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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. [ABSTRACT FROM AUTHOR]

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.

Laboratory experiments play a key role in deciphering the chemistry of the interstellar medium (ISM) and the formation of complex organic molecules (COMs) relevant to life. To date, however, most studies in experimental astrochemistry have made use of a reductionist approach to experimental design in which chemical responses to variations in a single parameter are investigated while all other parameters are held constant. Although such work does afford insight into the chemistry of the ISM, it is likely that several important points (e.g., the possible influence of experimental parameter interaction) remain ambiguous. In light of this, we propose the adoption of a new “systems astrochemistry” approach for experimental studies and present the basic tenants and advantages of this approach in this perspective article. Such an approach has already been used for some time now and to great effect in the field of prebiotic chemistry, and so we anticipate that its application to experimental astrochemistry will uncover new data hitherto unknown which could aid in better linking laboratory work to observations and models.

Experiments designed to reveal the low-temperature reactivity on the surfaces of interstellar dust grains are used to probe the heterogeneous reaction between oxygen atoms and acrylonitrile (C2H3CN, H2C=CH-CN). The reaction is studied at a series of fixed surface temperatures between 14 and 100 K. After dosing the reactants on to the surface, temperature-programmed desorption, coupled with time-of-flight mass spectrometry, reveals the formation of a product with themolecular formula C3H3NO. This product results from the addition of a single oxygen atom to the acrylonitrile reactant. The oxygen atom attack appears to occur exclusively at the C=C double bond, rather than involving the cyano(-CN) group. The absence of reactivity at the cyano site hints that full saturation of organic molecules on dust grains may not always occur in the interstellar medium. Modelling the experimental data provides a reaction probability of 0.007 ± 0.003 for a Langmuir--Hinshelwood style (diffusive) reaction mechanism. Desorption energies for acrylonitrile, oxygen atoms, and molecular oxygen, from the multilayer mixed ice their deposition forms, are also extracted from the kinetic model and are 22.7 ± 1.0 kJ mol-1 (2730 ± 120 K), 14.2 ± 1.0 kJ mol-1 (1710 ± 120 K), and 8.5 ± 0.8 kJ mol-1 (1020 ± 100 K), respectively. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and acrylonitrile could occur on interstellar dust grains on an astrophysical time-scale. [ABSTRACT FROM AUTHOR]

Studying the molecular phase of the interstellar medium in galaxies is fundamental for the understanding of the onset and evolution of compact and extended star formation, and of the growth of supermassive black holes. Molecular line emission is an excellent tracer of chemical, physical and dynamical conditions in the cold neutral gas. Key molecules in extragalactic studies are e.g. HCN, HCO+, HC3N, SiO, CH3OH, H2O. Furthermore, we can use IR excited molecular emission to probe the very inner regions of luminous infrared galaxies allowing us to get past the optically thick dust barrier of the compact obscured nuclei where lines of CO, HCN and HCO+ in their vibrational ground state (v=0) may be self-absorbed. Finally, molecular outflows and their chemistry are briefly discussed - including new ALMA results on for example the outflow of the lenticular galaxy NGC1377 and a study of the chemistry of the outflow of the quasar Mrk231. [ABSTRACT FROM AUTHOR]

Interstellar molecules with a peptide link (–NH–C(=O)–), like formamide (NH2CHO), acetamide (NH2COCH3) and isocyanic acid (HNCO), are particularly interesting for their potential role in pre-biotic chemistry. We have studied their emission in the protostellar shock regions L1157-B1 and L1157-B2, with the IRAM 30 m telescope, as part of the ASAI Large Programme. Analysis of the line profiles shows that the emission arises from the outflow cavities associated with B1 and B2. Molecular abundances of ≈(0.4–1.1) × 10−8 and (3.3–8.8) × 10−8 are derived for formamide and isocyanic acid, respectively, from a simple rotational diagram analysis. Conversely, NH2COCH3 was not detected down to a relative abundance of a few ≤10−10. B1 and B2 appear to be among the richest Galactic sources of HNCO and NH2CHO molecules. A tight linear correlation between their abundances is observed, suggesting that the two species are chemically related. Comparison with astrochemical models favours molecule formation on icy grain mantles, with NH2CHO generated from hydrogenation of HNCO. [ABSTRACT FROM PUBLISHER]

