro-ph.GA↗
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.

astro-ph.GA↗
arXiv2026

High-energy astrochemistry in the molecular interstellar medium

In the past decade, there has been a significant shift in astrochemistry with a renewed focus on the role of non-thermal processes on the molecular interstellar medium, in particular energetic particles (such as cosmic ray particles and fast electrons) and X-ray radiation. This has been brought about in large part due to new observations of interstellar complex organic molecules (iCOMS) in environments that would inhibit their formation, such as cold, dense gas in prestellar cores or in the highly energetic environments in galactic centers. In parallel, there has been a plethora of new laboratory investigations on the role of high-energy radiation and electrons on the chemistry of astrophysical ices, demonstrating the ability of this radiation to induce complex chemistry. In recent years, theoretical models have also begun to include newer cosmic-ray-driven processes in both the gas and ice phases. In this review, we unify aspects of the chemistry driven by X-ray radiation and energetic particles into a ``high-energy astrochemistry'', defining this term and reviewing the underlying chemical processes. We conclude by examining various laboratories where high-energy astrochemistry is at play and identify future issues to be tackled.

astro-ph.GA↗
arXiv2026

One-dimensional and time-dependent modelling of complex organic molecules in protostars

Complex organic molecules (COMs), the building blocks of life, have been extensively detected under various physical conditions, from quiescent clouds to star-forming regions. They therefore serve as excellent tracers for the local physical and chemical properties of these environments. Proper models that are capable of grasping the formation and destruction of COMs are crucial to understanding observations. However, given that distinct COMs may be detected from different locations and at varying times, we improve UCLCHEM - a gas-grain chemical code - to a one-dimensional, time-dependent model, tailored to protostars. In this update, we examine two stages of a protostar: the prestellar and heating stages, incorporating a simple radiative mechanism for both the internal and external radiation fields of the cloud. This approach relies on the key assumption that the dust and gas temperatures are completely coupled. Ultimately, we implement an updated version of our model to interpret observations obtained through both single-dish and interferometry under varying conditions, including a SgrB2(N1) hot core, massive Galactic clumps and a hot core in Orion. We show that our model could reproduce these observations well, highlighting that some COMs are positioned at a higher temperature in the envelope, whereas others are from the lower temperature, potentially leading to misinterpretation when using a single-point model. In a particular case of SgrB2(N1), the best model indicates that the cosmic-ray ionisation rate significantly exceeds the value typically used for the standard interstellar medium. Our model shows as an efficient computational tool particularly useful for better insights into observations of COMs.

astro-ph.GA↗
arXiv2025

The Emergence of Prebiotic Chemistry in the ISM

Contrary to popular belief, the interstellar medium (ISM) is not empty; it is filled with atoms, dust particles, and molecules. Some of these molecules may have been the very building blocks of life that, delivered to Earth via comets and meteorites, could have given rise to Life itself. A large-area single-dish telescope with superb sensitivity, field-of-view and multi-band instruments will allow us to explore the limits of chemical complexity in the interstellar medium, across our Galaxy and in external galaxies, determining whether amino acids, sugars, or RNA/DNA nucleobases can form in space.

astro-ph.IM↗
arXiv2025

A sensitivity analysis of interstellar ice chemistry in astrochemical models

Astrochemical models are essential to bridge the gap between the timescales of reactions, experiments, and observations. Ice chemistry in these models experiences a large computational complexity as a result of the many parameters required for the modeling of chemistry occurring on these ices, such as binding energies and reaction energy barriers. Many of these parameters are poorly constrained, and accurately determining all would be too costly. We aim to find out which parameters describing ice chemistry have a large effect on the calculated abundances of ices for different prestellar objects. Using Monte Carlo sampled binding energies, diffusion barriers, desorption and diffusion prefactors, and reaction energy barriers, we determined the sensitivity of the abundances of the main ice species calculated with UCLCHEM, an astrochemical modeling code, on each of these parameters. We do this for a large grid of physical conditions across temperature, density, cosmic ray ionization rate and UV field strength. We find that, regardless of the physical conditions, the main sensitivities of abundances of the main ice species are the diffusion barriers of small and relatively mobile reactive species such as H, N, O, HCO, and CH$_3$. Thus, these parameters should be determined more accurately to increase the accuracy of models, paving the way to a better understanding of observations of ices. In many cases, accurate reaction energy barriers are not essential due to the treatment of competition between reactions and diffusion.

