We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the timedependent gas-phase chemistry of 0D and 1D interstellar sources.

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reactions and rate coefficients are partially compiled from data in the literature, when available. We present in this paper kida.uva.2014, a new updated version of the kida.uva public gas-phase network first released in 2012. In addition to a description of the many specific updates, we illustrate changes in the predicted abundances of molecules for cold dense cloud conditions as compared with the results of the previous version of our network, kida.uva.2011.

Context. A 3.4 μm absorption band (around 2900 cm −1 ), assigned to aliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in the diffuse interstellar medium, but disappears or is modified in dense clouds. This spectral difference between different phases of the interstellar medium reflects the processing of dust in different environments. Cosmic ray bombardment is one of the interstellar processes that make carbonaceous dust evolve. Aims. We investigate the effects of cosmic rays on the interstellar 3.4 μm absorption band carriers. Methods. Samples of carbonaceous interstellar analogues (a-C:H and soot) were irradiated at room temperature by swift ions with energy in the MeV range (from 0.2 to 160 MeV). The dehydrogenation and chemical bonding modifications that occurred during irradiation were studied with IR spectroscopy. Results. For all samples and all ions/energies used, we observed a decrease of the aliphatic C-H absorption bands intensity with the ion fluence. This evolution agrees with a model that describes the hydrogen loss as caused by the molecular recombination of two free H atoms created by the breaking of C-H bonds by the impinging ions. The corresponding destruction cross section and asymptotic hydrogen content are obtained for each experiment and their behaviour over a large range of ion stopping powers are inferred. Using elemental abundances and energy distributions of galactic cosmic rays, we investigated the implications of these results in different astrophysical environments. The results are compared to the processing by UV photons and H atoms in different regions of the interstellar medium. Conclusions. The destruction of aliphatic C-H bonds by cosmic rays occurs in characteristic times of a few 10 8 years, and it appears that even at longer time scales, cosmic rays alone cannot explain the observed disappearance of this spectral signature in dense regions. In diffuse interstellar medium, the formation by atomic hydrogen prevails over the destruction by UV photons (destruction by cosmic rays is negligible in these regions). Only the cosmic rays can penetrate into dense clouds and process the corresponding dust. However, they are not efficient enough to completely dehydrogenate the 3.4 μm carriers during the cloud lifetime. This interstellar component should be destroyed in interfaces between diffuse and dense interstellar regions where photons still penetrate but hydrogen is in molecular form.

Context. Hydrogenated amorphous carbons (a-C:H) are a major component of the carbonaceous solids present in the interstellar medium. The production and existence of these grains is connected in particular with the balance between their photolysis, radiolysis, and hydrogenation. During grain processing, H 2 and other small organic molecules, radicals, and fragments are released into the gas phase. Aims. We perform photolytic experiments on laboratory produced interstellar a-C:H analogues to monitor and quantify the release of species and compare to relevant observations in the interstellar medium. Methods. Hydrogenated amorphous carbon analogues at low temperature are exposed to ultraviolet (UV) photons, under ultra-high vacuum conditions. The species produced are monitored using mass spectrometry and post irradiation temperature-programmed desorption. Additional experiments are performed using deuterated analogues and the species produced are unambiguously separated from background contributions. We implement the laboratory measured yields for the released species in a time dependent model to investigate the effect of the UV photon irradiation of hydrogenated amorphous carbons in a photon dominated region, and estimate the associated time scale. Results. The UV photolysis of hydrogenated amorphous carbons leads to the production of H 2 molecules and small hydrocarbons. The model shows that the photolytic evolution of a-C:Hs in photon dominated regions, such as the Horsehead Nebula, can raise the abundance of carbonaceous molecules by several orders of magnitude at intermediate visual extinctions, i.e., after the C + maximum and before the dense cloud conditions prevail where models generally show a minimum abundance for such carbonaceous species. The injection time peak ranges from a thousand to ten thousand years in the models, considering only the destruction of such grains and no re-hydrogenation. This time scale is consistent with the estimated advection front of a photon dominated region, which replenishes it with freshly exposed material.

