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The protonated form of hydrogen cyanide, HCNH+, holds significant importance in astrochemistry, serving as an intermediate species in ion-neutral reactions occurring in the cold molecular clouds. Although it plays a crucial role in the chemistry of HCN and HNC, the excitation rate coefficients of this molecular cation by the dominant interstellar colliders have not been thoroughly investigated, leading to limitations in the radiative transfer models used to derive its abundance. In this work, we present the first hyperfine-resolved excitation rate coefficients for HCNH+ induced by collisions with both He and H2 at low temperatures, addressing a crucial requirement for precise modeling of HCNH+ abundance in typical cold dense molecular clouds. Using non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations, we reproduced the 1 → 0 and 2 → 1 observational spectra of HCNH+ fairly well and derived updated molecular column densities. For the TMC-1 molecular cloud, the new HCNH+ abundance is twice as large as suggested by previous LTE modeling, whereas the column density of this molecular cation is improved only by 10% in the case of the L483 proto-star. The factor of two in the case of TMC-1 most likely arises from an error in the early analysis of observational spectra rather than an effect of the LTE assumption, given that the HCNH+ lines are predominantly thermalized at densities higher than 2 × 104 cm−3. For multiline studies of clouds of moderate densities, we strongly recommend using the collisional rate coefficients reported in this work.

Context. Indene, a small PAH, has been detected in the gas phase in the cold dense cloud TMC-1. Due to the low temperature in the cloud, below indene condensation temperature, its presence in the ice mantles of dust grains is likely. Aims. The aim of this work is to study the stability of indene against the energetic processing by VUV photons or cosmic rays in the ice mantles of dense molecular clouds. Methods. Ice layers of pure indene or indene diluted in water ice were grown by vapor deposition on a cold surface held at 10 K, 100 K, or 140 K. The samples were processed with VUV photons (120–180 nm) and 5 keV electrons and the destruction of indene was monitored by following the decay of its absorption bands in the infrared (IR) spectrum. Finally, we recorded the mass spectra of the decomposition products sublimated by thermal programmed desorption of the processed samples. Results. Photolysis and radiolysis cross-sections, along with half-life energy doses for indene in the solid form and diluted in water ice matrices at 10 K were derived. Ketones and alcohols were identified as the main processing products in indene and water–ice mixtures Conclusions. Dilution in water ice enhances the destruction rate of indene under energetic processing. The molecule is expected to survive for more than 107 yr in the ice mantles of grains in the interior of dense clouds, but it will end up getting destroyed within the next few hundred years by the intense VUV field in the diffuse region at the edges of the cloud.

Context. A currently unsolved question in supernova (SN) research is the origin of stripped-envelope supernovae (SESNe). Such SNe lack spectral signatures of hydrogen (Type Ib), or hydrogen and helium (Type Ic), indicating that the outer stellar layers have been stripped during their evolution. The mechanism for this is not well understood, and to disentangle the different scenarios’ determination of nucleosynthesis yields from observed spectra can be attempted. However, the interpretation of observations depends on the adopted spectral models. A previously missing ingredient in these is the inclusion of molecular effects, which can be significant. Aims. We aim to investigate how the molecular chemistry in SESNe affect physical conditions and optical spectra, and produce ro-vibrational emission in the mid-infrared (MIR). We also aim to assess the diagnostic potential of observations of such MIR emission with JWST. Methods. We coupled a chemical kinetic network including carbon, oxygen, silicon, and sulfur-bearing molecules into the nonlocal thermal equilibrium (NLTE) spectral synthesis code SUMO. We let four species – CO, SiO, SiS, and SO – participate in NLTE cooling of the gas to achieve self-consistency between the molecule formation and the temperature. We applied the new framework to model the spectrum of a Type Ic SN in the 100–600 days time range. Results. Molecules are predicted to form in SESN ejecta in significant quantities (typical mass 10−3 M⊙) throughout the 100–600 days interval. The impact on the temperature and optical emission depends on the density of the oxygen zones and varies with epoch. For example, the [O I] 6300, 6364 feature can be quenched by molecules from 200 to 450 days depending on density. The MIR predictions show strong emission in the fundamental bands of CO, SiO, and SiS, and in the CO and SiO overtones. Conclusions. Type Ibc SN ejecta have a rich chemistry and considering the effect of molecules is important for modeling the tempe

Material outgassing in a vacuum leads to molecular contamination, a well-known problem in spaceflight. Water is the most common contaminant in cryogenic spacecraft, altering numerous properties of optical systems. Too much ice means that Euclida's calibration requirements cannot be met anymore. Euclid must then be thermally decontaminated, which is a month-long risky operation. We need to understand how ice affects our data to build adequate calibration and survey plans. A comprehensive analysis in the context of an astrophysical space survey has not been done before. In this paper we look at other spacecraft with well-documented outgassing records. We then review the formation of thin ice films, and find that for Euclid a mix of amorphous and crystalline ices is expected. Their surface topography-and thus optical properties-depend on the competing energetic needs of the substrate-water and the water-water interfaces, and they are hard to predict with current theories. We illustrate that with scanning-tunnelling and atomic-force microscope images of thin ice films. Sophisticated tools exist to compute contamination rates, and we must understand their underlying physical principles and uncertainties. We find considerable knowledge errors on the diffusion and sublimation coefficients, limiting the accuracy of outgassing estimates. We developed a water transport model to compute contamination rates in Euclid, and find agreement with industry estimates within the uncertainties. Tests of the Euclid flight hardware in space simulators did not pick up significant contamination signals, but they were also not geared towards this purpose; our in-flight calibration observations will be much more sensitive. To derive a calibration and decontamination strategy, we need to understand the link between the amount of ice in the optics and its effect on the data. There is little research about this, possibly because other spacecraft can decontaminate more easily, quenching the ne

We carried out an observational search for the recently discovered molecule H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544, L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in different types of interstellar regions and shed light on its formation mechanism. We detected H2NC in most of the cold dense clouds targeted, 7 out of 8, while H2CN was only detected in 5 out of 8 clouds. The column densities derived for both H2NC and H2CN are in the range 1e11-1e12 cm-2 and the abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer H2NC is therefore widespread in cold dense clouds where it is present with an abundance similar to that of H2CN. We did not detect either H2NC or H2CN in any of the two diffuse clouds targeted, which does not allow to shed light on how the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We found that the column density of H2NC is correlated with that of NH3, which strongly suggests that these two molecules are chemically linked, most likely ammonia being a precursor of H2NC through the C + NH3 reaction. We performed electronic structure and statistical calculations which show that both H2CN and H2NC can be formed in the C + NH3 reaction through two different channels involving two different transition states which lie very close in energy. The predicted product branching ratio H2NC/H2CN is very method dependent but values between 0.5 and 0.8 are the most likely ones. Therefore, both the astronomical observations and the theoretical calculations support that the reaction C + NH3 is the main source of H2NC in interstellar clouds.