astro-ph.GA↗
arXiv2025

Carbox: an end-to-end differentiable astrochemical simulation framework

Since the first observations of interstellar molecules, astrochemical simulations have been employed to model and understand its formation and destruction path- ways. With the advent of high-resolution telescopes such as JWST and ALMA, the number of detected molecules has increased significantly, thereby creating a need for increasingly complex chemical reaction networks. To model such complex systems, we have developed Carbox, a new astrochemical simulation code that leverages the modern high-performance transformation framework Jax. With Jax enabling computational efficiency and differentiability, Carbox can easily utilize GPU acceleration, be used to study sensitivity and uncertainty, and interface with advances in Scientific Machine Learning. All of these features are crucial for modeling the molecules observed by current and next-generation telescopes.

astro-ph.GA↗
arXiv2025

Chemical templates of the Central Molecular Zone. Shock and protostellar object signatures under Galactic Center conditions

(Abridged) The Central Molecular Zone (CMZ) of the Milky Way exhibits extreme conditions, including high gas densities, elevated temperatures, enhanced cosmic-ray ionization rates, and large-scale dynamics. Large-scale molecular surveys reveal increasing chemical and physical complexity in the CMZ. A key step to interpreting the molecular richness found in the CMZ is to build chemical templates tailored to its diverse conditions. The combined impact of high ionization, elevated temperatures, and dense gas remains insufficiently explored for observable tracers. In this study, we utilized UCLCHEM, a gas-grain time-dependent chemical model, to link physical conditions with their corresponding molecular signatures and identify key tracers of temperature, density, ionization, and shock activity. We ran a grid of models of shocks and protostellar objects representative of typical CMZ conditions, focusing on twenty-four species, including complex organic molecules. Shocked and protostellar environments show distinct evolutionary timescales ($\lesssim 10^4$ vs. $\gtrsim 10^4$ years), with 300 K emerging as a key temperature threshold for chemical differentiation. We find that cosmic-ray ionization and temperature are the main drivers of chemical trends. HCO$^+$, H$_2$CO, and CH$_3$SH trace ionization, while HCO, HCO$^+$, CH$_3$SH, CH$_3$NCO, and HCOOCH$_3$ show consistent abundance contrasts between shocks and protostellar regions over similar temperature ranges. While our models underpredict some complex organics in shocks, they reproduce observed trends for most species, supporting scenarios involving recurring shocks in Galactic Center clouds and enhanced ionization towards Sgr B2(N2). Future work should assess the role of shock recurrence and metallicity in shaping chemistry.

astro-ph.GA↗
arXiv2025

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.

astro-ph.GA↗
arXiv2025

NeuralPDR: Neural Differential Equations as surrogate models for Photodissociation Regions

Computational astrochemical models are essential for helping us interpret and understand the observations of different astrophysical environments. In the age of high-resolution telescopes such as JWST and ALMA, the substructure of many objects can be resolved, raising the need for astrochemical modeling at these smaller scales, meaning that the simulations of these objects need to include both the physics and chemistry to accurately model the observations. The computational cost of the simulations coupling both the three-dimensional hydrodynamics and chemistry is enormous, creating an opportunity for surrogate models that can effectively substitute the chemical solver. In this work we present surrogate models that can replace the original chemical code, namely Latent Augmented Neural Ordinary Differential Equations. We train these surrogate architectures on three datasets of increasing physical complexity, with the last dataset derived directly from a three-dimensional simulation of a molecular cloud using a Photodissociation Region (PDR) code, 3D-PDR. We show that these surrogate models can provide speedup and reproduce the original observable column density maps of the dataset. This enables the rapid inference of the chemistry (on the GPU), allowing for the faster statistical inference of observations or increasing the resolution in hydrodynamical simulations of astrophysical environments.

