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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., Preprint of the original submitted version more...

The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C$_{16}$H$_{10}$$^{.+}$; Py$^+$ and C$_{16}$H$_{8}$$^{.+}$; ddPy$^+$) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py$^+$ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy$^+$ is dominated by absorption features from two isomers (4,5-ddPy$^+$ and 1,2-ddPy$^+$) with, at most, minor contributions from other isomers. These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results suggest that this process favours the loss of adjacent hydrogen atoms., Comment: 13 pages, 7 figures, supplementary information included more...

Context. Infrared spectroscopy of star and planet forming regions is at the dawn of a new age with the upcoming James Webb Space Telescope. In support of these observations, laboratory spectra are required to identify complex organic molecules in the ices that cover the dust grains in these regions. Aims. This study aims to provide reference spectra to firmly detect icy methyl formate in the different stages of star and planet forming regions. Methyl formate is mixed in astronomically relevant matrices, and the peak positions, FWHMs, and relative band intensities are characterized for different temperatures to provide an analytical tool for astronomers. Methods. Methyl formate is deposited at 15 K under high-vacuum conditions. Specifically, methyl formate is deposited pure and mixed with CO, H$_2$CO, CH$_3$OH, H$_2$O, and CO:H$_2$CO:CH$_3$OH combined. Throughout the experiment infrared spectra are acquired with a FTIR spectrometer in the range from 4000-500 cm$^{-1}$ (2.5-20 $\mu$m) at a spectral resolution of 0.5 cm$^{-1}$. Results. We present the characterization of five solid-state methyl formate vibrational modes in pure and astronomically relevant ice matrices. The five selected vibrational modes, namely the C=O stretch, C$-$O stretch, CH$_3$ rocking, O$-$CH$_3$ stretching, and OCO deformation, are best suited for a JWST identification of methyl formate. For each of these vibrational modes, and each of the mixtures the TvS heatmaps, peak position versus FWHM, and relative band intensities are given. Additionally, the acquired reference spectra of methyl formate are compared with Spitzer observations of HH 46. A tentative detection of methyl formate provides an upper limit to the column density of $1.7\times10^{17}$ cm$^{-2}$, corresponding to an upper limit relative to water of $\leq 2.2\%$ and $\leq 40\%$ with respect to methanol., Comment: Accepted for publication in A&A more...

The simultaneous detection of organic molecules of the form C$_2$H$_{\text{n}}$O, such as ketene (CH$_2$CO), acetaldehyde (CH$_3$CHO), and ethanol (CH$_3$CH$_2$OH), toward early star-forming regions offers hints of 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 C$_2$H$_2$ ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH$_2$CHOH) along the H$_2$O formation sequence on grain surfaces. In this work, the non-energetic formation scheme is extended with laboratory measurements focusing on the energetic counterpart, induced by cosmic rays penetrating the H$_2$O-rich ice mantle. The focus here is on the H$^+$ radiolysis of interstellar C$_2$H$_2$:H$_2$O ice analogs at 17 K. Ultra-high vacuum experiments were performed to investigate the 200 keV H$^+$ radiolysis chemistry of predeposited C$_2$H$_2$:H$_2$O 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 (IR) spectral assignments are further confirmed in isotope labeling experiments using H$_2$$^{18}$O. The energetic processing of C$_2$H$_2$:H$_2$O ice not only results in the formation of (semi-) saturated hydrocarbons (C$_2$H$_4$ and C$_2$H$_6$) and polyynes as well as cumulenes (C$_4$H$_2$ and C$_4$H$_4$), 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., Comment: 14 pages, 7 figures, 2 tables more...

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. Acknowledgements. M.B. and H.L. acknowledge the European Union (EU) and Horizon 2020 funding awarded under the Marie Skłodowska-Curie action to the EUROPAH consortium (grant number 722346) as well as NOVA 5 funding. Additional funding has been realized through a NWO-VICI grant. This work has been supported by the Danish National Research Foundation through the Center of Excellence “InterCat” (Grant agreement no.: DNRF150). We thank N. F. W. Ligterink, A. G. G. M. Tielens, J. Terwischa van Scheltinga, J. Bouwman and T. Lamberts for helpful discussions. more...

Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra Solar Systems. In the Solar System, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between Solar System comets and exocomets. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. Exocomets likely vary in their composition depending on their formation environment like Solar System comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of Solar System comets. Observations of gas around main sequence stars, spectroscopic observations of "polluted" white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with Solar System comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of Solar System comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and Solar System comets., Comment: 25 pages, 3 figures. To be published in PASP. This paper is the product of a workshop at the Lorentz Centre in Leiden, the Netherlands more...

Context. Young stellar objects (YSOs) and their environments are generally geometrically and dynamically challenging to model, and the corresponding chemistry is often dominated by regions in non-thermal equilibrium. In addition, modern astrochemical models have to consider not only gas-phase reactions, but also solid-state reactions on icy dust grains. Solving the geometrical, physical, and chemical boundary conditions simultaneously requires a high computational effort and still runs the risk of false predictions due to the intrinsically non-linear effects that can occur. As a first step, solving problems of reduced complexity is helpful to guide more sophisticated approaches. Aims. The objective of this work is to test a model that uses shell-like structures (i.e., assuming a power-law number density and temperature gradient of the environment surrounding the YSO) to approximate the geometry and physical structure of YSOs, that in turn utilizes an advanced chemical model that includes gas-phase and solid-state reactions to model the chemical abundances of key species. A special focus is set on formaldehyde (H2CO) and methanol (CH3OH) as these molecules can be traced in the gas phase but are produced on icy dust grains. Furthermore, this kind of molecule is believed to be key to understanding the abundance of more complex species. We compare the influence of the geometry of the object on the molecular abundances with the effect induced by its chemistry. Methods. We set up a model that combines a grain-gas phase chemical model with a physical model of YSOs. The model ignores jets, shocks, and external radiation fields and concentrates on the physical conditions of spherically symmetric YSOs with a density and temperature gradient derived from available spectral energy distribution observations in the infrared. In addition, new observational data are presented using the APEX 12 m and the IRAM 30 m telescopes. Formaldehyde and methanol transitions have been searched for in three YSOs (R CrA-IRS 5A, C1333-IRAS 2A, and L1551-IRS 5) that can be categorized as Class 0 and Class 1 objects, and in the pre-stellar core L1544. The observed abundances of H2CO and CH3OH are compared with those calculated by the spherical physical-chemical model. Results. Compared to a standard “ρ and T constant” model, i.e., a homogeneous (flat) density and temperature distribution, using number density and temperature gradients results in reduced abundances for the CO hydrogenation products formaldehyde and methanol. However, this geometric effect is generally not large, and depends on the source and on the molecular species under investigation. Although the current model uses simplified geometric assumptions the observed abundances of H2CO and CH3OH are well reproduced for the quiescent Class 1 object R CrA-IRS 5A. Our model tends to overestimate formaldehyde and methanol abundances for sources in early evolutionary stages, like the pre-stellar core L1544 or NGC 1333-IRS 2A (Class 0). Observational results on hydrogen peroxide and water that have also been predicted by our model are discussed elsewhere. more...

Complex organic molecules (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on solar system scales in the Class 0/I stage is relevant. ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained of seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using 'LTE' models, the excitation temperature and column densities are determined for several O-bearing COMs. B1-c, B1-bS, and Serpens S68N show COM emission, i.e, three out of the seven sources. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs with respect to CH3OH are remarkably similar for the three COM-rich sources, and to IRAS 16293-2422B and HH 212. For other COMs the abundances differ by up to an order of magnitude, indicating that local source conditions are case determining. B1-c hosts a cold ($T_{ex}\approx60$ K), more extended component of COM emission with a column density of typically a few % of the warm/hot ($T_{ex}\sim 200$ K), central component. A D/H ratio of 1-3 % is derived based on the CH2DOH/CH3OH ratio suggesting a temperature of $\sim$15~K during the formation of methanol. This ratio is consistent with other low-mass protostars. Future mid-infrared facilities such as JWST/MIRI will be essential to directly observe COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow for directly linking ice and gas-phase abundances in order to constrain the routes that produce and maintain chemical complexity during the star formation process., Accepted to A&A, 44 pages, 17 figures more...

