arXiv subjects

Serena Viti

Publications and source records attributed to Serena Viti.

At least 55 records · Page 3Linked to original sources

arXiv2023

GMC Collisions As Triggers of Star Formation. IX. Chemical Evolution

Collisions between giant molecular clouds (GMCs) have been proposed as a mechanism to trigger massive star and star cluster formation. To investigate the astrochemical signatures of such collisions, we carry out 3D magnetohydrodynamics simulations of colliding and non-colliding clouds exposed to a variety of cosmic ray ionization rates (CRIRs), $ζ$, following chemical evolution including gas and ice-phase components. At the GMC scale, carbon starts mostly in $\rm{C^+}$, but then transitions into C, CO, followed by ice-phase CO and $\rm{CH_3OH}$ as dense, cooler filaments, clumps and cores form from the clouds. The oxygen budget is dominated by O, CO and water ice. In dense regions, we explore the gas phase CO depletion factor, $f_D$, that measures the extent of its freeze-out onto dust grains, including dependence on CRIR and observables of mass surface density and temperature. We also identify dense clumps and analyze their physical and chemical properties, including after synthetic line emission modeling, investigating metrics used in studies of infrared dark clouds (IRDCs), especially abundances of CO, $\rm HCO^+$ and $\rm N_2H^+$. For the colliding case, we find clumps have typical densities of $n_{\rm H}\sim10^5\:{\rm{cm}}^{-3}$ and temperatures of $\sim20\:$K, while those in non-colliding GMCs are cooler. Depending on $ζ$ and GMC dynamical history, we find CO depletion factors of up to $f_D\sim10$, and abundances of HCO$^+\sim 10^{-9}$ to $10^{-8}$ and $\rm{N_2H^+}\sim10^{-11}$ to $10^{-10}$. Comparison with observed IRDC clumps indicates a preference for low CRIRs ($\sim10^{-18}\:{\rm{s}}^{-1}$) and a more quiescent (non-colliding), cooler and evolved chemodynamical history. We discuss the general implications of our results and their caveats for interpretation of molecular cloud observations.

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arXiv2023

Tracing the chemical footprint of shocks in AGN-host and starburst galaxies with ALMA multi-line molecular studies

Multi-line molecular observations are an ideal tool for a systematic study of the physico-chemical processes in the Interstellar Medium (ISM), given the wide range of critical densities associated with different molecules and their transitions, and the dependencies of chemical reactions on the energy budget of the system. Recently high spatial resolution of typical shock tracers - SiO, HNCO, and CH3OH - have been studied in the potentially shocked regions in two nearby galaxies: NGC 1068 (an AGN-host galaxy) (Huang et al., Astron. Astrophys., 2022, 666, A102; Huang et al., in prep.) and NGC 253 (a starburst galaxy) (K.-Y. Huang et al., arXiv, 2023, preprint, arXiv:2303.12685, DOI: 10.48550/arXiv.2303.12685). This paper is dedicated to the comparative study of these two distinctively different galaxies, with the aim of determining the differences in their energetics and understanding large-scale shocks in dfferent types of galaxies.

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arXiv2023

Hydrodynamic simulations of the Disk of Gas Around Supermassive black holes (HDGAS) -I; Molecular Gas Dynamics

We present hydrodynamic simulations of the interstellar medium (ISM) within the circumnuclear disk (CND) of a typical AGN-dominated galaxy influenced by mechanical feedback from an active galactic nucleus(AGN). The simulations are coupled with the CHIMES non-equilibrium chemistry network to treat the radiative-cooling and AGN-heating. A focus is placed on the central 100 pc scale where AGN outflows are coupled to the ISM and constrained by observational Seyfert-2 galaxies. AGN-feedback models are implemented with different wind-velocity and mass-loading factors. We post-process the simulation snapshots with a radiative-transfer code to obtain the molecular emission lines. We find that the inclusion of an AGN promotes the formation of CO in clumpy and dense regions surrounding supermassive-blackholes (SMBH). The CO(1-0) intensity maps ($<$6 Myr) in the CND seem to match well with observations of NGC 1068 with a best match for a model with 5000 $\rm km/s$ wind-velocity and a high mass-loading factor. We attempt to discern between competing explanations for the apparent counter-rotating gas disk in the NGC 1068 through an analysis of kinematic maps of the CO line emission. We suggest that mechanical AGN-feedback could explain the alignment-stability of position-angle across the different CND radii around the SMBH through momentum and energy loading of the wind. It is the wind-velocity that drives the disk out of alignment on a 100 pc scale for a long period of time. The position-velocity diagrams are in broad agreement with the predicted Keplerian rotation-curve in the model without-AGN, but the AGN models exhibit a larger degree of scatter, in better agreement with NGC 1068 observations.