Amorphous, astrophysically relevant silicates were prepared by laser ablation of siliceous targets and subsequent quenching of the evaporated atoms and clusters in a helium/oxygen gas atmosphere. The described gas-phase condensation method can be used to synthesize homogeneous and astrophysically relevant silicates with different compositions ranging from nonstoichiometric magnesium iron silicates to pyroxene- and olivine-type stoichiometry. Analytical tools have been used to characterize the morphology, composition, and spectral properties of the condensates. The nanometer-sized silicate condensates represent a new family of cosmic dust analogs that can generally be used for laboratory studies of cosmic processes related to condensation, processing, and destruction of cosmic dust in different astrophysical environments. The well-characterized silicates comprising amorphous Mg2SiO4 and Fe2SiO4, as well as the corresponding crystalline silicates forsterite and fayalite, produced by thermal annealing of the amorphous condensates, have been used as real grain surfaces for H2 formation experiments. A specifically developed ultra-high vacuum apparatus has been used for the investigation of molecule formation experiments. The results of these molecular formation experiments on differently structured Mg2SiO4 and Fe2SiO4 described in this paper will be the topic of the next paper of this series. [ABSTRACT FROM AUTHOR]

Context. Planet formation is closely related to the structure and dispersal of protoplanetary disks. A certain class of disks, called transition disks, exhibit cavities in dust images at scales of up to a few 10s of AU. The formation mechanism of the cavities is still unclear. The gas content of such cavities can be spatially resolved for the first time using the Atacama Large Millimeter/submillimeter Array (ALMA). Aims. We develop a new series of models to simulate the physical conditions and chemical abundances of the gas in cavities to address the question whether the gas is primarily atomic or molecular inside the dust free cavities exposed to intense UV radiation. Molecular/atomic line emission by carbon monoxide (CO), its isotopologues (13CO, C18O, C17O, and 13C18O) and related species ([C i], [C ii], and [Oi]) is predicted for comparison with ALMA and the Herschel Space Observatory. Methods. We use a thermo-chemical model, which calculates the radiative transfer both in lines and the continuum, and solves for the chemical abundances and gas temperature. The model is based on our previous work, but includes several improvements. We study the dependence of CO abundances and lines on several parameters such as gas mass in the cavity, disk mass and luminosity of the star. Results. The gas can remain in molecular form down to very low amounts of gas in the cavity (~1% of MEarth). Shielding of the stellar radiation by a dusty inner disk ("pre-transition disk") allows CO to survive down to lower gas masses in the cavity. The column densities of H2 and CO in the cavity scale almost linearly with the amount of gas in the cavity down to the mass where photodissociation becomes important. The main parameter for the CO emission from cavity is the gas mass. Other parameters such as the outer disk mass, bolometric luminosity, shape of the stellar spectrum or PAH abundance are less important. Since the CO pure rotational lines readily become optically thick, the CO isotopologues need to be observed in order to quantitatively determine the amount of gas in the cavity. Determining gas masses in the cavity from atomic lines ([Ci], [C ii], and [Oi]) is challenging. Conclusions. A wide range of gas masses in the cavity of transition disks (~4 orders of magnitude) can be probed using combined observations of CO isotopologue lines with ALMA. Measuring the gas mass in the cavity will ultimately help to distinguish between different cavity formation theories. [ABSTRACT FROM AUTHOR]

We present far-infrared (50-200 μm) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [O I], [C II], CO, OH, H2O, CH+. The most common feature is the [O I] 63 μm line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 1013 < NOH < 1016 cm-2, emitting radii 15 < r < 100AU and excitation temperatures 100 < Tex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n ⩾ 1010 cm-3) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (Tex ~ 100K) and compact (r ~ 60AU) emission in the case of HD 100546. Off-source [O I] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([O I] 63 μm, [O I] 145 μm, [CII]) are analyzed with PDR models and require high gas density (n ⩾ 105 cm-3) and high UV fluxes (Go ~ 103-107), consistent with a disk origin for the oxygen lines for most of the sources. [ABSTRACT FROM AUTHOR]

We present far-infrared (50-200 μm) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [O I], [C II], CO, OH, H2O, CH+. The most common feature is the [O I] 63 μm line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 1013 < NOH < 1016 cm-2, emitting radii 15 < r < 100AU and excitation temperatures 100 < Tex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n ⩾ 1010 cm-3) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (Tex ~ 100K) and compact (r ~ 60AU) emission in the case of HD 100546. Off-source [O I] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([O I] 63 μm, [O I] 145 μm, [CII]) are analyzed with PDR models and require high gas density (n ⩾ 105 cm-3) and high UV fluxes (Go ~ 103-107), consistent with a disk origin for the oxygen lines for most of the sources. [ABSTRACT FROM AUTHOR]