Context: In the laboratory, hydrogen peroxide (HOOH) was proven to be an intermediate product in the solid-state reaction scheme that leads to the formation of water on icy dust grains. When HOOH desorbs from the icy grains, it can be detected in the gas phase. In combination with water detections, it may provide additional information on the water reaction network. Hydrogen peroxide has previously been found toward $\rho$ Oph A. However, further searches for this molecule in other sources failed. Hydrogen peroxide plays a fundamental role in the understanding of solid-state water formation and the overall water reservoir in young stellar objects (YSOs). Without further HOOH detections, it is difficult to assess and develop suitable chemical models that properly take into account the formation of water on icy surfaces. Aims: The objective of this work is to identify HOOH in YSOs and thereby constrain the grain surface water formation hypothesis. Methods: Using an astrochemical model based on previous work in combination with a physical model of YSOs, the sources R CrA-IRS\,5A, NGCC1333-IRAS\,2A, L1551-IRS\,5, and L1544 were identified as suitable candidates for an HOOH detection. Long integration times on the APEX 12m and IRAM 30m telescopes were applied to search for HOOH signatures in these sources. Results: None of the four sources under investigation showed convincing spectral signatures of HOOH. The upper limit for HOOH abundance based on the noise level at the frequency positions of this molecule for the source R CrA-IRS\,5A was close to the predicted value. For NGC1333-IRAS 2A, L1544, and L1551-IRS\,5, the model overestimated the hydrogen peroxide abundances., Comment: 13 pages, 4 figures more...

Infrared bands at 3.3, 6.2, 7.6, 7.8, 8.6, and 11.2 μm have been attributed to polycyclic aromatic hydrocarbons (PAHs) and are observed toward a large number of galactic and extragalactic sources. Some interstellar PAHs possibly contain five-membered rings in their honeycomb carbon structure. The inclusion of such pentagon defects can occur during PAH formation, or as large PAHs are eroded by photo-dissociation to ultimately yield fullerenes. Pentagon formation is a process that is associated with the bowling of the PAH plane, that is, the ability to identify PAH pentagons in space holds the potential to directly link PAHs to cage and fullerene structures. It has been hypothesized that infrared (IR) activity around 1100 cm−1 may be a spectral marker for interstellar pentagons. We present an experimentally measured gas-phase IR absorption spectrum of the pentagon-containing rubicene cation (C26H14•+) to investigate if this band is present. The NASA Ames PAH IR Spectroscopic Database is scrutinized to see whether other rubicene-like species show IR activity in this wavelength range. We find that a specific molecular characteristic is responsible for this IR band. Namely, the vibrational motion attributed to this IR activity involves pentagon-containing harbors. An attempt to find this specific mode in Spitzer observations is undertaken and tentative detections around 9.3 μm are made toward the reflection nebula NGC 7023 and the H II-region IRAS 12063-6259. Simulated emission spectra are used to derive upper limits for the contributions of rubicene-like pentagonal PAH species to the IR band at 6.2 μm toward these sources. more...

Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. It is thought to have partial origin from interstellar molecular clouds, which are near the beginning of the star formation cycle. Observational surveys of CH$_4$ ice towards low- and high-mass young stellar objects showed that much of the CH$_4$ is expected to be formed by the hydrogenation of C on dust grains, and that CH$_4$ ice is strongly correlated with solid H$_2$O. Yet, this has not been investigated under controlled laboratory conditions, as carbon-atom chemistry of interstellar ice analogues has not been experimentally realized. In this study, we successfully demonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus the formation of CH$_4$ ice in two separate co-deposition experiments: C + H on a 10 K surface to mimic CH$_4$ formation right before H$_2$O ice is formed on the dust grain, and C + H + H$_2$O on a 10 K surface to mimic CH$_4$ formed simultaneously with H$_2$O ice. We confirm that CH$_4$ can be formed by the reaction of atomic C and H, and that the CH$_4$ formation rate is 2 times greater when CH$_4$ is formed within a H$_2$O-rich ice. This is in agreement with the observational finding that interstellar CH$_4$ and H$_2$O form together in the polar ice phase, i.e., when C- and H-atoms simultaneously accrete with O-atoms on dust grains. For the first time, the conditions that lead to interstellar CH$_4$ (and CD$_4$) ice formation are reported, and can be incorporated into astrochemical models to further constrain CH$_4$ chemistry in the interstellar medium and in other regions where CH$_4$ is inherited. more...

Context. In cold regions of the interstellar medium with intense ultraviolet radiation fields, photodesorption has been suggested as a nonthermal desorption mechanism promoting the transition of molecules from the solid state to the gas phase. Laboratory experiments measuring photodesorption rates are crucial in attempting to explain high molecular gas phase abundances of species that are expected to form in the solid state, such as methane, methanol, and acetonitrile, and to aid astrochemical modeling. Due to the convoluted competition between photodesorption and photoconversion, it is far from trivial to derive accurate photodesorption rates. Aims. The aim of this study is to apply a new methodology to discriminate between the two processes. The method has been validated using the well-studied case of CO and extended to CH4, CH3OH, and CH3CN. Methods. Vacuum ultraviolet (VUV; photon energy of 7–10.2 eV) irradiated ices at 20 K are studied, first as a pure CH4, CH3OH, or CH3CN ice and subsequently with an Ar coating on top. The latter is transparent to the VUV photons (wavelength below 200 nm), but it quenches the photodesorption process. Comparing the laser desorption post ionization time-of-flight mass spectrometry of the ices with and without the Ar coating provides information on the different interactions of the VUV photons with the ice. Results. The newly developed experimental technique allowed for a derivation of photodesorption rates for ices at 20 K of: CO (3.1 ± 0.3)×10−3 mol. photon−1, CH4 (3.1 ± 0.5)×10−2 mol. photon−1, and upper limits for CH3OH (< 6 × 10−5 mol. photon−1) and CH3CN (< 7.4 × 10−4 mol. photon−1); in the latter case, no literature values have been reported yet. The newly introduced approach provides more insight into the photodesorption process, in particular, for commonly observed complex organic molecules (COMs). Photoconversion cross sections are presented in the 7–10.2 eV range. The possible role of photodesorption and photoconversion in the formation of interstellar COMs is discussed. more...

Molecules with an amide functional group resemble peptide bonds, the molecular bridges that connect amino acids, and may thus be relevant in processes that lead to the formation of life. In this study, the solid state formation of some of the smallest amides is investigated in the laboratory. To this end, CH$_{4}$:HNCO ice mixtures at 20 K are irradiated with far-UV photons, where the radiation is used as a tool to produce the radicals required for the formation of the amides. Products are identified and investigated with infrared spectroscopy and temperature programmed desorption mass spectrometry. The laboratory data show that NH$_{2}$CHO, CH$_{3}$NCO, NH$_{2}$C(O)NH$_{2}$, CH$_{3}$C(O)NH$_{2}$ and CH$_{3}$NH$_{2}$ can simultaneously be formed. The NH$_{2}$CO radical is found to be key in the formation of larger amides. In parallel, ALMA observations towards the low-mass protostar IRAS 16293-2422B are analysed in search of CH$_{3}$NHCHO (N-methylformamide) and CH$_{3}$C(O)NH$_{2}$ (acetamide). CH$_{3}$C(O)NH$_{2}$ is tentatively detected towards IRAS 16293-2422B at an abundance comparable with those found towards high-mass sources. The combined laboratory and observational data indicates that NH$_{2}$CHO and CH$_{3}$C(O)NH$_{2}$ are chemically linked and form in the ice mantles of interstellar dust grains. A solid-state reaction network for the formation of these amides is proposed., Comment: Accepted for publication in MNRAS more...

