arXiv subjects

Thanja Lamberts

Publications and source records attributed to Thanja Lamberts.

At least 19 recordsLinked to original sources

arXiv2026

Methanimine as a sink in the HCN and HNC solid state hydrogenation network

We aim to provide a systematic and quantitative description of the hydrogenation network connecting HCN and HNC to methylamine on interstellar water ices, while identifying dominant pathways and bottlenecks. To this end, we performed a comprehensive quantum-chemical investigation of H-addition, H-abstraction, reactions with H2, and water-assisted H-transfer isomerization, covering intermediates linking HCN and HNC to CH3NH2. Calculations were carried out on amorphous solid water clusters of 14 molecules. Using benchmarked density functional theory, we derived activation barriers, elucidated mechanisms, and determined the binding energy distribution of H2CN and CNH2, also assessing deuterium substitution effects. H-addition reactions generally involve activation barriers, except for radical species. Considering both barrier heights and tunneling crossover temperatures, the most favorable sequence originates from HNC rather than HCN. The network evolves toward methanimine (H2CNH), the central species, or the singlet carbene HC:NH2, from which further hydrogenation leads to methylamine. Along these paths, several reactions are barrierless, while some H-abstraction processes compete with addition. Reactions involving H2 are uncommon, as most are endoergic. Deuterium substitution weakly affects classical barriers but significantly influences tunneling efficiencies. Our results support efficient formation of methanimine and methylamine from HNC on cold interstellar ices, with methanimine acting as a chemical sink, whereas HCN is less reactive and more likely to persist. These findings provide quantitative constraints for astrochemical models.

astro-ph.GA↗

arXiv2026

Valence and Rydberg excited state bond dissociation curves of CO2 from orbital-optimized density functional calculations

Calculations of the lowest valence π* as well as the 3s and higher energy 3pσ Rydberg excited states of the CO2 molecule are carried out using density functionals with variational optimization of the orbitals, an approach involving relatively little computational effort. Five functionals with varying degree of exchange are used in combination with real or complex-valued orbitals that are optimized by finding saddle points on the electronic energy surface corresponding to the excited states. When the PBE functional is used in combination with complex orbitals, the calculated excitation energy is found to be within 0.3 eV of multireference configuration interaction reference values, and the results are further improved with hybrid functionals. In contrast, linear-response time-dependent density functional theory calculations give errors up to 1.9 eV for the most diffuse 3pσ excitation and exhibit stronger dependence on both the excitation character and the functional used. Calculated C-O dissociation curves using the PBE functional and the orbital-optimized approach compare remarkably well with the reported multireference configuration interaction and equation-of-motion coupled-cluster singles and doubles calculations. Thanks to the low computational cost, these results demonstrate that orbital-optimized density functional calculations can be a promising route for modelling photorelaxation in condensed-phase CO2, for example in the context of interstellar cosmic-ray radiation driven process involving high-energy Rydberg states.

physics.chem-ph↗

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

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

Interstellar Chemistry of CN Radicals on Ices: The formation of CH3CN and CH3NC and potential connection to acetamide

Context. Among the most significant chemical functional groups of interstellar molecules are the class of nitriles, which are proposed as key prebiotic molecules due to their chemical connection to the peptide bond after hydrolysis. CN radicals, the simplest representative of this group, have been shown to exhibit strong interactions with interstellar water ices, potentially impacting their reactivity with other radicals nearby. Aims. This study explores (a) whether CN and CH3 radicals can readily react to form methyl cyanide (CH3CN) and its isomer methyl isocyanide (CH3NC); and (b) the feasibility of the reaction (CN...H2O)hemi -> C(OH)=NH and its potential role in the formation of acetamide. Methods. Following a benchmark, density functional theory was employed to map the potential energy surfaces of these chemical processes, focusing on their reactivity on water and carbon monoxide ices. Results. The results show that CN reacts with CH3 radicals on water ices, forming CH3CN and CH3NC efficiently. However, these reactions are driven by diffusion of CH3 towards the reactive site and subsequently compete with back-diffusion of CH3 from that site. The formation of the radical intermediate C(OH)=NH on water ice requires quantum tunnelling and assuming that acetimidic acid forms via CH3 + C(OH)=NH -> CH3C(OH)=NH, it can also only isomerize into acetamide through a sizable barrier thanks to quantum tunnelling. Both quantum tunnelling-driven reactions are highly dependent on the local structure of the water ice. Finally, radical coupling reactions on carbon monoxide ices are found to be barrierless for all cases and again, both the cyanide and the isocyanide are formed. Conclusions. This work reinforces the conclusion that CN radicals on interstellar grain surfaces are highly reactive and unlikely to persist unaltered.