We revise the rates of reactions CH + S -> CS + H and C_2 + S -> CS + C, important CS formation routes in dark and diffuse warm gas. We performed ab initio calculations to characterize the main features of all the electronic states correlating to the open shell reactants. For CH+S we have calculated the full potential energy surfaces for the lowest doublet states and the reaction rate constant with a quasi-classical method. For C_2+S, the reaction can only take place through the three lower triplet states, which all present deep insertion wells. A detailed study of the long-range interactions for these triplet states allowed to apply a statistic adiabatic method to determine the rate constants. This study of the CH + S reaction shows that its rate is nearly independent on the temperature in a range of 10-500 K with an almost constant value of 5.5 10^{-11} cm^3/s at temperatures above 100~K. This is a factor \sim 2-3 lower than the value obtained with the capture model. The rate of the reaction C_2 + S depends on the temperature taking values close to 2.0 10^{-10} cm^3/s at low temperatures and increasing to 5. 10^{-10} cm^3/s for temperatures higher than 200~K. Our modeling provides a rate higher than the one currently used by factor of \sim 2. These reactions were selected for involving open-shell species with many degenerate electronic states, and the results obtained in the present detailed calculations provide values which differ a factor of \sim 2-3 from the simpler classical capture method. We have updated the sulphur network with these new rates and compare our results in the prototypical case of TMC1 (CP). We find a reasonable agreement between model predictions and observations with a sulphur depletion factor of 20 relative to the sulphur cosmic abundance, but it is not possible to fit all sulphur-bearing molecules better than a factor of 10 at the same chemical time.

Aims. We investigate the outcome of an electrical discharge of naphthalene and search for the resulting products in the Taurus Molecular Cloud (TMC-1). Methods. Using chirped pulse Fourier transform microwave spectroscopy paired with an electric discharge source, we investigated products resulting from the naphthalene discharge. Quantum chemical calculations were used to help assign species and investigate potential reaction pathways relevant to the interstellar medium. These products were searched for in TMC-1 using the QUIJOTE line survey, covering 31.0-50.3 GHz. Results. We confirm the detection of ethynylbenzene in TMC-1, and we also present a new molecular species, 1,2-diethynylbenzene, which could play an important role in the formation of naphthalene. Over ten products have been identified as resulting from the discharge, with only one of these species found in a previous IR-UV discharge study of naphthalene. Conclusions. Ethynylbenzene has definitively been detected in TMC-1, and while we have identified a potentially important species for the formation of naphthalene and an exothermic reaction pathway, there is no current indication of its presence in TMC-1.

We report new observations of molecular anions with the Yebes 40m and IRAM 30m telescopes toward the cold dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We detected for the first time C3N- and C5N- in Lupus-1A and C4H- and C6H- in L483. In addition, we report new lines of C6H- toward the six targeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- and C3N- in TMC-1 CP. Excitation calculations indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H cannot explain various observational facts, which advises for a revisitation of the collision data for this species. The observations presented here, together with observational data from the literature, are used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree within 50 % (a factor of two at most) with literature values, when available, except for the C4H-/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H-/C6H ratio shows a tentative trend in which it increases with increasing H2 density, as expected from theoretical grounds. And second, it is incontestable that the higher the molecular size the higher the anion-to-neutral ratio, which supports a formation mechanism based on radiative electron attachment. Nonetheless, calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude.

Material outgassing in a vacuum leads to molecular contamination, a well-known problem in spaceflight. Water is the most common contaminant in cryogenic spacecraft, altering numerous properties of optical systems. Too much ice means that Euclid’s calibration requirements cannot be met anymore. Euclid must then be thermally decontaminated, which is a month-long risky operation. We need to understand how ice affects our data to build adequate calibration and survey plans. A comprehensive analysis in the context of an astrophysical space survey has not been done before. In this paper we look at other spacecraft with well-documented outgassing records. We then review the formation of thin ice films, and find that for Euclid a mix of amorphous and crystalline ices is expected. Their surface topography – and thus optical properties – depend on the competing energetic needs of the substrate-water and the water-water interfaces, and they are hard to predict with current theories. We illustrate that with scanning-tunnelling and atomic-force microscope images of thin ice films. Sophisticated tools exist to compute contamination rates, and we must understand their underlying physical principles and uncertainties. We find considerable knowledge errors on the diffusion and sublimation coefficients, limiting the accuracy of outgassing estimates. We developed a water transport model to compute contamination rates in Euclid, and find agreement with industry estimates within the uncertainties. Tests of the Euclid flight hardware in space simulators did not pick up significant contamination signals, but they were also not geared towards this purpose; our in-flight calibration observations will be much more sensitive. To derive a calibration and decontamination strategy, we need to understand the link between the amount of ice in the optics and its effect on the data. There is little research about this, possibly because other spacecraft can decontaminate more easily, quenching the ne

The Lyman–Werner (LW) radiation field is a key ingredient in the chemothermal evolution of gas in the early Universe, as it dissociates H2 molecules, the primary cooling channel in an environment devoid of metals and dust. Despite its important role, it is still not implemented in cosmological simulations on a regular basis, in contrast to the ionizing UV background. This is in part due to uncertainty in the source modelling, their spectra and abundance, as well as the detailed physics involved in the propagation of the photons and their interactions with the molecules. The goal of this work is to produce an accurate model of the LW radiation field at z ≥ 6, by post-processing the physics-rich high-resolution FiBY simulation. Our novelties include updated cross-sections for H2, H− and H+2 chemical species, IGM absorption by neutral Hydrogen and various spectral models for Population III and Population II stars. With our fiducial set of parameters, we show that the mean LW intensity steadily increases by three orders of magnitude from z ∼ 23 to z ∼ 6, while spatial inhomogeneities originate from massive star-forming galaxies that dominate the photon budget up to a distance of ∼100 proper kpc. Our model can be easily applied to other simulations or semi-analytical models as an external radiation field that regulates the formation of stars and massive black hole seeds in high-z low-mass haloes.