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arXiv2023

A statistical and machine learning approach to the study of astrochemistry

In order to obtain a good understanding of astrochemistry, it is crucial to better understand the key parameters that govern grain-surface chemistry. For many chemical networks, these crucial parameters are the binding energies of the species. However, there exists much disagreement regarding these values in the literature. In this work, a Bayesian inference approach is taken to estimate these values. It is found that this is difficult to do in the absence of enough data. The Massive Optimised Parameter Estimation and Data (MOPED) compression algorithm is then used to help determine which species should be prioritised for future detections in order to better constrain the values of binding energies. Finally, an interpretable machine learning approach is taken in order to better understand the non-linear relationship between binding energies and the final abundances of specific species of interest.

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arXiv2023

Investigating the impact of reactions of C and CH with molecular hydrogen on a glycine gas-grain network

The impact of including the reactions of C and CH with molecular hydrogen in a gas-grain network is assessed via a sensitivity analysis. To this end, we vary 3 parameters, namely, the efficiency for the reaction \ce{C + H2 -> CH2}, and the cosmic ray ionisation rate, with the third parameter being the final density of the collapsing dark cloud. A grid of 12 models is run to investigate the effect of all parameters on the final molecular abundances of the chemical network. We find that including reactions with molecular hydrogen alters the hydrogen economy of the network; since some species are hydrogenated by molecular hydrogen, atomic hydrogen is freed up. The abundances of simple molecules produced from hydrogenation, such as \ce{CH4}, \ce{CH3OH} and \ce{NH3}, increase, and at the same time, more complex species such as glycine and its precursors see a significant decrease in their final abundances. We find that the precursors of glycine are being preferentially hydrogenated, and therefore glycine itself is produced less efficiently.

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arXiv2022

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple HCN and HNC rotational transitions observed using ALMA via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models which are constrained using our HCN and HNC measurements. We find that the volume density $n_{\text{H}_{2}}$ and cosmic ray ionization rate (CRIR) $ζ$ are enhanced by about an order of magnitude in the galaxy's central regions as compared to those further from the nucleus. In NGC 253's central GMCs, where observed HCN/HNC abundance ratios are lowest, $n \sim 10^{5.5}$ cm$^{-3}$ and $ζ\sim 10^{-12}$ s$^{-1}$ (greater than $10^4$ times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, HII regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where $ζ$ is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.

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arXiv2022

Identifying the most constraining ice observations to infer molecular binding energies

In order to understand grain-surface chemistry, one must have a good understanding of the reaction rate parameters. For diffusion-based reactions, these parameters are binding energies of the reacting species. However, attempts to estimate these values from grain-surface abundances using Bayesian inference are inhibited by a lack of enough sufficiently constraining data. In this work, we use the Massive Optimised Parameter Estimation and Data (MOPED) compression algorithm to determine which species should be prioritised for future ice observations to better constrain molecular binding energies. Using the results from this algorithm, we make recommendations for which species future observations should focus on.

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arXiv2022

The Effects of Cosmic Rays on the Chemistry of Dense Cores

Cosmic rays are crucial for the chemistry of molecular clouds and their evolution. They provide essential ionizations, dissociations, heating and energy to the cold, dense cores. As cosmic rays pierce through the clouds they are attenuated and lose energy, which leads to a dependency on the column density of a system. The detailed effects these particles have on the central regions still needs to be fully understood. Here, we revisit how cosmic rays are treated in the UCLCHEM chemical modeling code by including both ionization rate and H2 dissociation rate dependencies alongside the production of cosmic ray induced excited species and we study in detail the effects of these treatments on the chemistry of pre-stellar cores. We find that these treatments can have significant effects on chemical abundances, up to several orders of magnitude, depending on physical conditions. The ionization dependency is the most significant treatment, influencing chemical abundances through increased presence of ionized species, grain desorptions and enhanced chemical reactions. Comparisons to chemical abundances derived from observations show the new treatments reproduce these observations better than the standard handling. It is clear that more advanced treatments of cosmic rays are essential to chemical models and that including this type of dependency provides more accurate chemical representations.