Context. Cosmic ray ion irradiation affects the chemical composition of and triggers physical changes in interstellar ice mantles in space. One of the primary structural changes induced is the loss of porosity, and the mantles evolve toward a more compact amorphous state. Previously, ice compaction was monitored at low to moderate ion energies. The existence of a compaction threshold in stopping power has been suggested. Aims. In this article we experimentally study the effect of heavy ion irradiation at energies closer to true cosmic rays. This minimises extrapolation and allows a regime where electronic interaction always dominates to be explored, providing the ice compaction cross section over a wide range of electronic stopping power. Methods. High-energy ion irradiations provided by the GANIL accelerator, from the MeV up to the GeV range, are combined with in-situ infrared spectroscopy monitoring of ice mantles. We follow the IR spectral evolution of the ice as a function of increasing fluence (induced compaction of the initial microporous amorphous ice into a more compact amorphous phase). We use the number of OH dangling bonds of the water molecule, i.e. pending OH bonds not engaged in a hydrogen bond in the initially porous ice structure as a probe of the phase transition. These high-energy experiments are combined with lower energy experiments using light ions (H, He) from other facilities in Catania, Italy, and Washington, USA. Results. We evaluated the cross section for the disappearance of OH dangling bonds as a function of electronic stopping power. A cross-section law in a large energy range that includes data from different ice deposition setups is established. The relevant phase structuring time scale for the ice network is compared to interstellar chemical time scales using an astrophysical model. Conclusions. The presence of a threshold in compaction at low stopping power suggested in some previous works seems not to be confirmed for the high-energy cosmic rays encountered in interstellar space. Ice mantle porosity or pending bonds monitored by the OH dangling bonds is removed efficiently by cosmic rays. As a consequence, this considerably reduces the specific surface area available for surface chemical reactions.

γ -ray production cross sections for proton and α-particle interactions with 12 C, 16 O, 24 Mg, and Fe have been measured in the energy range 5-25 MeV with proton beams and 5-40 MeV with α-particle beams. Isotopically pure foils of 24 Mg and foils of natural isotopical composition of C, MgO, and Fe have been used. γ -ray angular distributions were obtained with five high-purity Ge detectors with bismuth germanate Compton shields placed at angles of 45 • to 157.5 • . Cross sections for more than 50 different γ -ray transitions were extracted, and for many of them no data have been published before. Comparison of present data with data available in the literature shows mostly good to excellent agreement. In addition to the production cross sections, high-statistics, low-background line shapes of the 4.438 MeV 12 C γ ray from inelastic scattering off 12 C and spallation of 16 O were obtained. Comparison with nuclear reaction calculations shows that these data place interesting constraints on nuclear reaction models.

Interstellar grain mantles present in dense interstellar clouds are in constant exchange with the gas phase via accretion and desorption mechanisms such as UV, X-ray photodesorption, cosmic ray induced sputtering, grain thermal fluctuations, and chemical reaction energy release. The relative importance of the various desorption mechanisms is of uttermost importance for astrophysical models to constrain the chemical evolution in such high density dense cloud regions. The sputtering yields for swift ions simulating the effects of cosmic rays are most often measured in the semi-infinite limit using thick ice targets with the determination of the effective yield per incident ion. In this experimental work we investigated the sputtering yield as a function of ice mantle thickness, exposed to Xe ions at 95MeV. The ion induced ice phase transformation and the sputtering yield were simultaneously monitored by infrared spectroscopy and mass spectrometry. The sputtering yield is constant above a characteristic ice layer thickness and then starts to decrease below this thickness. An estimate of the typical sputtering depth corresponding to this length can be evaluated by comparing the infinite thickness yield to the column density where the onset of the sputtering yield decrease occurs. In these experiments the measured characteristic desorption depth corresponds to ≈ 30 ice layers. Assuming an effective cylindrical shape for the volume of sputtered molecules, the aspect ratio is close to unity; in the semi-infinite ice film case this ratio is the diameter to height of the cylinder. This result shows that most ejected molecules arise from a rather compact volume. The measured infinite thickness sputtering yield for water ice mantles scales as the square of the ion electronic stopping power (S e , deposited energy per unit path length). Considering the experiments on insulators, we expect that the desorption depth dependence varies with S α e , where α∼1. Astrophysical models should take into account the thickness dependence constraints of these ice mantles in the interface regions when ices are close to their extinction threshold. In the very dense cloud regions, most of the water ice mantles are above this limit for the bulk of the cosmic rays.

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