Context. Sulfur is a biogenic element used as a tracer of the evolution of interstellar clouds to stellar systems. However, most of the expected sulfur in molecular clouds remains undetected. Sulfur disappears from the gas phase in two steps. The first depletion occurs during the translucent phase, reducing the gas-phase sulfur by 7-40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. This long-standing question awaits an explanation. Aims. The aim of this study is to understand under what form the missing sulfur is hiding in molecular clouds. The possibility that sulfur is depleted onto dust grains is considered. Methods. Experimental simulations mimicking HS ice UV photoprocessing in molecular clouds were conducted at 8 K under ultra-high vacuum. The ice was subsequently warmed up to room temperature. The ice was monitored using infrared spectroscopy, and the desorbing molecules were measured by quadrupole mass spectrometry in the gas phase. Theoretical Monte Carlo simulations were performed for interpretation of the experimental results and extrapolation to the astrophysical and planetary conditions. Results. HS formation was observed during irradiation at 8 K. Molecules HS x with x > 2 were also identified and found to desorb during warm-up, along with S to S 4 species. Larger S x molecules up to S 8 are refractory at room temperature and remained on the substrate forming a residue. Monte Carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains of sulfur consisting of 6-7 atoms. Conclusions. Based on the interpretation of the experimental results using our theoretical model, it is proposed that S + in translucent clouds contributes notoriously to S depletion in denser regions by forming long S chains on dust grains in a few times 10 4 yr. We suggest that the S to S 4 molecules observed in comets are not produced by fragmentation of, Astrodynamics & Space Missions

Context. Methoxymethanol (CH3OCH2OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products. Aims. The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel athe recombination of CH2OH and CH3O radicals afor 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. We also investigate an alternative hydrogenation channel starting from methyl formate. Methods. Hydrogen atoms and CO or H2CO molecules were co-deposited on top of predeposited H2O ice to mimic the conditions associated with the beginning of arapida CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs. Results. The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H2CO+H experiments. The resulting abundance of methoxymethanol with respect to CH3OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH3OH, with values ranging between 0.0

Context. The Rosetta and Giotto missions investigated the composition of the cometary comae of 67P/Churyumov-Gerasimenko and 1P/Halley, respectively. In both cases, a surprisingly large amount of molecular oxygen (O2) was detected and was well correlated with the observed abundances of H2O. Laboratory experiments simulating chemical processing for various astronomical environments already showed that formation of solid state O2 is linked to water. However, a quantitative study of O2 formation upon UV photolysis of pure H2O and H2O dominated interstellar ice analogues is still missing. Aims. The goal of this work is to investigate whether the UV irradiation of H2O-rich ice produced at the earliest stages of star formation is efficient enough to explain the observed abundance of cometary O2. Methods. The photochemistry of pure H216O (H218O) as well as mixed H2O:CO2 (ratio of 100:11, 100:22, 100:44) and H2O:CO2:O2 (100:22:2) ices was quantified during UV photolysis. Laser desorption post-ionisation time of flight mass spectrometry (LDPI TOF MS) was used to probe molecular abundances in the ice as a function of UV fluence. Results. Upon UV photolysis of pure amorphous H2O ice, deposited at 20 K, formation of O2 and H2O2 is observed at abundances of, respectively, (0.9 ± 0.2)% (O2/H2O) and (1.3 ± 0.3)% (H2O2/H2O). To the best of our knowledge, this is the first quantitative characterisation of the kinetics of this process. During the UV photolysis of mixed H2O:CO2 ices, the formation of the relative amount of O2 compared to H2O increases to a level of (1.6 ± 0.4)% (for H2O:CO2 ratio of 100:22), while the (H2O2/H2O) yield remains similar to experiments with pure water. In an ice enriched with O2 (2%), the O2 level increases up to 7% with regard to H2O, at low UV fluence, which is higher than expected on the basis of the enrichment alone. The resulting O2/H2O values derived for the H2O and H2O:CO2 ices may account for a (substantial) part of the high oxygen amounts found i

The previous detection of two species related to the non polar molecule cyanogen (NCCN), its protonated form (NCCNH+) and one metastable isomer (CNCN), in cold dense clouds supported the hypothesis that dicyanopolyynes are abundant in space. Here we report the first identification in space of NC4NH+, which is the protonated form of NC4N, the second member of the series of dicyanopolyynes after NCCN. The detection was based on the observation of six harmonically related lines within the Yebes 40m line survey of TMC-1 QUIJOTE. The six lines can be fitted to a rotational constant B = 1293.90840 +/- 0.00060 MHz and a centrifugal distortion constant D = 28.59 +/- 1.21 Hz. We confidently assign this series of lines to NC4NH+ based on high-level ab initio calculations, which supports the previous identification of HC5NH+ by Marcelino et al. (2020) from the observation of a series of lines with a rotational constant 2 MHz lower than that derived here. The column density of NC4NH+ in TMC-1 is (1.1 +1.4 -0.6)e10 cm-2, which implies that NC4NH+ is eight times less abundant than NCCNH+. The species CNCN, previously reported toward L483 and tentatively in TMC-1, is confirmed in this latter source. We estimate that NCCN and NC4N are present in TMC-1 with abundances a few times to one order of magnitude lower than HC3N and HC5N, respectively. This means that dicyanopolyynes NC-(CC)n-CN are present at a lower level than the corresponding monocyanopolyynes HCC-(CC)n-CN. The reactions of the radicals CN and C3N with HNC arise as the most likely formation pathways to NCCN and NC4N in cold dense clouds.

The formation of hydrides by gas-phase reactions between H2 and a heavy element atom is a very selective process. Reactions with ground-state neutral carbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have high energy barriers because the H2 molecule has to be fragmented before a hydride bond is formed. In cold interstellar clouds, these barriers exclude the formation of CH, OH, NH, and SH radicals through hydrogen abstraction reactions. Here we study a very energetically unfavorable process, the reaction of N(4S) atoms with H2 molecules. We calculated the reaction rate coefficient for H2 in different vibrational levels, using quantum methods for v=0-7 and quasi-classical methods up to v=12. Owing to the high energy barrier, these rate coefficients increase with v and also with the gas temperature. We implemented the new rates in the Meudon PDR code and studied their effect on models with different ultraviolet (UV) illumination conditions. In strongly UV-irradiated dense gas (Orion Bar conditions), the presence of H2 in highly vibrationally excited levels (v>7) enhances the NH abundance by two orders of magnitude (at the PDR surface) compared to models that use the thermal rate coefficient for reaction N(4S) + H2 -> NH + H. The increase in NH column density across the PDR is a factor of ~25. We explore existing Herschel/HIFI observations of the Orion Bar and Horsehead PDRs. We report a 3-sigma emission feature at the ~974 GHz frequency of the NH N_J=1_2-0_1 line toward the Bar. The emission level implies N(NH)~10^13 cm^-2, which is consistent with PDR models using the new rate coefficients for reactions between N and UV-pumped H2. This formation route dominates over hydrogenation reactions involving the less abundant N+ ion. JWST observations will soon quantify the amount and reactivity of UV-pumped H2 in many interstellar and circumstellar environments.