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arXiv2022

ALMA imaging of the cold molecular and dusty disk in the type 2 active nucleus of the Circinus galaxy

We aim to shed light on the physical properties and kinematics of the molecular material in the nucleus of one of the closest type 2 active galaxies. To this end, we obtained high angular resolution ALMA observations of the nucleus of the Circinus galaxy. The observations map the emission at 350GHz and 690GHz with spatial resolutions of ~3.8pc and ~2.2pc, respectively. The continuum emission traces cold ($T\lesssim100$K) dust in a circumnuclear disk with spiral arms on scales of 25pc, plus a marginally resolved nuclear emission peak. The latter is not extended in polar direction as claimed based on earlier ALMA observations. A significant amount (of the order of 40%) of the 350GHz emission is not related to dust, but most likely free-free emission instead. We detect CO(3-2) and CO(6-5) as well as HCO$^+$(4-3), HCN(4-3), and CS(4-3). The CO emission is extended, showing a spiral pattern, similar to the extended dust emission. Towards the nucleus, CO is excited to higher transitions and its emission is self-absorbed, leading to an apparent hole in the CO(3-2) but not the CO(6-5) emission. On the other hand, the high gas density tracers HCO$^+$, HCN, and CS show a strong, yet unresolved (($\lesssim4$pc) concentration of the emission at the nucleus, pointing at a very small 'torus'. The kinematics are dominated by rotation and point at a geometrically thin disk down to the resolution limit of our observations. In contrast to several other AGNs, no HCN enhancement is found towards the nucleus. The Circinus nucleus is therefore composed of at least two distinct components: (1) an optically thin, warm outflow of ionised gas containing clouds of dust; and (2) a cold molecular and dusty disk. These findings support the most recent radiative transfer calculations of the obscuring structures in AGNs, which find a similar two-component structure. (Abridged)

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arXiv2022

Energizing Star Formation: The Cosmic Ray Ionization Rate in NGC 253 Derived From ALCHEMI Measurements of H$_3$O$^+$ and SO

The cosmic ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star formation regions, which impacts the physical and chemical processes which drive the formation of stars. Previous studies of the circum-molecular zone (CMZ) of the starburst galaxy NGC 253 have found evidence for a high CRIR value; $10^3-10^6$ times the average cosmic ray ionization rate within the Milky Way. This is a broad constraint and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations towards the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H$_3$O$^+$ and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 due to emission from their transitions. We find that each of the four regions modelled has a CRIR in the range $(1-80)\times10^{-14}$ s$^{-1}$ and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays including C$_2$H, HCO$^+$, HOC$^+$, and CO. From shock and PDR/XDR models, we further find that neither UV/X-ray driven nor shock dominated chemistry are a viable single alternative as none of these processes can adequately fit the abundances of all of these species.

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arXiv2022

Using Statistical Emulation and Knowledge of Grain-Surface Diffusion for Bayesian Inference of Reaction Rate Parameters: An Application to a Glycine Network

There exists much uncertainty surrounding interstellar grain-surface chemistry. One of the major reaction mechanisms is grain-surface diffusion for which the the binding energy parameter for each species needs to be known. However, these values vary significantly across the literature which can lead to debate as to whether or not a particular reaction takes place via diffusion. In this work we employ Bayesian inference to use available ice abundances to estimate the reaction rates of the reactions in a chemical network that produces glycine. Using this we estimate the binding energy of a variety of important species in the network, by assuming that the reactions take place via diffusion. We use our understanding of the diffusion mechanism to reduce the dimensionality of the inference problem from 49 to 14, by demonstrating that reactions can be separated into classes. This dimensionality reduction makes the problem computationally feasible. A neural network statistical emulator is used to also help accelerate the Bayesian inference process substantially. The binding energies of most of the diffusive species of interest are found to match some of the disparate literature values, with the exceptions of atomic and diatomic hydrogen. The discrepancies with these two species are related to limitations of the physical and chemical model. However, the use of a dummy reaction of the form H + X -> HX is found to somewhat reduce the discrepancy with the binding energy of atomic hydrogen. Using the inferred binding energies in the full gas-grain version of UCLCHEM results in almost all the molecular abundances being recovered.