astro-ph.GA↗
arXiv2025

The Rich JWST Spectrum of the Western Nucleus of Arp 220: Shocked Hot Core Chemistry Dominates the Inner Disk

We present full 3-28 $\mathrm{μm}$ JWST MIRI/MRS and NIRSpec/IFU spectra of the western nucleus of Arp 220, the nearest ultraluminous infrared galaxy. This nucleus has long been suggested to possibly host an embedded Compton-thick AGN. Millimeter observations of the dust continuum suggest the presence of a distinct 20 pc core with a dust temperature of $T_\mathrm{d} \gtrsim 500~\mathrm{K}$, in addition to a 100 pc circumnuclear starburst disk. However, unambiguously identifying the nature of this core is challenging, due to the immense obscuration, the nuclear starburst activity, and the nearby eastern nucleus. With the JWST integral field spectrographs, we can, for the first time, separate the two nuclei across this full wavelength range, revealing a wealth of molecular absorption features towards the western nucleus. We analyse the rovibrational bands detected at 4-22 $\mathrm{μm}$, deriving column densities and rotational temperatures for 10 distinct species. Optically thick features of C$_2$H$_2$, HCN and HNC suggest that this molecular gas is hidden behind a curtain of cooler dust, and indicate that the column densities of C$_2$H$_2$ and HCN are an order of magnitude higher than previously derived from Spitzer observations. We identify a warm HCN component with rotational temperature $T_\mathrm{rot} = 330~\mathrm{K}$, which we associate with radiative excitation by the hot inner nucleus. We propose a geometry where the detected molecular gas is located in the inner regions of the starburst disk, directly surrounding the hot 20 $\mathrm{pc}$ core. The chemical footprint of the western nucleus is reminiscent of that of hot cores, with additional evidence for shocks. Despite the molecular material's close proximity to the central source, no evidence for the presence of an AGN in the form of X-ray-driven chemistry or extreme excitation is found.

astro-ph.GA↗
arXiv2025

Understanding molecular ratios in the carbon and oxygen poor outer Milky Way with interpretable machine learning

Context. The outer Milky Way has a lower metallicity than our solar neighbourhood, but still many molecules are detected in the region. Molecular line ratios can serve as probes to better understand the chemistry and physics in these regions. Aims. We use interpretable machine learning to study 9 different molecular ratios, helping us understand the forward connection between the physics of these environments and the carbon and oxygen chemistries. Methods. Using a large grid of astrochemical models generated using UCLCHEM, we study the properties of molecular clouds of low oxygen and carbon initial abundance. We first try to understand the line ratios using a classical analysis. We then move on to using interpretable machine learning, namely Shapley Additive Explanations (SHAP), to understand the higher order dependencies of the ratios over the entire parameter grid. Lastly we use the Uniform Manifold Approximation and Projection technique (UMAP) as a reduction method to create intuitive groupings of models. Results. We find that the parameter space is well covered by the line ratios, allowing us to investigate all input parameters. SHAP analysis shows that the temperature and density are the most important features, but the carbon and oxygen abundances are important in parts of the parameter space. Lastly, we find that we can group different types of ratios using UMAP. Conclusions. We show the chosen ratios are mostly sensitive to changes in the carbon initial abundance, together with the temperature and density. Especially the CN/HCN and HNC/HCN ratio are shown to be sensitive to the initial carbon abundance, making them excellent probes for this parameter. Out of the ratios, only CS/SO shows a sensitivity to the oxygen abundance.

astro-ph.GA↗