Hydrogen cyanide (HCN) is considered a good tracer of the dense molecular gas that serves as fuel for star formation. However, recent large-scale surveys of giant molecular clouds (GMCs) have detected extended HCN rotational line emission far from star-forming cores. Such observations often spectroscopically resolve the HCN J = 1-0 (partially also the J = 2-1 and 3-2) hyperfine structure (HFS). A precise determination of the physical conditions of the gas requires treating the HFS line overlap effects. Here, we study the HCN HFS excitation and line emission using nonlocal radiative transfer models that include line overlaps and new HFS-resolved collisional rate coefficients for inelastic collisions of HCN with both para-H2 and ortho-H2 (computed via the scaled-infinite order sudden approximation up to Tk = 500 K). In addition, we account for the role of electron collisions in the HFS level excitation. We find that line overlap and opacity effects frequently produce anomalous HCN J = 1-0 HFS line intensity ratios (i.e., inconsistent with the common assumption of the same Tex for all HFS lines) as well as anomalous HFS line width ratios. Line overlap and electron collisions also enhance the excitation of the higher J rotational lines. Our models explain the anomalous HCN J = 1-0 HFS spectra observed in the Orion Bar and Horsehead photodissociation regions. As shown in previous studies, electron excitation becomes important for molecular gas with H2 densities below a few 105 cm-3 and electron abundances above ∼10-5. We find that when electron collisions are dominant, the relative intensities of the HCN J = 1-0 HFS lines can be anomalous too. In particular, electron excitation can produce low-surface-brightness HCN emission from very extended but low-density gas in GMCs. The existence of such a widespread HCN emission component may affect the interpretation of the extragalactic relationship HCN luminosity versus star-formation rate. Alternatively, extended HCN emission

In recent years we have seen an important increase in the number of protonated molecules detected in cold dense clouds. Here we report the detection in TMC-1 of HCCNCH+, the protonated form of HCCNC, which is a metastable isomer of HC3N. This is the first protonated form of a metastable isomer detected in a cold dense cloud. The detection was based on observations carried out with the Yebes 40m and IRAM 30m telescopes, which revealed four harmonically related lines. We derive a rotational constant B = 4664.431891 +/- 0.000692 MHz and a centrifugal distortion constant D = 519.14 +/- 4.14 Hz. From a high-level ab initio screening of potential carriers we confidently assign the series of lines to the ion HCCNCH+. We derive a column density of (3.0 +/- 0.5)e10 cm-2 for HCCNCH+, which results in a HCCNCH+/HCCNC abundance ratio of 0.010 +/- 0.002. This value is well reproduced by a state-of-the-art chemical model, which however is subject to important uncertainties regarding the chemistry of HCCNCH+. The observational and theoretical status of protonated molecules in cold dense clouds indicate that there exists a global trend in which protonated-to-neutral abundance ratios MH+/M increase with increasing proton affinity of the neutral M, although if one restricts to species M with high proton affinities (>700 kJ/mol), MH+/M ratios fall in the range 0.001-0.1, with no apparent correlation with proton affinity. We suggest various protonated molecules that are good candidates for detection in cold dense clouds in the near future.

Context. Dust grains play an important role in the synthesis of molecules in the interstellar medium, from the simplest species, such as H2, to complex organic molecules. How some of these solid-state molecules are converted into gas-phase species is still a matter of debate. Aims. Our aim is to directly compare ice and gas abundances of methanol (CH3OH) and carbon monoxide (CO) obtained from near-infrared (2.5-5 μm) and millimetre (1.3 mm) observations and to investigate the relationship between ice, dust, and gas in low-mass protostellar envelopes. Methods. We present Submillimeter Array (SMA) and Atacama Pathfinder EXperiment (APEX) observations of gas-phase CH3OH (JK = 5K-4K), 13CO, and C18O (J = 2-1) towards the multiple protostellar system IRAS 05417+0907, which is located in the B35A cloud, λ Orionis region. We use archival IRAM 30 m data and AKARI H2O, CO, and CH3OH ice observations towards the same target to compare ice and gas abundances and directly calculate CH3OH and CO gas-to-ice ratios. Results. The CO isotopologue emissions are extended, whereas the CH3OH emission is compact and traces the giant molecular outflow emanating from IRAS 05417+0907. A discrepancy between sub-millimetre dust emission and H2O ice column density is found for B35A-4 and B35A-5, similar to what has previously been reported. B35A-2 and B35A-3 are located where the sub-millimetre dust emission peaks and show H2O column densities lower than that of B35A-4. Conclusions. The difference between the sub-millimetre continuum emission and the infrared H2O ice observations suggests that the distributions of dust and H2O ice differ around the young stellar objects in this dense cloud. The reason for this may be that the four sources are located in different environments resolved by the interferometric observations: B35A-2, B35A-3, and, in particular, B35A-5 are situated in a shocked region that is plausibly affected by sputtering and heating, which in turn impacts the sub-millimetre d

Context: Interstellar grains are known to be important actors in the formation of interstellar molecules such as H2, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. Aims: In this work, we aim to investigate the robustness or weakness of this assumption. In particular, we consider the case of acetaldehyde (CH3CHO), one of the most abundant and commonly identified iCOMs, as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH3. Here we report new theoretical computations on the efficiency of its formation. Methods: To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH3 + HCO, which can lead to the formation of CH3CHO or CO + CH4. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger-Kassel-Marcus theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Results: Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio f of the CH3 radical. If the ratio f is ≥0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if f is smaller, the efficiency dramatically crashes: with f = 0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH4. Conclusions: Given the poor understanding of the diffusion over binding energy ratio f and the dramatic effect it has on the formation, or not, of

Context. Many C-, O-, and H-containing complex organic molecules (COMs) have been observed in the interstellar medium (ISM) and their formation has been investigated in laboratory experiments. An increasing number of recent detections of large N-bearing COMs motivates our experimental investigation of their chemical origin. Aims. We investigate the potential role of acetonitrile (CH3CN) as a parent molecule to N-bearing COMs, motivated by its omnipresence in the ISM and structural similarity to another well-known precursor species, CH3OH. The aim of the present work is to characterize the chemical complexity that can result from vacuum UV photolysis of a pure CH3CN ice and a more realistic mixture of H2O:CH3CN. Methods. The CH3CN ice and H2O:CH3CN ice mixtures were UV irradiated at 20 K. Laser desorption post ionization time-of-flight mass spectrometry was used to detect the newly formed COMs in situ. We examined the role of water in the chemistry of interstellar ices through an analysis of two different ratios of H2O:CH3CN (1:1 and 20:1). Results. We find that CH3CN is an excellent precursor to the formation of larger nitrogen-containing COMs, including CH3CH2CN, NCCN/CNCN, and NCCH2CH2CN. During the UV photolysis of H2O:CH3CN ice, the water derivatives play a key role in the formation of molecules with functional groups of: imines, amines, amides, large nitriles, carboxylic acids, and alcohols. We discuss possible formation pathways for molecules recently detected in the ISM. © ESO 2021.

