Dense Interstellar Clouds Research Articles

Nearby Supernova and Cloud Crossing Effects on the Orbits of Small Bodies in the Solar System

Supernova (SN) blasts envelop many surrounding stellar systems, transferring kinetic energy to small bodies in the systems. Geologic evidence from 60Fe points to recent nearby SN activity within the past several Myr. Here, we model the transfer of energy and resulting orbital changes from these SN blasts to the Oort Cloud, the Kuiper Belt, and Saturn’s Phoebe ring. For the Oort Cloud, an impulse approximation shows that a 50 pc SN can eject approximately half of all objects less than 1 cm while altering the trajectories of larger ones, depending on their orbital parameters. For stars closest to SNe, objects up to ∼100 m can be ejected. Turning to the explored solar system, we find that SNe closer than 50 pc may affect Saturn’s Phoebe ring and can sweep away Kuiper Belt dust. It is also possible that the passage of the solar system through a dense interstellar cloud could have a similar effect; a numerical trajectory simulation shows that the location of the dust grains and the direction of the wind (from an SN or interstellar cloud) has a significant impact on whether or not the grains will become unbound from their orbit in the Kuiper Belt. Overall, nearby SNe sweep micron-sized dust from the solar system, though whether the grains are ultimately cast toward the Sun or altogether ejected depends on various factors. Evidence of SN-modified dust grain trajectories may be observed by New Horizons, though further modeling efforts are required.

Earth's Mesosphere During Possible Encounters With Massive Interstellar Clouds 2 and 7 Million Years Ago

AbstractOur solar system's path has recently been shown to potentially intersect dense interstellar clouds 2 and 7 million years ago: the Local Lynx of Cold Cloud and the edge of the Local Bubble. These clouds compressed the heliosphere, directly exposing Earth to the interstellar medium. Previous studies that examined climate effects of these encounters argued for an induced ice age due to the formation of global noctilucent clouds (NLCs). Here, we revisit such studies with a modern 2D atmospheric chemistry model using parameters of global heliospheric magnetohydrodynamic models as input. We show that NLCs remain confined to polar latitudes and short seasonal lifetimes during these dense cloud crossings lasting ∼105 years. Polar mesospheric ozone becomes significantly depleted, but the total ozone column broadly increases. Furthermore, we show that the densest NLCs lessen the amount of sunlight reaching the surface instantaneously by up to 7% while halving outgoing longwave radiation.

Product-specific reaction kinetics in continuous uniform supersonic flows probed by chirped-pulse microwave spectroscopy.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30K, detecting the HC3N product, and with C2H6 at 10K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

Stability of solid-state formamide under Lyα irradiation

Context. Formamide (NH2CHO) plays a pivotal role as a crucial precursor to various prebiotic molecules, including sugars and nucleobases. To gain a deeper understanding of the chemical processes involving formamide formation in astrophysical settings, it becomes imperative to refine our comprehension through astrochemical models. These models necessitate not only the inclusion of pathways for formamide formation across diverse environments, but also the elucidation of mechanisms that lead to its degradation. Aims. The primary objective of this study is to scrutinize the influence of the underlying amorphous silicate substrate and the phase of formamide ice on the kinetics of its destruction and the resulting products upon exposure to Lyα (121.6 nm) radiation at 16 K. Methods. To achieve this, we conducted an examination of the photodestruction of NH2CHO ice, employing Fourier transform infrared spectroscopy. Results. Our findings reveal that, while the destruction rates of amorphous formamide ice remain consistent, regardless of the presence of an underlying amorphous olivine substrate, this substrate effectively reduces the formation of NH3, HNCO, and HCN within the ice following UV irradiation. On the other hand, contrary to common knowledge, crystalline formamide ice exhibits a considerably faster destruction rate, by an order of magnitude, than its amorphous counterpart under photo processing. Conclusions. In the interstellar medium, molecular ices often undergo phase changes depending on the environmental conditions. Our results indicate that crystalline formamide ice is more susceptible to rapid destruction than its amorphous counterpart, rendering it more elusive for detection within the lifetime of dense interstellar clouds. Furthermore, our findings emphasise the crucial significance of accounting for the influence of underlying dust grain surfaces in astrochemical models, as they have an effect on product formation during the degradation of molecular ices.