The chemical changes of high-mass star-forming regions provide a potential method for classifying their evolutionary stages and, ultimately, ages. In this study, we search for correlations between molecular abundances and the evolutionary stages of dense molecular clumps associated with high-mass star formation. We use the molecular line maps from Year 1 of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey. The survey mapped several hundred individual star-forming clumps chosen from the ATLASGAL survey to span the complete range of evolution, from prestellar to protostellar to H II regions. The evolutionary stage of each clump is classified using the Spitzer GLIMPSE/MIPSGAL mid-IR surveys. Where possible, we determine the dust temperatures and H2 column densities for each clump from Herschel/Hi-GAL continuum data. From MALT90 data, we measure the integrated intensities of the N2H+, HCO+, HCN and HNC (1-0) lines, and derive the column densities and abundances of N2H+ and HCO+. The Herschel dust temperatures increase as a function of the IR-based Spitzer evolutionary classification scheme, with the youngest clumps being the coldest, which gives confidence that this classification method provides a reliable way to assign evolutionary stages to clumps. Both N2H+ and HCO+ abundances increase as a function of evolutionary stage, whereas the N2H+ (1-0) to HCO+ (1-0) integrated intensity ratios show no discernable trend. The HCN (1-0) to HNC(1-0) integrated intensity ratios show marginal evidence of an increase as the clumps evolve. [ABSTRACT FROM AUTHOR]

ABSTRACT The form of depleted sulphur in dense clouds is still unknown. Until now, only two molecules, OCS and SO2, have been detected in interstellar ices but cannot account for the elemental abundance of sulphur observed in the diffuse medium. Chemical models suggest that solid H2S is the main form of sulphur in denser sources but observational constraints exist that greatly weaken this hypothesis. We have used the Nautilus gas-grain code in which new chemical reactions have been added, based on recent experiments of H2S ice irradiation with UV photons and high energy protons. In particular, we included the new species S n, H2S n and C2S. We found that at the low temperature observed in dense clouds, i.e. 10 K, these new molecules are not efficiently produced and our modifications of the network do not change the previous predictions. At slightly higher temperature, 20 K in less dense clouds or in the proximity of protostars, H2S abundance on the surfaces is strongly decreased in favour of the polysulphanes H2S3. Such a result can also be obtained if the diffusion barriers on the grains are less important. In the context of the life cycle of interstellar clouds and the mixing between diffuse and denser parts of the clouds, the depletion of sulphur in the form of polysulphanes or other sulphur polymers may have occurred in regions where the temperature is slightly higher than the cold inner parts of the clouds. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate systematic chemical differentiation of molecules in regions of high-mass star formation (HMSF). We observed five prominent sites of HMSF in HCN, HNC, HCO+, their isotopes, C18O, C34S and some other molecular lines, for some sources both at 3 and 1.3 mm and in continuum at 1.3 mm. Taking into account earlier obtained data for N2H+, we derive molecular abundances and physical parameters of the sources (mass, density, ionization fraction, etc.). The kinetic temperature is estimated from CH3C2H observations. Then, we analyse correlations between molecular abundances and physical parameters and discuss chemical models applicable to these species. The typical physical parameters for the sources in our sample are the following: kinetic temperature in the range ∼30–50 K (it is systematically higher than that obtained from ammonia observations and is rather close to dust temperature), masses from tens to hundreds solar masses, gas densities and ionization fraction . In most cases, the ionization fraction slightly (a few times) increases towards the embedded young stellar objects (YSOs). The observed clumps are close to gravitational equilibrium. There are systematic differences in distributions of various molecules. The abundances of CO, CS and HCN are more or less constant. There is no sign of CO and/or CS depletion as in cold cores. At the same time, the abundances of HCO+, HNC and especially N2H+ strongly vary in these objects. They anticorrelate with the ionization fraction and as a result decrease towards the embedded YSOs. For N2H+ this can be explained by dissociative recombination to be the dominant destroying process. N2H+, HCO+ and HNC are valuable indicators of massive protostars. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cationic isomers of methanimine generated via dissociative photoionization. • Isomers react with methane by H atom transfer to give protonated methanimine. • Reactive cross sections are given as a function of collision energy and photon energy. • Different cation isomers react differently and calculations help understanding. • Reactions are relevant for Titan's atmosphere. Experimental and theoretical studies are presented on the reactions of the isomeric radical cations H 2 CNH + and HCNH 2 + with CH 4. Ionic isomers were generated selectively by VUV dissociative photoionization of azetidine and cyclopropylamine precursors respectively. Both exclusively give H 2 CNH 2 + plus CH 3 as products, but differences are observed related to a competition between stripping and complex-mediated H-transfer. Astrochemical implications for Titan's atmosphere are briefly discussed, where the presence of methanimine (H 2 CNH), a key prebiotic molecule and a potential precursor for tholins, is proposed on the basis of atmospheric models and the observation of CH 2 NH 2 + ions in Cassini mass spectrometric data. [ABSTRACT FROM AUTHOR]