astro-ph.GA↗

arXiv2025

Toward Unraveling Cyanopolyyne Surface Chemistry: A Preview on Isolated Systems From HC_{3}N to Ethyl Cyanide and Propylamine

Cyanopolyynes, a family of nitrogen containing carbon chains, are common in the interstellar medium and possibly form the backbone of species relevant to prebiotic chemistry. Following their gas phase formation, they are expected to freeze out on ice grains in cold interstellar regions. In this work we present the hydrogenation reaction network of isolated HC_{3}N, the smallest cyanopolyyne, that consists over-a-barrier radical-neutral reactions and barrierless radical-radical reactions. We employ density functional theory, coupled cluster and multiconfigurational methods to obtain activation and reaction energies for the hydrogenation network of HC_{3}N. This work explores the reaction network of the isolated molecule and constitutes a preview on the reactions occurring on the ice grain surface. We find that the reactions where the hydrogen atom adds to the carbon chain at carbon atom opposite of the cyano-group give the lowest and most narrow barriers. Subsequent hydrogenation leads to the astrochemically relevant vinyl cyanide and ethyl cyanide. Alternatively, the cyano-group can hydrogenate via radical-radical reactions, leading to the fully saturated propylamine. These results can be extrapolated to give insight into the general reactivity of carbon chains on interstellar ices.

astro-ph.GA↗

arXiv2025

Hydrogenation of HOCO and formation of interstellar CO$_2$: A not so straightforward relation

Carbon dioxide (CO$_2$) is one of the most important interstellar molecules. While it is considered that it forms on the surface of interstellar dust grains, the exact contribution of different chemical mechanisms is still poorly constrained. Traditionally it is deemed that the CO + OH reaction occurring on top of ices is the main reaction path for its formation. Recent investigations showed that in reality the reaction presents a more complex mechanism, requiring an additional H-abstraction step. Building on our previous works, we carried out a detailed investigation of such H abstraction reactions with the hydrogen atom as a reactant for the abstraction reaction. We found an unconventional chemistry for this reaction, markedly depending on the isomeric form of the HOCO radical prior to reaction. The favored reactions are t-HOCO + H -> CO + H$_2$O, c-HOCO + H -> CO$_2$ + H$_2$ and t/c-HOCO + H -> c/t-HCOOH. We estimate bounds for the rate constants of the less favored reaction channels, t-HOCO + H -> CO$_2$ + H and c-HOCO + H -> CO + H$_2$O, to be approximately 10$^{4-6}$ s$^{-1}$. However, these estimates should be interpreted cautiously due to the significant role of quantum tunneling in these reactions and the complex electronic structure of the involved molecules, which complicates their study. Our findings underscore the need for detailed investigation into the chemistry of interstellar CO$_2$ and pave the way for a reevaluation of its primary formation mechanisms in the interstellar medium.

astro-ph.GA↗

arXiv2025

Modelling methanol and hydride formation in the JWST Ice Age era

(Abridged) JWST observations have measured the ice composition toward two highly-extinguished field stars in the Chamaeleon I cloud. The observed extinction excess on the long-wavelength side of the H2O ice band at 3 micron has been attributed to a mixture of CH3OH with ammonia hydrates, which suggests that CH3OH ice could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH3OH could form via the grain surface reactions CH3+OH and/or C+H2O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency and dependence on the astrochemical code employed. We model the ice chemistry in the Chamaeleon I cloud using a set of astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations) to test the effects of the different code architectures and of the assumed ice chemistry. Our models show that the JWST ice observations are better reproduced for gas densities >1e5 cm-3 and collapse times >1e5 yr. CH3OH ice forms predominantly (>99%) via CO hydrogenation. The contribution of reactions CH3+OH and C+H2O, is negligible. The CO2 ice may form either via CO+OH or CO+O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate constant of the CO+O surface reaction. CH4 is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the initial translucent cloud phase. Large differences in the ice abundances are found at Tdust<12 K between diffusive and non-diffusive chemistry codes. This is due to the fact that non-diffusive chemistry takes over diffusive chemistry at such low Tdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.

astro-ph.GA↗

arXiv2024

Proposed importance of HOCO chemistry: Inefficient formation of CO$_2$ from CO and OH reactions on ice dust