Context. The simultaneous detection of organic molecules of the form C2HnO, such as ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH), toward early star-forming regions offers hints of a shared chemical history. Several reaction routes have been proposed and experimentally verified under various interstellar conditions to explain the formation pathways involved. Most noticeably, the non-energetic processing of C2H2 ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH2CHOH) along the H2O formation sequence on grain surfaces in translucent clouds. Aims. In this work, the non-energetic formation scheme is extended with laboratory measurements focusing on the energetic counterpart, induced by cosmic rays penetrating the H2O-rich ice mantle. The focus here is on the H+ radiolysis of interstellar C2H2:H2O ice analogs at 17 K. Methods. Ultra-high vacuum experiments were performed to investigate the 200 keV H+ radiolysis chemistry of predeposited C2H2:H2O ices, both as mixed and layered geometries. Fourier-transform infrared spectroscopy was used to monitor in situ newly formed species as a function of the accumulated energy dose (or H+ fluence). The infrared spectral assignments are further confirmed in isotope labeling experiments using H218O. Results. The energetic processing of C2H2:H2O ice not only results in the formation of (semi-) saturated hydrocarbons (C2H4 and C2H6) and polyynes as well as cumulenes (C4H2 and C4H4), but it also efficiently forms O-bearing COMs, including vinyl alcohol, ketene, acetaldehyde, and ethanol, for which the reaction cross-section and product composition are derived. A clear composition transition of the product, from H-poor to H-rich species, is observed as a function of the accumulated energy dose. Furthermore, the astronomical relevance of the resulting reaction network is discussed. © K.-J. Chuang et al. 2021.

Context. Modelling the formation of cloud condensation nuclei (CCNs) is key for predicting cloud properties in planet and brown dwarf atmospheres. The large diversity of exoplanets (rocky planets, mini-Neptunes, giant gas planets) requires a fundamental approach to cloud formation modelling in order to allow a full analysis of observational data contributing to exoplanet characterisation. Aims. We aim to understand the onset of cloud formation and study the formation of TiO2-CCNs. The formation of (TiO2)N clusters as precursors to extrasolar cloud formation is modelled by two different methods in order to understand their potential, identify underlying shortcomings, and to validate our methods. We propose potential spectral tracers for TiO2-CCN formation. Methods. We applied three-dimensional Monte Carlo (3D MC) simulations to model the collision-induced growth of TiO2-molecules to (TiO2)N-clusters in the free molecular flow regime of an atmospheric gas. We derived individual, time-dependent (TiO2)N cluster number densities. For T = 1000 K, the results are compared to a kinetic approach that utilises thermodynamic data for individual (TiO2)N clusters. Results. The (TiO2)N cluster size distribution is temperature dependent and evolves in time until a steady state is reached. For T = 1000 K, the 3D MC and the kinetic approach agree well regarding the cluster number densities for N = 1 … 10, the vivid onset of cluster formation, and the long transition into a steady state. Collision-induced growth and evaporation simulated using a 3D MC approach enables a faster onset of cluster growth through nucleation bursts. Different size distributions develop for monomer-cluster and for cluster-cluster growth, with the largest cluste

We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD2CCH and CH2DCCD were identified in the dense core L483 through, respectively, nine and eight rotational lines in the 72-116 GHz range using the 30 m telescope of the Institut de Radioastronomie Millimétrique (IRAM). We combined the astronomical frequencies observed here with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 ± 0.5) × 1012 cm-2 for CHD2CCH and (2.2 ± 0.4) × 1012 cm-2 for CH2DCCD, which translate to abundance ratios of CH3CCH/CHD2CCH = 34 ± 10 and CH3CCH/CH2DCCD = 42 ± 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, specifically around 2.4 times less abundant than CH3CCD. The abundances of the different deuterated isotopologues with respect to CH3CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C3H6D+ and C3H5D+, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as being responsible for the formation and deuterium fractionation of CH3CCH in L483. The abundance ratios CH2DCCH/CH3CCD = 3.0 ± 0.9 and CHD2CCH/CH2DCCD = 1.25 ± 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD2CCH compared to CH2DCCD being well explained by the chemical model.

The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground-state oxygen atoms limits its efficiency in cold gas. In this study we calculated the dependence of the reaction rate coefficient on the rotational and vibrational state of H2, and evaluated the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons where H2 can be efficiently pumped to excited vibrational states. We used a recently calculated potential energy surface and carried out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O (3P) + H2 (v, j) → OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2. However, even for high vibrational states of H2 (v = 4-5), for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and to evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to AV = 1.3-2.3 mag, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, about 60%. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.

We report the first identification in space of H2NC, a high-energy isomer of H2CN that has been largely ignored in chemical and astrochemical studies. The observation of various unidentified lines around 72.2 GHz in the cold dark cloud L483 motivated the search and successful detection of additional groups of lines in harmonic relation. Following an exhaustive high-level ab initio screening of possible carriers, we confidently assign the unidentified lines to H2NC based on the good agreement between the astronomical and theoretical spectroscopic parameters alongside sound spectroscopic and astrochemical arguments. The observed frequencies are used to precisely characterize the rotational spectrum of H2NC. This species is also detected in the cold dark cloud B1-b and the z = 0.89 galaxy in front of the quasar PKS 1830-211. We derive H2NC/H2CN abundance ratios ∼1 in L483 and B1-b and 0.27 toward PKS 1830-211. Neither H2NC nor H2CN are detected in the dark cloud TMC-1, which seriously undermines the previous identification of H2CN in this source. We suggest that the H2NC/H2CN ratio behaves as the HNC/HCN ratio, with values close to one in cold dense clouds and below one in diffuse clouds. The reactions N + CH3 and C + NH3 emerge as strong candidates for the production of H2NC in interstellar clouds. Further studies on these two reactions are needed to evaluate the yield of H2NC. Due to the small number of atoms involved, it should be feasible to constrain the chemistry behind H2NC and H2CN, just as has been done for HNC and HCN, as this could allow for the H2NC/H2CN ratio to be applied as a probe of chemical or physical conditions of the host clouds.

We have computed the rate of collisional cooling of a gas by H2 molecules under conditions appropriate to the primordial and interstellar media. We incorporated the results of recent calculations of the rate coefficients for collisional excitation of H2 by H and H+, which are essential to a reliable evaluation of the ortho:para H2 ratio and the cooling rate. Comparison is made with the results of previous calculations of the cooling function. The data are made available for grids of values of the kinetic temperature, density, H:H2 ratio, and the fractional abundance of H+, together with a programme to perform linear interpolation of the data sets for any given set of values of these parameters, within the ranges of the grids.