Dust and Ice in the Interstellar Medium

In this paper, we present an introduction to bare dust particles and grains covered by ice sheets in diffuse and dense interstellar clouds. It follows the life cycle of dust from its formation in stellar environments to its incorporation into the body of the Solar system. Dust particles play an important role in the physics and chemistry of many space environments. Regarding dust composition, two different populations are generally observed: silicate and carbonaceous dust. Energetic processing of ice layers with UV rays, X – rays or cosmic rays, after which the heat leads to the formation of complex organic molecules; some of them have prebiotic properties. Delivery of this organic fraction to the early Earth by meteorites and asteroids may have contributed to the origin of life.

Molecular growth of PANH via intermolecular Coulombic decay.

Nitrogen-bearing polycyclic aromatic hydrocarbons (PANHs) are ubiquitous in space. They are considered precursors to advanced biomolecules identified in meteorites. However, their chemical evolution into biomolecules in photodestructive astrophysical mediums remains a paradox. Here, we show that light can efficiently initiate the molecular mass growth of PANHs. Ultraviolet-photoexcited quinoline monomers, the smallest PANH, were observed to associate and intermolecular Coulombic decay between the associating monomers formed the cations of quinoline-dimer. Molecular rearrangements in the dimer cation lead to a dominant formation of cations heavier than quinoline. The enrichment of these heavier cations over all the other cations reveals the efficiency of this route for the mass growth of PANHs in space. This mechanism also leads to a highly reactive unsaturated PANH-ring via CH loss, a hitherto unknown channel in any photon-driven process. The occurrence of this efficient pathway toward complex molecules points to a rich chemistry in dense interstellar clouds.

An Ice Age JWST inventory of dense molecular cloud ices

Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and the composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, before the onset of star formation. With the exquisite sensitivity of the James Webb Space Telescope, this critical stage of ice evolution is now accessible for detailed study. Here we show initial results of the Early Release Science programme Ice Age that reveal the rich composition of these dense cloud ices. Weak ice features, including 13CO2, OCN−, 13CO, OCS and complex organic molecule functional groups, are now detected along two pre-stellar lines of sight. The 12CO2 ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2% and 19% of the bulk budgets of the key C, O, N and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice-rich environment. Using JWST, the molecules seen in planetary atmospheres can be traced back to their cold origins in ices formed in dense interstellar clouds, before the onset of star formation, revealing that chemical diversity and complexity is achieved early.

Proton and Electron Irradiations of CH4:H2O Mixed Ices

The organic chemistry occurring in interstellar environments may lead to the production of complex molecules that are relevant to the emergence of life. Therefore, in order to understand the origins of life itself, it is necessary to probe the chemistry of carbon-bearing molecules under conditions that simulate interstellar space. Several of these regions, such as dense molecular cores, are exposed to ionizing radiation in the form of galactic cosmic rays, which may act as an important driver of molecular destruction and synthesis. In this paper, we report the results of a comparative and systematic study of the irradiation of CH4:H2O ice mixtures by 1 MeV protons and 2 keV electrons at 20 K. We demonstrate that our irradiations result in the formation of a number of new products, including both simple and complex daughter molecules such as C2H6, C3H8, C2H2, CH3OH, CO, CO2, and probably also H2CO. A comparison of the different irradiation regimes has also revealed that proton irradiation resulted in a greater abundance of radiolytic daughter molecules compared to electron irradiation, despite a lower radiation dose having been administered. These results are important in the context of the radiation astrochemistry occurring within the molecular cores of dense interstellar clouds, as well as on outer Solar System objects.

Low-temperature kinetics for the N + NO reaction: experiment guides the way.