With the advent of JWST ice observations, dedicated studies on the formation reactions of detected molecules are becoming increasingly important. One of the most interesting molecules in interstellar ice is CO$_2$. Despite its simplicity, the main formation reaction considered, CO + OH -> CO$_2$ + H through the energetic HOCO* intermediate on ice dust, is subject to uncertainty because it directly competes with the stabilization of HOCO as a final product which is formed through energy dissipation of HOCO* to the water ice. When energy dissipation to the surface is effective during reaction, HOCO can be a dominant product. In this study, we experimentally demonstrate that the major product of the reaction is indeed not CO$_2$, but rather the highly reactive radical HOCO. The HOCO radical can later evolve into CO$_2$ through H-abstraction reactions, but these reactions compete with addition reactions, leading to the formation of carboxylic acids (R-COOH). Our results highlight the importance of HOCO chemistry and encourage further exploration of the chemistry of this radical.

astro-ph.GA↗

arXiv2024

Formation of S-bearing complex organic molecules in interstellar clouds via ice reactions with C2H2, HS, and atomic H

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH3CH2SH and CH2CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C2H2 molecules and SH radicals -- both thought to be present in interstellar icy mantles -- at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH3CH2SH, CH2CHSH, HSCH2CH2SH, H2S2, and tentatively CH3CHS and CH2CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH3CH2SH and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed.

astro-ph.GA↗

arXiv2023

Detection of Interstellar $E$-1-cyano-1,3-butadiene in GOTHAM Observations of TMC-1

We report the detection of the lowest energy conformer of $E$-1-cyano-1,3-butadiene ($E$-1-C$_4$H$_5$CN), a linear isomer of pyridine, using the fourth data reduction of the GOTHAM deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1$σ$ level. We derive a total column density of $3.8^{+1.0}_{-0.9}\times 10^{10}$ cm$^{-2}$, which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of $2.34 \times 10^{10}\ \mathrm{cm}^{-2}$ for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by 1--4 orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitudes. We discuss implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2022

First experimental confirmation of the CH3O + H2CO -> CH3OH + HCO reaction: expanding the CH3OH formation mechanism in interstellar ices

The successive addition of H atoms to CO in the solid phase has been hitherto regarded as the primary route to form methanol in dark molecular clouds. However, recent Monte Carlo simulations of interstellar ices alternatively suggested the radical-molecule H-atom abstraction reaction CH3O + H2CO -> CH3OH + HCO, in addition to CH3O + H -> CH3OH, as a very promising and possibly dominating (70 - 90 %) final step to form CH3OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H2CO and D2CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD2O + D2CO -> CHD2OD + DCO, is significantly hampered by the isotope effect in the D-abstraction process, and can thus be used as an artifice to probe the efficiency of this step. We observe a significantly smaller yield of D2CO + H products in comparison to H2CO + H, implying that the CH3O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-Absorption InfraRed Spectroscopy (RAIRS) and Temperature Programmed Desorption-Quadrupole Mass Spectrometry (TPD-QMS) analyses are used to quantify the species in the ice. Both analytical techniques indicate constant contributions of ~80 % for the abstraction route in the 10 - 16 K interval, which agrees well with the Monte Carlo conclusions. Additional H2CO + D experiments confirm these conclusions.

astro-ph.GA↗

arXiv2021

Methoxymethanol Formation Starting from CO-Hydrogenation

Methoxymethanol (CH3OCH2OH, MM) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. This molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products. The goal of this work is to investigate experimentally and theoretically the most likely solid-state MM reaction channel -- the recombination of CH2OH and CH3O radicals -- for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. Hydrogen atoms and CO or H2CO molecules are co-deposited on top of the predeposited H2O ice to mimic the conditions associated with the beginning of 'rapid' CO freeze-out. Quadrupole mass spectrometry is used to analyze the gas-phase COM composition following a temperature programmed desorption. Monte Carlo simulations are used for an astrochemical model comparing the MM formation efficiency with that of other COMs. Unambiguous detection of newly formed MM has been possible both in CO+H and H2CO+H experiments. The resulting abundance of MM with respect to CH3OH is about 0.05, which is about 6 times less than the value observed toward NGC 6334I and about 3 times less than the value reported for IRAS 16293B. The results of astrochemical simulations predict a similar value for the MM abundance with respect to CH3OH factors ranging between 0.06 to 0.03. We find that MM is formed by co-deposition of CO and H2CO with H atoms through the recombination of CH2OH and CH3O radicals. In both the experimental and modeling studies, the efficiency of this channel alone is not sufficient to explain the observed abundance of MM. These results indicate an incomplete knowledge of the reaction network or the presence of alternative solid-state or gas-phase formation mechanisms.