Context. Pre-stellar cores (PSCs) are units of star formation. Besides representing early stages of the dynamical evolution leading to the formation of stars and planets, PSCs also provide a substrate for incipient chemical complexity in the interstellar space. Aims. Our aim is to understand the influence of external conditions on the chemical composition of PSCs. For this purpose, we compared molecular column densities in two typical PSCs, L183 and L1544, which are embedded in different environments. Methods. A single-pointing survey of L183 at λ = 3 mm was conducted using the IRAM 30-m single-dish antenna. This led to the detection of more than 100 emission lines from 46 molecular species. The molecular column densities and excitation temperatures derived from these lines were compared to the corresponding parameters in L1544. The data for L1544 were obtained from literature or publicly available surveys, and they were analysed using the same procedure as adopted for L183. An astrochemical model, previously developed for the interpretation of organic molecule emissions towards the methanol peak of L1544, was used to interpret the combined data. Results. Our analysis reveals clear chemical differences between the two PSCs. While L1544 is richer in carbon-bearing species, in particular carbon chains, oxygen-containing species are generally more abundant in L183. The results are well-reproduced by our chemical model. Conclusions. The observed chemical differentiation between the two PSCs is caused by the different environmental conditions: the core of L183 is deeply buried in the surrounding cloud, whereas L1544 lies close to the edge of the Taurus Molecular Cloud. The obscuration of L183 from the interstellar radiation field (ISRF) allows the carbon atoms to be locked in carbon monoxide, which ultimately leads to a large abundance of O-bearing species. In contrast, L1544, being more affected by the ISRF, can keep a fraction of carbon in atomic form, which is needed

Context. The inclusion of molecular physics is an important piece that tends to be missing from the puzzle when modeling the spectra of supernovae (SNe). Molecules have both a direct impact on the spectra, particularly in the infrared, and an indirect one as a result of their influence on certain physical conditions, such as temperature. Aims. In this paper, we aim to investigate molecular formation and non-local thermodynamic equilibrium (NLTE) cooling, with a particular focus on CO, the most commonly detected molecule in supernovae. We also aim to determine the dependency of supernova chemistry on physical parameters and the relative sensitivity to rate uncertainties. Methods. We implemented a chemical kinetic description of the destruction and formation of molecules into the SN spectral synthesis code SUMO. In addition, selected molecules were coupled into the full NLTE level population framework and, thus, we incorporated molecular NLTE cooling into the temperature equation. We produced a test model of the CO formation in SN 1987A between 150 and 600 days and investigated the sensitivity of the resulting molecular masses to the input parameters. Results. We find that there is a close inter-dependency between the thermal evolution and the amount of CO formed, mainly through an important temperature-sensitive CO destruction process with O+. After a few hundred days, CO completely dominates the cooling of the oxygen-carbon zone of the supernova which, therefore, contributes little optical emission. The uncertainty of the calculated CO mass scales approximately linearly with the typical uncertainty factor for individual rates. We demonstrate how molecular masses can potentially be used to constrain various physical parameters of the supernova.

Context. Polycyclic aromatic hydrocarbons (PAHs) and fullerenes are the largest molecules found in the interstellar medium (ISM). They are abundant and widespread in various astronomical environments. However, the detailed connection between these two species is unknown; in particular, no quantum chemical studies have been performed.Aims. In this work, we investigate a vital step in transforming planar PAHs to fullerenes, that is, the tubulation processes of PAHs.Methods. We used density functional theory for this study. The molecular structures and vibrational frequencies were calculated using the hybrid density functional B3LYP method. To better describe intermolecular forces, we considered Grimme's dispersion correction in the calculations for this work. Intrinsic reaction coordinate calculations were also performed to confirm that the transition state structures are connected to their corresponding local potential energy surface minima.Results. As expected, we find that it is easier to bend a molecule as it gets longer, whereas it is harder to bend the molecule if it gets "wider" (i.e., with more rows of benzene rings). The change of multiplicity slightly alters the bending energies, while (a complete) dehydrogenation alleviates the bending barrier significantly and facilitates the formation of pentagons, which may act as an indispensable step in the formation of fullerenes in the ISM., QC 20210112

Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives have been poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations were performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K. Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H-2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would completely break into carbon chains. The transformation from a hexagon to a pentagon or a heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) would facilitate the transformation., QC 20200217

Context: The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time. Aims: Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions. Methods: We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance. Results: We confirm the presence in situ of predicted cations at comets, such as CHm+ (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO2++.

The aim of the presented work is to analyze the impact of experimentally evaluated reactions of hydrogen abstraction on surfaces of interstellar grains on the chemical evolution of methanol and its precursors on grains and in the gas phase under conditions of a cold dark cloud and during the collapse of a translucent cloud into a dark cloud. Analysis of simulation results shows that those reactions are highly efficient destruction channels for HCO and H2CO on grain surfaces, and significantly impact the abundances of almost all molecules participating in the formation of CH3OH. Next, in models with those reactions, maximum abundances of methanol in gas and on grain surfaces decrease by more than 2-3 orders of magnitude in comparison to models without surface abstraction reactions of hydrogen. Finally, we study the impact of binding energies of CH2OH and CH3O radicals on methanol chemistry. © 2020 National Astronomical Observatories, CAS and IOP Publishing Ltd.

Context. Phosphorus-bearing species are essential to the formation of life on Earth, however they have barely been detected in the interstellar medium. In particular, towards star-forming regions only PN and PO have been identified so far. Since only a small number of detections of P-bearing molecules are available, their chemical formation pathways are not easy to constrain and are thus highly debatable. An important factor still missing in the chemical models is the initial elemental abundance of phosphorus, that is, the depletion level of P at the start of chemical models of dense clouds. Aims. In order to overcome this problem, we study P-bearing species in diffuse and translucent clouds. In these objects phosphorus is expected to be mainly in the gas phase and therefore the elemental initial abundance needed in our chemical simulations corresponds to the cosmic one and is well constrained. Methods. For the study of P-bearing chemistry we used an advanced chemical model. We updated and significantly extended the P-chemistry network based on chemical databases and previous literature. We performed single-pointing observations with the IRAM 30 m telescope in the 3 mm range towards the line of sight to the strong continuum source B0355+508 aiming for the (2-1) transitions of PN, PO, HCP, and CP. This line of sight incorporates five diffuse and/or translucent clouds. Results. The (2-1) transitions of the PN, PO, HCP, and CP were not detected. We report high signal-to-noise-ratio detections of the (1-0) lines of 13CO, HNC, and CN along with a first detection of C34S towards this line of sight. We have attempted to reproduce the observations of HNC, CN, CS, and CO in every cloud with our model by applying typical physical conditions for diffuse or translucent clouds. We find that towards the densest clouds with vLSR = -10, - 17 km s-1 the best-fit model is given by the parameters (n(H), AV, Tgas) = (300 cm-3, 3 mag, 40 K). Conclusions. According to our best-fit model