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arXiv2022

UCLCHEMCMC: A MCMC Inference tool for Physical Parameters of Molecular Clouds

We present the publicly available, open source code UCLCHEMCMC, designed to estimate physical parameters of an observed cloud of gas by combining Monte Carlo Markov Chain (MCMC) sampling with chemical and radiative transfer modeling. When given the observed values of different emission lines, UCLCHEMCMC runs a Bayesian parameter inference, using a MCMC algorithm to sample the likelihood and produce an estimate of the posterior probability distribution of the parameters. UCLCHEMCMC takes a full forward modeling approach, generating model observables from the physical parameters via chemical and radiative transfer modeling. While running UCLCHEMCMC, the created chemical models and radiative transfer code results are stored in an SQL database, preventing redundant model calculations in future inferences. This means that the more UCLCHEMCMC is used, the more efficient it becomes. Using UCLCHEM and RADEX, the increase of efficiency is nearly two orders of magnitude, going from 5185.33 \pm 1041.96 s for ten walkers to take one thousand steps when the database is empty, to 68.89 \pm 45.39 s when nearly all models requested are in the database. In order to demonstrate its usefulness we provide an example inference of UCLCHEMCMC to estimate the physical parameters of mock data, and perform two inferences on the well studied prestellar core, L1544, one of which show that it is important to consider the substructures of an object when determining which emission lines to use.

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arXiv2022

Interstellar Hydrocarbons: Degradation Chemistry in Diffuse Clouds

Observations of diffuse clouds showed that they contain a number of simple hydrocarbons (e.g. CH, C$_2$H, (l- and c-)C$_3$H$_2$, and C$_4$H) in abundances that may be difficult to understand on the basis of conventional gas-phase chemical models. Recent experimental results revealed that the photo-decomposition mechanism of hydrogenated amorphous carbon (HAC) and of solid hexane releases a range of hydrocarbons into the gas, containing up to 6 C-atoms for the case of HAC decomposition. These findings motivated us to introduce a new potential input to interstellar chemistry; the "top-down" or degradation scheme, as opposed to the conventional "build-up" or synthesis scheme. In this work, we demonstrate the feasibility of the top-down approach in diffuse clouds using gas-grain chemical models. In order to examine this scheme, we derived an expression to account for the formation of hydrocarbons when HACs are photo-decomposed after their injection from grain mantles. Then, we calculated the actual formation rate of these species by knowing their injected fraction (from experimental work) and the average rate of mantle carbon injection into the ISM (from observations). Our preliminary results are promising and reveal that the degradation scheme can be considered as an efficient mechanism for the formation of some simple hydrocarbons in diffuse clouds. However, an actual proof of the efficiency of this process and its rate constants would require comprehensive experimental determination.

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arXiv2022

FAUST III. Misaligned rotations of the envelope, outflow, and disks in the multiple protostellar system of VLA 1623$-$2417

We report a study of the low-mass Class-0 multiple system VLA 1623AB in the Ophiuchus star-forming region, using H$^{13}$CO$^+$ ($J=3-2$), CS ($J=5-4$), and CCH ($N=3-2$) lines as part of the ALMA Large Program FAUST. The analysis of the velocity fields revealed the rotation motion in the envelope and the velocity gradients in the outflows (about 2000 au down to 50 au). We further investigated the rotation of the circum-binary VLA 1623A disk as well as the VLA 1623B disk. We found that the minor axis of the circum-binary disk of VLA 1623A is misaligned by about 12 degrees with respect to the large-scale outflow and the rotation axis of the envelope. In contrast, the minor axis of the circum-binary disk is parallel to the large-scale magnetic field according to previous dust polarization observations, suggesting that the misalignment may be caused by the different directions of the envelope rotation and the magnetic field. If the velocity gradient of the outflow is caused by rotation, the outflow has a constant angular momentum and the launching radius is estimated to be $5-16$ au, although it cannot be ruled out that the velocity gradient is driven by entrainments of the two high-velocity outflows. Furthermore, we detected for the first time a velocity gradient associated with rotation toward the VLA 16293B disk. The velocity gradient is opposite to the one from the large-scale envelope, outflow, and circum-binary disk. The origin of its opposite gradient is also discussed.