The reaction N(4S) + NO(X2Π) → O(3P) + N2(X1Σ+g) plays a pivotal role in the conversion of atomic to molecular nitrogen in dense interstellar clouds and in the atmosphere. Here we report a joint experimental and computational investigation of the N + NO reaction with the aim of providing improved constraints on its low temperature reactivity. Thermal rates were measured over the 50 to 296 K range in a continuous supersonic flow reactor coupled with pulsed laser photolysis and laser induced fluorescence for the production and detection of N(4S) atoms, respectively. With decreasing temperature, the experimentally measured reaction rate was found to monotonously increase up to a value of (6.6 ± 1.3) × 10-11 cm3 s-1 at 50 K. To confirm this finding, quasi-classical trajectory simulations were carried out on a previously validated, full-dimensional potential energy surface (PES). However, around 50 K the computed rates decreased which required re-evaluation of the reactive PES in the long-range part due to a small spurious barrier with a height of ∼40 K in the entrance channel. By exploring different correction schemes the measured thermal rates can be adequately reproduced, displaying a clear negative temperature dependence over the entire temperature range. The possible astrochemical implications of an increased reaction rate at low temperature are also discussed.

Mechanism of Prebiotic Uracil Synthesis from Urea and HC3O+ in Space.

The potential energy surface for the formation of protonated uracil (UH+) from urea and HC3O+ was explored by performing quantum chemical complete basis set-QB3 calculations. A barrierless pathway was found for the formation of UH+, which was estimated to occur in the interstellar medium (ISM) much faster than the timescale of chemical revolution of typical dense interstellar clouds. Investigation of further reactions of UH+ formed through the obtained pathway led to the conclusion that uracil could be produced on icy grain surfaces but not in the gas phase of the ISM.

Formation of periodic FRB in binary systems with eccentricity

ABSTRACT Long-term periodicity in the rate of flares is observed for two repeating sources of fast radio bursts (FRBs). In this paper, we present a hydrodynamical modelling of a massive binary consisting of a magnetar and an early-type star. We model the interaction of the pulsar wind from the magnetar with an intense stellar wind. It is shown that only during a fraction of the orbital period, radio emission can escape the system. This explains the duty cycle of the two repeating FRB sources with periodic activity. The width of the transparency window depends on the eccentricity, stellar wind properties, and the viewing angle. To describe the properties of the known sources, it is necessary to assume large eccentricities ≳0.5. We apply the maser cyclotron mechanism of the radio emission generation to model spectral properties of the sources. The produced spectrum is not wide: Δν/ν ∼ 0.2 and the typical frequency depends on the radius of the shock where the emission is generated. The shock radius changes along the orbit. This, together with changing parameters of the medium, allows us to explain the frequency drift during the phase of visibility. Frequency dependence of the degree of polarization at few GHz can be a consequence of a small-scale turbulence in the shocked stellar wind. It is much more difficult to explain huge (∼105 rad/m2) and variable value of the rotation measure observed for FRB 121102. We suggest that this can be explained if the supernova explosion, which produced the magnetar happened near a dense interstellar cloud with n ∼ 100 cm−3.

Rotational excitation of NS+ by H2 revisited: A new global potential energy surface and rate coefficients.

Due to the lack of specific collisional data, the abundance of NS+ in cold dense interstellar clouds was determined using collisional rate coefficients of CS as a substitute. To better understand the chemistry of sulfur in the interstellar medium, further abundance modeling using the actual NS+ collisional rate coefficients is needed. For this purpose, we have computed the first full 4D potential energy surface of the NS+-H2 van der Waals complex using the explicitly correlated coupled cluster approach with single, double, and non-iterative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. The potential energy surface exhibits a global minimum of 848.24 cm-1 for a planar configuration of the complex. The long-range interaction energy, described using multipolar moments, is sensitive to the orientation of H2 up to radial distances of ∼50 a0. From this new interaction potential, we derived excitation cross sections, induced by collision with ortho- and para-H2, for the 15 low-lying rotational levels of NS+ using the quantum mechanical close-coupling approach. By thermally averaging these data, we determined downward rate coefficients for temperatures up to 50K. By comparing them with the previous NS+-H2 data, we demonstrated that reduced dimensional approaches are not suited for this system. In addition, we found that the CS collisional data underestimate our results by up to an order of magnitude. The differences clearly indicate that the abundance of NS+, in cold dense clouds retrieved from observational spectra, must be reassessed using these new collisional rate coefficients.