Context. The interaction between dust, ice, and gas during the formation of stars produces complex organic molecules. While observations indicate that several species are formed on ice-covered dust grains and are released into the gas phase, the exact chemical interplay between solid and gas phases and their relative importance remain unclear. Aims. Our goal is to study the interplay between dust, ice, and gas in regions of low-mass star formation through ice- and gas-mapping and by directly measuring gas-to-ice ratios. This provides constraints on the routes that lead to the chemical complexity that is observed in solid and gas phases. Methods. We present observations of gas-phase methanol (CH3OH) and carbon monoxide (13CO and C18O) at 1.3 mm towards ten low-mass young protostars in the Serpens SVS 4 cluster from the SubMillimeter Array (SMA) and the Atacama Pathfinder EXperiment (APEX) telescope. We used archival data from the Very Large Telescope (VLT) to derive abundances of ice H2O, CO, and CH3OH towards the same region. Finally, we constructed gas-ice maps of SVS 4 and directly measured CO and CH3OH gas-to-ice ratios. Results. The SVS 4 cluster is characterised by a global temperature of 15 ± 5 K. At this temperature, the chemical behaviours of CH3OH and CO are anti-correlated: larger variations are observed for CH3OH gas than for CH3OH ice, whereas the opposite is seen for CO. The gas-to-ice ratios (Ngas/Nice) range from 1-6 for CO and 1.4 × 10-4-3.7 × 10-3 for CH3OH. The CO gas-maps trace an extended gaseous component that is not sensitive to the effect of freeze-out. Because of temperature variations and dust heating around 20 K, the frozen CO is efficiently desorbed. The CH3OH gas-maps, in contrast, probe regions where methanol is predominantly formed and present in ices and is released into the gas phase through non-thermal desorption mechanisms. Conclusions. Combining gas- and ice-mapping techniques, we measure gas-to-ice ratios of CO and CH3OH in the

H2D+ and D2H+ are important chemical tracers of prestellar cores due to their pure rotational spectra that can be excited at the ∼ 20 K temperature of these environments. The use of these molecules as probes of prestellar cores requires understanding the chemistry that forms and destroys these molecules. Of the eight key reactions that have been identified (Albertsson et al. 2013), five are thought to be well understood. The remaining three are the isotope exchange reactions of atomic D with H+ 3 ,H2D+, and D2H+. Semi-classical results differ from the classical Langevin calculations by an order of magnitude (Moyano et al. 2004). To resolve this discrepancy, we have carried out laboratory measurements for these three reactions. Absolute cross sections were measured using a dual-source, merged fast-beams apparatus for relative collision energies between ∼ 10 meV to ∼ 10 eV (Hillenbrand et al. 2019). A semi-empirical model was developed incorporating high level quantum mechanical ab initio calculations for the zero-point-energycorrected potential energy barrier in order to generate thermal rate coefficients for astrochemical models. Based on our studies, we find that these three reactions proceed too slowly at prestellar core temperatures to play a significant role in the deuteration of H+ 3 isotopologues., SCOPUS: ar.k, info:eu-repo/semantics/published

Context: Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form. Aims: The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft. Methods: We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed. Results: Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov-Gerasimenko. It has been found that at low activity and within several tens of kilometers from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015 November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.

In a cometary coma, the ion-neutral decoupling distance, sometimes referred to as the ion exobase or collisionopause, can be defined as the cometocentric distance, r(in), where ions, initially moving with the neutral outgassing speed, have a probability of 1/e of not colliding with neutrals on their subsequent journey radially outwards. We present an analytical model for calculating this decoupling distance in the presence of a static radial electric field. We show that for a logarithmically decaying potential, the value of r(in) can even decrease to similar to 15 per cent of its field-free case value. Moreover, already at this distance, the effective ion speed can be expected to markedly exceed the neutral expansion velocity. These analytical results are in line with previous numerical calculations, adapting similar but not identical field profiles. The presence of a non-negligible ambipolar electric field and limited importance of ion-neutral collisional coupling are further supported by observations in the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko by plasma instruments onboard Rosetta that reveal ion speeds several times higher than the neutral expansion velocity.

Context. The 3.3 mu m aromatic C-H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at similar to 3.4-3.6 mu m. One of these sources, IRAS 21282 +5050, a planetary nebula, also exhibits a weak band at similar to 1.68 mu m. While the satellite features at similar to 3.4-3.6 mu m are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 mu m feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6-1.7 mu m wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang-Landau random walk technique was employed. Results. All the dominant bands in the 1.6-1.7 mu m wavelength range and in the 3.1-3.5 mu m C-H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6-1.7 mu m region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1-3.5 mu m C-H stretching features to the bands in the 1.6-1.7 mu m region, I3.1-3:5/I1.6-1.7, for both ground and vibrationally excited states. On average, we obtain < I3.1-3.5=/I1.6-1.7 > approximate to 12.6 and < I3.1-3.5=/I1.6-1.7 > approximate to 17:6 for PAHs at ground states and at vibrationally excited states, respectively., QC 20200117

Context. As the fourth most abundant element in the universe, carbon (C) is widespread in the interstellar medium (ISM) in various allotropic forms (e.g. fullerenes have been identified unambiguously in many astronomical environments, the presence of polycyclic aromatic hydrocarbon molecules in space has been commonly acknowledged, and presolar graphite, as well as nanodiamonds, have been identified in meteorites). As stable allotropes of these species, whether carbon nanotubes (CNTs) and their hydrogenated counterparts are also present in the ISM or not is unknown. Aims. The aim of the present works is to explore the possible routes for the formation of CNTs in the ISM and calculate their fingerprint vibrational spectral features in the infrared (IR). Methods. We studied the hydrogen-abstraction and acetylene-addition (HACA) mechanism and investigated the synthesis of nanotubes using density functional theory (DFT). The IR vibrational spectra of CNTs and hydrogenated nanotubes (HNTs), as well as their cations, were obtained with DFT. Results. We find that CNTs could be synthesized in space through a feasible formation pathway. CNTs and cationic CNTs, as well as their hydrogenated counterparts, exhibit intense vibrational transitions in the IR. Their possible presence in the ISM could be investigated by comparing the calculated vibrational spectra with astronomical observations made by the Infrared Space Observatory, Spitzer Space Telescope, and particularly the upcoming James Webb Space Telescope., QC 20191217

In this work, we perform molecular dynamic (MD) simulations to investigate the stability and fragmentation processes of vibrationally excited linear polycyclic aromatic hydrocarbons (PAHs). The program of CP2K in combination with the semi-empirical method PM3 is utilized for the MD simulations. The simulations show that the formation of molecular hydrogens (H-2) is different than previous studies, in particular, different than compact PAHs. At high temperatures, linear PAHs tend to open aromatic rings and convert the sp(3) C-C or sp(2) C=C bonds to sp C C bonds by removing H-2; i.e. polyynes are formed in such process. Besides polyynes, PAHs attached with sp-bonded polyyne chains are commonly observed at high temperatures. We notice that due to the addition of flexible tails (polyynes), the ring-polyyne molecules do not dissociate for a long period of time at high temperatures. Such structures facilitate the molecules to survive in the harsh environment of the interstellar medium. In addition, the ring-polyyne structures induce dipole moments that could, in principle, be detected by radio astronomy., QC 20190918