astro-ph.GA↗

arXiv2021

Hydrogenation of accreting C-atoms and CO molecules -- simulating ketene and acetaldehyde formation under dark and translucent cloud conditions

Simple and complex organic molecules (COMs) are observed along different phases of star and planet formation and have been successfully identified in prestellar environments such as dark and translucent clouds. Yet the picture of organic molecule formation at those earliest stages of star formation is not complete and an important reason is the lack of specific laboratory experiments that simulate carbon atom addition reactions on icy surfaces of interstellar grains. Here we present experiments in which CO molecules as well as C- and H-atoms are co-deposited with H$_2$O molecules on a 10 K surface mimicking the ongoing formation of an "H$_2$O-rich" ice mantle. To simulate the effect of impacting C-atoms and resulting surface reactions with ice components, a specialized C-atom beam source is used, implemented on SURFRESIDE$^3$, an UHV cryogenic setup. Formation of ketene (CH$_2$CO) in the solid state is observed "in situ" by means of reflection absorption IR spectroscopy. C$^1$$^8$O and D isotope labelled experiments are performed to further validate the formation of ketene. Data analysis supports that CH$_2$CO is formed through C-atom addition to a CO-molecule, followed by successive hydrogenation transferring the formed :CCO into ketene. Efficient formation of ketene is in line with the absence of an activation barrier in C+CO reaction reported in the literature. We also discuss and provide experimental evidence for the formation of acetaldehyde (CH$_3$CHO) and possible formation of ethanol (CH$_3$CH$_2$OH), two COM derivatives of CH$_2$CO hydrogenation. The underlying reaction network is presented and the astrochemical implications of the derived pathways are discussed.

astro-ph.GA↗

arXiv2021

Carbon Atom Reactivity with Amorphous Solid Water: H$_2$O Catalyzed Formation of H$_2$CO

We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H$_2$CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds, and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H$_2$CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions $^3$C + H$_2$O -> $^3$HCOH and $^1$HCOH -> $^1$H$_2$CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers or absence thereof explain the very small kinetic isotope effect in our experiments when comparing the formation of H$_2$CO to D$_2$CO. Our results reconcile the disagreement found in the literature on the reaction route: C + H$_2$O -> H$_2$CO.

astro-ph.GA↗

arXiv2021

Methane formation in cold regions from carbon atoms and molecular hydrogen

Methane is typically thought to be formed in the solid state on the surface of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we investigate the potential role of molecular hydrogen in the CH$_4$ reaction network. We make use of an ultra-high vacuum cryogenic setup combining an atomic carbon atom beam and both atomic and/or molecular beams of hydrogen and deuterium on a H$_2$O ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH$_4$ is formed in an experiment combining C atoms with H$_2$ on amorphous solid water, albeit slower than in experiments with H atoms present. Furthermore, CH$_2$D$_2$ is detected in an experiment of C atoms with H$_2$ and D$_2$ on H$_2$O ice. CD$_4$, however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational chemical insights. This leads to the following conclusions: a) the reaction C + H$_2$ -> CH$_2$ can take place, although not barrierless in the presence of water, b) the reaction CH + H$_2$ -> CH$_3$ is barrierless, but has not yet been included in astrochemical models, c) the reactions CH$_2$ + H$_2$ -> CH$_3$ + H and CH$_3$ + H$_2$ -> CH$_4$ + H can take place only via a tunneling mechanism and d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.

astro-ph.GA↗

arXiv2019

Efficient Production of S$_8$ in Interstellar Ices: The effects of cosmic ray-driven radiation chemistry and non-diffusive bulk reactions

In this work, we reexamine sulfur chemistry occurring on and in the ice mantles of interstellar dust grains, and report the effects of two new modifications to standard astrochemical models; namely, (a) the incorporation of cosmic ray-driven radiation chemistry and (b) the assumption of fast, non-diffusive reactions for key radicals in the bulk. Results from our models of dense molecular clouds show that these changes can have a profound influence on the abundances of sulfur-bearing species in ice mantles, including a reduction in the abundance of solid-phase H$_2$S and HS, and a significant increase in the abundances of OCS, SO$_2$, as well as pure allotropes of sulfur, especially S$_8$. These pure-sulfur species - though nearly impossible to observe directly - have long been speculated to be potential sulfur reservoirs and our results represent possibly the most accurate estimates yet of their abundances in the dense ISM. Moreover, the results of these updated models are found to be in good agreement with available observational data. Finally, we examine the implications of our findings with regard to the as-yet-unknown sulfur reservoir thought to exist in dense interstellar environments.

astro-ph.GA↗