Effect of Microhydration on the Temporary Anion States of Pyrene.

The influence of incremental hydration (≤4) on the electronic resonances of the pyrene anion is studied using two-dimensional photoelectron spectroscopy. The photoexcitation energies of the resonances do not change; therefore, from the anion’s perspective, the resonances remain the same, but from the neutral’s perspective of the electron–molecule reaction, the resonances decrease in energy by the binding energy of the water molecules. The autodetachment of the resonances shows that hydration has very little effect, showing that even the dynamics of most of the resonances are not impacted by hydration. Two specific resonances do show changes that are explained by the closing of specific autodetachment channels. The lowest-energy resonance leads to efficient electron capture as observed through thermionic emission and evaporation of water molecules (dissociative electron attachment). The implications of low-energy electron capture in dense molecular interstellar clouds are discussed.

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications.

The reaction between atomic carbon in its ground electronic state, C(3P), and nitrous oxide, N2O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(3P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the 3A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (kC(3P)+N2O (296 K) = (3.4 ± 0.3) × 10-11 cm3 s-1), reaching a value of kC(3P)+N2O (50 K) = (7.9 ± 0.8) × 10-11 cm3 s-1 at 50 K. As current astrochemical models do not include the C + N2O reaction, we tested the influence of this process on interstellar N2O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N2O abundances decrease significantly at intermediate times (103 - 105 years) when gas-phase C(3P) abundances are high.

Chemical analysis of prestellar cores in Ophiuchus yields short timescales and rapid collapse

Sun-like stars form from the contraction of cold and dense interstellar clouds. How the collapse proceeds and what the main physical processes are driving it, however, is still under debate and a final consensus on the timescale of the process has not been reached. If the contraction proceeds slowly, supported by strong magnetic fields and mediated by ambipolar diffusion, or is driven by fast collapse with gravity dominating the entire process is still an open question. One way to answer this question is to measure the age of prestellar cores through statistical methods based on observations or via reliable chemical chronometers, which should better reflect the physical conditions of the cores. Here we report Atacama Pathfinder EXperiment observations of ortho-H2D+ and para-D2H+ for six cores in the Ophiuchus complex, and we combined them with detailed three-dimensional magneto-hydrodynamical simulations including chemistry, providing a range of ages for the observed cores of up to 200 kyr. The outcome of our simulations and subsequent analysis provides a good matching with the observational results in terms of physical parameters (core masses and volume densities) and dynamical parameters such as the Mach number and the virial parameter. We show that models of fast collapse successfully reproduce the observed range of chemical abundance ratios since the timescales to reach the observed stages is comparable to the dynamical time of the cores (i.e. the free-fall time) and much shorter than the ambipolar diffusion time, measured from the electron fraction in the simulations. To confirm that this ratio can be used to distinguish between different star-formation scenarios, a larger (statistically relevant) sample of star-forming cores should be explored.

Kinetic Study of the Gas-Phase C(3P) + CH3CN Reaction at Low Temperatures: Rate Constants, H-Atom Product Yields, and Astrochemical Implications

Rate constants have been measured for the C(3P) + CH3CN reaction between 50 and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultraviolet laser-induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts, and transition states relevant to the title reaction over the 3A″ potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range of (3–4) × 10–10 cm3 s–1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in the current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.