SN 1987A is the only young supernova (SN) in which H-2 has been detected in the ejecta. The properties of H-2 are important for understanding the explosion and the ejecta chemistry. Here we present new Very Large Telescope/SINFONI observations of H-2 in SN 1987A, focusing on the 2.12 mu m (1,0)S(1) line. We find that the 3D emissivity is dominated by a single clump in the southern ejecta, with weaker emission being present in the north along the plane of the circumstellar ring. The lowest observed velocities are in the range of 400-800 km s(-1), in agreement with previous limits on inward mixing of H. The brightest regions of H-2 coincide with faint regions of H alpha, which can be explained by H alpha being powered by X-ray emission from the ring, while the H-2 is powered by Ti-44. A comparison with ALMA observations of other molecules and dust shows that the brightest regions of H-2, CO, and SiO occupy different parts of the inner ejecta and that the brightest H-2 clump coincides with a region of very weak dust emission. The latter is consistent with theoretical predictions that the H-2 should form in the gas phase rather than on dust grains., QC 20190403

We propose a new mechanism of desorption of molecules from dust surface heated by exothermic reactions and derive a formula for the desorption probability. This theory includes no parameter that is physically ambiguous. It can predict the desorption probabilities not only for one-product reactions but also for multiproduct reactions. Furthermore, it can predict desorption probability of a pre-adsorbed molecule induced by a reaction at a nearby site. This characteristic will be helpful to verify the theory by the experiments which involve complex reaction networks. We develop a quantitative method of comparing the predicted desorption probability with the experiments. This method is also applied to the theories proposed so far. It is shown that each of them reproduces the experiments with similar precision, although the amount of systematic experimental data that give definite desorption probability are limited at present. We point out the importance of clarifying the nature of the substrate used in the experiment, in particular, its thermal diffusivity. We show a way to estimate the substrate properties from systematic desorption experiments without their direct measurements.

Many current astrochemical models explicitly consider the species that comprise the bulk of interstellar dust grain ice mantles separately from those in the top few monolayers. Bombardment of these ices by ionizing radiation - whether in the form of cosmic rays, stellar winds, or radionuclide emission - represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate-equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O2 ice at 5 K and (b) pure H2O ice at 16 K and 77 K, were bombarded by keV H+ ions. Our aims were twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine how bulk chemistry in such a well-constrained system is best handled using the same gas-grain codes that are used to model the interstellar medium. We found that our modified astrochemical model was able to reproduce both the abundance of O3 in the 5 K pure O2 ice, as well as both the abundance of H2O2 in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice. © 2019. The American Astronomical Society. All rights reserved.

We propose a new mechanism of desorption of molecules from dust surface heated by exothermic reactions and derive a formula for the desorption probability. This theory includes no parameter that is physically ambiguous. It can predict the desorption probabilities not only for one-product reactions but also for multiproduct reactions. Furthermore, it can predict desorption probability of a pre-adsorbed molecule induced by a reaction at a nearby site. This characteristic will be helpful to verify the theory by the experiments which involve complex reaction networks. We develop a quantitative method of comparing the predicted desorption probability with the experiments. This method is also applied to the theories proposed so far. It is shown that each of them reproduces the experiments with similar precision, although the amount of systematic experimental data that give definite desorption probability are limited at present. We point out the importance of clarifying the nature of the substrate used in the experiment, in particular, its thermal diffusivity. We show a way to estimate the substrate properties from systematic desorption experiments without their direct measurements.

Water is one of the most abundant molecules in the form of solid ice phase in the different regions of the interstellar medium (ISM). This large abundance cannot be properly explained by using only traditional low-temperature gas-phase reactions. Thus, surface chemical reactions are believed to be major synthetic channels for the formation of interstellar water ice. Among the different proposals, hydrogenation of atomic O (i.e. 2H + O ¿ H2O) is a chemically `simple¿ and plausible reaction toward water formation occurring on the surfaces of interstellar grains. Here, novel theoretical results concerning the formation of water adopting this mechanism on the crystalline (010) Mg2SiO4 surface (a unequivocally identified interstellar silicate) are presented. The investigated reaction aims to simulate the formation of the first water ice layer covering the silicate core of dust grains. Adsorption of the atomic O as a first step of the reaction has been computed, results indicating that a peroxo (O2¿2¿) group is formed. The following steps involve the adsorption, diffusion, and reaction of two successive H atoms with the adsorbed O atom. Results indicate that H diffusion on the surface has barriers of 4¿6 kcal mol¿1, while actual formation of OH and H2O present energy barriers of 22¿23 kcal mol¿1. Kinetic study results show that tunneling is crucial for the occurrence of the reactions and that formation of OH and H2O are the bottlenecks of the overall process. Several astrophysical implications derived from the theoretical results are provided as concluding remarks.

Anions are very reactive species that can play important roles in the chemical evolution of the interstellar medium. Following the detection of CH carbon chains, investigations on detectable anions led to the discovery of five new species during the last decade. The CH anion was first observed in the circumstellar envelope of IRC + 10216 and in TMC-1. In these cool and low-density regions, an accurate modelling of the chemical and physical conditions of the observed emission lines requires the knowledge of both radiative and collisional excitation rates. We present here the first interaction potential energy surface and inelastic rate coefficients for CH in collision with para-H(j = 0). The ab initio interaction energies were computed using highly correlated coupled cluster procedures. Quantum scattering calculations were performed with the ab initio potential energy surface. The close-coupling approach was used to compute rotational excitation cross-sections of CH for the first 21 j rotational levels (up to j = 20) and for collisional energies up to 500 cm. State-to-state rate coefficients were obtained for the temperature range 2-100 K.

We present a detailed simulation of a dust grain covered by a decamer of (CHOH)-ice-mantle, bombarded by an OH closed-shell molecule with kinetic energies from 10-22 eV. The chemical pathways are studied through Born-Oppenheimer (ab initio) molecular dynamics. The simulations show that methanol ice-mantles can be a key generator of complex organic molecules (COMs). We report the formation of COMs such as methylene glycol (CH(OH)) and the OCHOH radical, which have not been detected yet in the interstellar medium (ISM). We discuss the chemical formation of new species through the reaction of CHOH with the hydroxyl projectile. The dependence of the outcome on the kinetic energy of the projectile and the implications for the observation and detection of these molecules might explain why the methoxy radical (CH·) has been observed in a wider range of astrophysical environments than the hydroxymethyl (CH·) isomer. Because of the projectile kinetic energies required for these reactions to occur, we suggest that these processes are likely relevant in the production of COMs in photodissociation and shock regions produced by high-velocity jets and outflows from young stellar objects.