Irradiation of solid nitrogen at 4 K with far-ultraviolet light from a synchrotron caused excitation to the upper state of the Vegard–Kaplan (VK) system; the emission in that system was simultaneously recorded in wavelength region 200–440 nm. The lifetimes of emission lines for VK (0, 1) to (0, 12) transitions were measured in the range of 2.12 ∼ 2.65 s. The threshold wavelength to observe the VK emission was 175.0 ± 3.5 nm, corresponding to energy 7.08 ± 0.14 eV. This investigation of the generation of icy VK nitrogen enhances our understanding of its photochemistry in space. [ABSTRACT FROM AUTHOR]

CO2 ice is an important reservoir of carbon and oxygen in star- and planet-forming regions. Together with water and CO, CO2 sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO2 ice spectroscopy is a prerequisite to characterize CO2 interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO2 longitudinal optical (LO) phonon mode in pure CO2 ice and in CO2 ice mixtures with H2O, CO, and O2 components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of the James Webb Space Telescope, this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible. [ABSTRACT FROM AUTHOR]

Complex organic molecules (COMs) are known to be abundant toward some low-mass young stellar objects (YSOs), but how these detections relate to typical COM abundance are not yet understood. We aim to constrain the frequency distribution of COMs during low-mass star formation, beginning with this pilot survey of COM lines toward six embedded YSOs using the IRAM 30 m Telescope. The sample was selected from the Spitzerc2d ice sample and covers a range of ice abundances. We detect multiple COMs, including CH3CN, toward two of the YSOs, and tentatively toward a third. Abundances with respect to CH3OH vary between 0.7% and 10%. This sample is combined with previous COM observations and upper limits to obtain a frequency distributions of CH3CN, HCOOCH3, CH3OCH3, and CH3CHO. We find that for all molecules more than 50% of the sample have detections or upper limits of 1%-10% with respect to CH3OH. Moderate abundances of COMs thus appear common during the early stages of low-mass star formation. A larger sample is required, however, to quantify the COM distributions, as well as to constrain the origins of observed variations across the sample. [ABSTRACT FROM AUTHOR]

We present observational data for two main components (S255IR and S255N) of the S255 high mass star-forming region in continuum and molecular lines obtained at 1.3 mm and 1.1 mm with the Submillimeter Array (SMA), at 1.3 cm with the Very Large Array, and at 23 and 50 cm with the Giant Metrewave Radio Telescope. The angular resolution was from ∼2″ to ∼5″ for all instruments. With the SMA we detected a total of about 50 spectral lines of 20 different molecules (including isotopologues). About half of the lines and half of the species (in particular N2H+, SiO, C34S, DCN, DNC, DCO+, HC3N, H2CO, H2CS, SO2) have not been previously reported in S255IR and partly in S255N at high angular resolution. Our data reveal several new clumps in the S255IR and S255N areas through their millimeter wave continuum emission. Masses of these clumps are estimated at a few solar masses. The line widths greatly exceed expected thermal widths. These clumps have practically no association with NIR or radio continuum sources, implying a very early stage of evolution. At the same time, our SiO data indicate the presence of high-velocity outflows related to some of these clumps. In some cases, strong molecular emission at velocities of the quiescent gas has no detectable counterpart in the continuum. We discuss the main features of the distribution of NH3, N2H+, and deuterated molecules. We estimate properties of decimeter wave radio continuum sources and their relationship with the molecular material. [ABSTRACT FROM AUTHOR]

We present 2″-10″ imaging of 11 transitions from 9 molecular species across the nuclear bar in Maffei 2. The data were obtained with the BIMA and OVRO interferometers. The 10 detected transitions are compared with existing CO isotopologues, HCN, CS, and millimeter continuum data. Dramatic spatial variations among the mapped species are observed across the nuclear bar. A principal component analysis is performed to characterize correlations between the transitions, star formation, and molecular column density. The analysis reveals that HCN, HNC, HCO+, and 3 mm continuum are tightly correlated, indicating a direct connection to massive star formation. We find two main morphologically distinct chemical groups, CH3OH, SiO, and HNCO comprising the grain chemistry molecules, versus HCN, HNC, HCO+, and C2H, molecules strong in the presence of star formation. The grain chemistry molecules, HNCO, CH3OH, and SiO, trace hydrodynamical bar shocks. The near constancy of the HNCO/CH3OH, SiO/CH3OH, and SiO/HNCO ratios argues that shock properties are uniform across the nucleus. HCN/HCO+, HCN/HNC, HCN/CS, and HCN/CO ratios are explained primarily by variations in density. High HCO+/N2H+ ratios are correlated with the C2H line, suggesting that this ratio may be a powerful new dense photon-dominated region probe in external galaxies. C2H reveals a molecular outflow along the minor axis. The morphology and kinematics of the outflow are consistent with an outflow age of 6-7 Myr. [ABSTRACT FROM AUTHOR]