New Bistable Solutions in Molecular Cloud Chemistry: Nitrogen and Carbon Autocatalysis

Abstract We have investigated the chemistry of dense interstellar clouds and found new bistable solutions in the nitrogen and carbon chemistries. We identify the autocatalytic processes that are present in the pure, reduced, chemical networks and find, as previously found for oxygen chemistry, that He+ plays an important role. The applicability of these results to astronomical environments is briefly discussed. The bistable solutions found for carbon chemistry occur for low densities and high ionization fractions that are not compatible with those found in cold, dense clouds. Bistability in the pure nitrogen chemistry occurs for conditions that are relevant for prestellar cores in which significant CO depletion has taken place. We conclude that several autocatalyses are embedded in gas-phase interstellar chemistry and that many more are potentially present.

Revisiting the OH + H2 → H2O + H reaction at the molecular level: the plausible catalytic role of ice in its own reconstruction

Context. In spite of the permanent damage suffered from the radiation field (cosmic rays, X-rays, and intense UV-visible radiations), interstellar grains are still covered by ices mantles whose role in interstellar chemistry is well beyond any doubt. This clearly means that the destruction of the ice cover has to be counterbalanced by efficient reconstruction mechanisms. Aims. Our goal is to determine whether the ice, which is still present after irradiation, has a catalytic role in the OH + H2 → H2O + H reaction for its own reconstruction. We focus on the three plausible reaction paths depending on the way reactants OH or H2 are adsorbed at the ice surface. Methods. Calculations were performed in both cluster and solid state approaches, using ab-initio post Hartree-Fock methods for small systems, standard density functional theory (DFT) for larger clusters, and periodic solid state DFT with specific formalisms accounting for weak interactions in systems of infinite dimensions. Results. Although the end product is the same, that is namely the reconstruction of one H2O on the subjacent ice, three different reaction paths are found depending on whether H2 reacts with adsorbed OH(ads), wether OH reacts with adsorbed H2(ads) or wether both OH(ads) and H2(ads) are adsorbed on the ice before reacting. In the first case, there is an activation barrier of ~6 kcal mol−1, requiring the tunneling effect for the reaction to proceed, which is in agreement with preceding studies. In the second case, the reaction is a barrierless process leading to the direct reconstruction of the ice. In the third case, the double adsorption increases the activation barrier due to the lowering of the starting energy. This is found regardless of the dimension of the supporting ice aggregates. Conclusions. Icy grain surfaces play a critical role for their own reconstruction in cold, dense interstellar clouds. The prevalence of tunneling over the direct mechanism should strongly depend on the temperature and local environment.

A non-energetic mechanism for glycine formation in the interstellar medium

The detection of the amino acid glycine and its amine precursor methylamine on the comet 67P/Churyumov-Gerasimenko by the Rosetta mission provides strong evidence for a cosmic origin of prebiotics on Earth. How and when such complex organic molecules form along the process of star- and planet-formation remains debated. We report the first laboratory detection of glycine formed in the solid phase through atom and radical-radical addition surface reactions under cold dense interstellar cloud conditions. Our experiments, supported by astrochemical models, suggest that glycine forms without the need for energetic irradiation, such as UV photons and cosmic rays, in interstellar water-rich ices, where it remains preserved, in a much earlier star-formation stage than previously assumed. We also confirm that solid methylamine is an important side-reaction product. A prestellar formation of glycine on ice grains provides the basis for a complex and ubiquitous prebiotic chemistry in space enriching the chemical content of planet-forming material.

Astrochemical Bistability: Autocatalysis in Oxygen Chemistry

Abstract The origin of bistable solutions in the kinetic equations describing the chemistry of dense interstellar clouds is explained as being due to the autocatalysis and feedback of oxygen nuclei from the oxygen dimer (O2). We identify four autocatalytic processes that can operate in dense molecular clouds, driven respectively by reactions of H+, He+, C+, and S+ with O2. We show that these processes can produce the bistable solutions found in previous studies, as well as the dependence on various model parameters such as the helium ionization rate, the sulfur depletion and the electron recombination rate. We also show that ion–grain neutralizations are unlikely to affect the occurrence of bistability in dense clouds. It is pointed out that many chemical models of astronomical sources should have the potential to show bistable solutions.