The first gas-phase infrared spectra of two isolated astronomically relevant and large PAH cations - diindenoperylene (DIP) and dicoronylene (DC) - in the 530$-$1800 cm$^{-1}$ (18.9$-$5.6 $\mu$m) range - are presented. Vibrational band positions are determined for comparison to the aromatic infrared bands (AIBs). The spectra are obtained via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap (QIT) using the intense and tunable radiation of the free electron laser for infrared experiments (FELIX). DIP$^{+}$ shows its main absorption peaks at 737 (13.57), 800 (12.50), 1001 (9.99), 1070 (9.35), 1115 (8.97), 1152 (8.68), 1278 (7.83), 1420 (7.04) and 1550 (6.45) cm$^{-1}$($\mu$m), in good agreement with DFT calculations that are uniformly scaled to take anharmonicities into account. DC$^+$ has its main absorption peaks at 853 (11.72), 876 (11.42), 1032 (9.69), 1168 (8.56), 1300 (7.69), 1427 (7.01) and 1566 (6.39) cm$^{-1}$($\mu$m), that also agree well with the scaled DFT results presented here. The DIP$^+$ and DC$^+$ spectra are compared with the prominent infrared features observed towards NGC 7023. This results both in matches and clear deviations. Moreover, in the 11.0$-$14.0 $\mu$m region, specific bands can be linked to CH out-of-plane (oop) bending modes of different CH edge structures in large PAHs. The molecular origin of these findings and their astronomical relevance are discussed. more...

A long standing problem in astrochemistry is the inability of many current models to account for missing sulfur content. Many relatively simple species that may be good candidates to sequester sulfur have not been measured experimentally at the high spectral resolution necessary to enable radioastronomical identification. On the basis of new laboratory data, we report searches for the rotational lines in the microwave, millimeter, and sub-millimeter regions of the sulfur-containing hydrocarbon HCCSH. This simple species would appear to be a promising candidate for detection in space owing to the large dipole moment along its $b$-inertial axis, and because the bimolecular reaction between two highly abundant astronomical fragments (CCH and SH radicals) may be rapid. An inspection of multiple line surveys from the centimeter to the far-infrared toward a range of sources from dark clouds to high-mass star-forming regions, however, resulted in non-detections. An analogous search for the lowest-energy isomer, H$_2$CCS, is presented for comparison, and also resulted in non-detections. Typical upper limits on the abundance of both species relative to hydrogen are $10^{-9}$-$10^{-10}$. We thus conclude that neither isomer is a major reservoir of interstellar sulfur in the range of environments studied. Both species may still be viable candidates for detection in other environments or at higher frequencies, providing laboratory frequencies are available., Accepted in the Astrophysical Journal more...