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arXiv2022

History-independent tracers: Forgetful molecular probes of the physical conditions of the dense interstellar medium

Molecular line emission is a powerful probe of the physical conditions of astrophysical objects but can be complex to model, and it is often unclear which transitions would be the best targets for observers who wish to constrain a given parameter. We therefore produce a list of molecular species for which the gas history can be ignored, removing a major modelling complexity. We then determine the best of these species to observe when attempting to constrain various physical parameters. To achieve this, we use a large set of chemical models with different chemical histories to determine which species have abundances at 1 MYr that are insensitive to the initial conditions. We then use radiative transfer modelling to produce the intensity of every transition of these molecules. We finally compute the mutual information between the physical parameters and all transitions and transition ratios in order to rank their usefulness in determining the value of a given parameter. We find 48 species that are insensitive to the chemical history of the gas, 23 of which have collisional data available. We produce a ranked list of all the transitions and ratios of these species using their mutual information with various gas properties. We show mutual information is an adequate measure of how well a transition can constrain a physical parameter by recovering known probes and demonstrating that random forest regression models become more accurate predictors when high-scoring features are included. Therefore, this list can be used to select target transitions for observations in order to maximize knowledge about those physical parameters.

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arXiv2021

Starburst Energy Feedback Seen Through HCO$^+$/HOC$^+$ Emission in NGC 253 from ALCHEMI

Molecular abundances are sensitive to UV-photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO$^+$ and its metastable isomer HOC$^+$ in the center of the starburst galaxy NGC 253 using data from the ALMA large program ALCHEMI. The distribution of the HOC$^+$(1-0) integrated intensity shows its association with "superbubbles", cavities created either by supernovae or expanding HII regions. The observed HCO$^+$/HOC$^+$ abundance ratios are $\sim 10-150$, and the fractional abundance of HOC$^+$ relative to H$_2$ is $\sim 1.5\times 10^{-11} - 6\times 10^{-10}$, which implies that the HOC$^+$ abundance in the center of NGC 253 is significantly higher than in quiescent spiral-arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of $G_0\gtrsim 10^3$ if the maximum visual extinction is $\gtrsim 5$, or a cosmic-ray ionization rate of $ζ\gtrsim 10^{-14}$ s$^{-1}$ (3-4 orders of magnitude higher than that within clouds in the Galactic spiral-arms) to reproduce the observed results. From the difference in formation routes of HOC$^+$, we propose that a low-excitation line of HOC$^+$ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV-photons and cosmic rays, sources of energy feedback.

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arXiv2021

On the Formation of Deuterated Methyl Formate in Hot Corinos

Methyl formate, HCOOCH$_3$, and many of its isotopologues have been detected in astrophysical regions with considerable abundances. However, the recipe for the formation of this molecule and its isotopologues is not yet known. In this work, we attempt to investigate, theoretically, the successful recipe for the formation of interstellar HCOOCH$_3$ and its deuterated isotopologues. We used the gas-grain chemical model, UCLCHEM, to examine the possible routes of formation of methyl formate on grain surfaces and in the gas-phase in low-mass star-forming regions. Our models show that radical-radical association on grains are necessary to explain the observed abundance of DCOOCH$_3$ in the protostar IRAS~16293--2422. H-D substitution reactions on grains significantly enhance the abundances of HCOOCHD$_2$, DCOOCHD$_2$, and HCOOCD$_3$. The observed abundance of HCOOCHD$_2$ in IRAS 16293--2422 can only be reproduced if H-D substitution reactions are taken into account. However, HCOOCH$_2$D remain underestimated in all of our models. The deuteration of methyl formate appears to be more complex than initially thought. Additional studies, both experimentally and theoretically, are needed for a better understanding of the interstellar formation of these species.

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arXiv2021

The complex organic molecular content in the L1498 starless core

Observations carried out toward starless and pre-stellar cores have revealed that complex organic molecules are prevalent in these objects, but it is unclear what chemical processes are involved in their formation. Recently, it has been shown that complex organics are preferentially produced at an intermediate-density shell within the L1544 pre-stellar core at radial distances of ~4000 au with respect to the core center. However, the spatial distribution of complex organics has only been inferred toward this core and it remains unknown whether these species present a similar behaviour in other cores. We report high-sensitivity observations carried out toward two positions in the L1498 pre-stellar core, the dust peak and a position located at a distance of ~11000 au from the center of the core where the emission of CH$_3$OH peaks. Similarly to L1544, our observations reveal that small O-bearing molecules and N-bearing species are enhanced by factors ~4-14 toward the outer shell of L1498. However, unlike L1544, large O-bearing organics such as CH3CHO, CH3OCH3 or CH3OCHO are not detected within our sensitivity limits. For N-bearing organics, these species are more abundant toward the outer shell of the L1498 pre-stellar core than toward the one in L1544. We propose that the differences observed between O-bearing and N-bearing species in L1498 and L1544 are due to the different physical structure of these cores, which in turn is a consequence of their evolutionary stage, with L1498 being younger than L1544.

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