A number of recent experimental studies have shown that solid-state complex organic molecules (COMs) can form under conditions that are relevant to the CO freeze-out stage in dense clouds. In this work, we show that alcohols can be formed well before the CO freeze-out stage (i.e., in the H2O-rich ice phase). This joint experimental and computational investigation shows that the isomers, n- and i-propanol (H3CCH2CH2OH and H3CCHOHCH3) and n- and i-propenol (H3CCHCHOH and H3CCOHCH2), can be formed in radical-addition reactions starting from propyne (H3CCCH) + OH at the low temperature of 10 K, where H3CCCH is one of the simplest representatives of stable carbon chains already identified in the interstellar medium (ISM). The resulting average abundance ratio of 1:1 for n-propanol:i-propanol is aligned with the conclusions from the computational work that the geometric orientation of strongly interacting species is influential to the extent of which 'mechanism' is participating, and that an assortment of geometries leads to an averaged-out effect. Three isomers of propanediol are also tentatively identified in the experiments. It is also shown that propene and propane (H3CCHCH2 and H3CCH2CH3) are formed from the hydrogenation of H3CCCH. Computationally-derived activation barriers give additional insight into what types of reactions and mechanisms are more likely to occur in the laboratory and in the ISM. Our findings not only suggest that the alcohols studied here share common chemical pathways and therefore can show up simultaneously in astronomical surveys, but also that their extended counterparts that derive from polyynes containing H3C(CC)nH structures may exist in the ISM. Such larger species, like fatty alcohols, are the possible constituents of simple lipids that primitive cell membranes on the early Earth are thought to be partially composed of., Published in ACS Earth and Space Chemistry, Complex Organic Molecules (COMs) in Star-Forming Regions special issue more...

1-propanol (CH3CH2CH2OH) is a three carbon-bearing representative of primary linear alcohols that may have its origin in the cold dark cores in interstellar space. To test this, we investigated in the laboratory whether 1-propanol ice can be formed along pathways possibly relevant to the prestellar core phase. We aim to show in a two-step approach that 1-propanol can be formed through reaction steps that are expected to take place during the heavy CO freeze-out stage by adding C2H2 into the CO + H hydrogenation network via the formation of propanal (CH3CH2CHO) as an intermediate and its subsequent hydrogenation. Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) is used to identify the newly formed propanal and 1-propanol. Reflection absorption infrared spectroscopy (RAIRS) is used as a complementary diagnostic tool. The mechanisms that can contribute to the formation of solid-state propanal and 1-propanol, as well as other organic compounds, during the heavy CO freeze-out stage are constrained by both laboratory experiments and theoretical calculations. Here it is shown that recombination of HCO radicals, formed upon CO hydrogenation, with radicals formed upon C2H2 processing - H2CCH and H3CCH2 - offers possible reaction pathways to solid-state propanal and 1-propanol formation. This extends the already important role of the CO hydrogenation chain in the formation of larger COMs (complex organic molecules). The results are used to compare with ALMA observations. The resulting 1-propanol:propanal ratio concludes an upper limit of < 0:35-0:55, which is complemented by computationally-derived activation barriers in addition to the experimental results., Accepted for publication in Astronomy and Astrophysics more...

We present the first infrared (IR) gas phase spectrum of a large and astronomically relevant PAH cation (C$_{42}$H$_{18}$$^+$, HBC$^+$) and its di-cation (C$_{42}$H$_{18}$$^{2+}$, HBC$^{2+}$). The spectra are recorded via infrared multi-photon dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense infrared radiation of a free electron laser in the 530$-$1800 cm$^{-1}$ (5.6$-$18.9 $\mu$m) range. HBC$^{+}$ shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25) and 1530 (6.54) cm$^{-1}$($\mu$m), in good agreement with DFT calculations that after scaling to take the anharmonicities effect into account. HBC$^{2+}$ has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm$^{-1}$($\mu$m). Given the similarity in the cationic and di-cationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC$^+$ and HBC$^{2+}$ at $\sim$ 6.5, 7.7, 8.4 and 13.1 $\mu$m coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 $\mu$m, squarely in the range predicted by theory and previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11$-$14 $\mu$m range as a diagnostic tool for the edge topology of large PAHs in space. more...

A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-fragmentation of methyl- and methoxy-substituted derivatives of HBC cations, (OCH3)6HBC + and (CH3)4(OCH3)2HBC + . Both substituted HBC cations fragment through sequential loss of CH3CO units upon laser (595nm) irradiation, resulting in a PAH-like derivative C36H12 + and a methyl-substituted PAH derivative C44H24 + , respectively. Upon ongoing irradiation, these species further fragment. For lower laser energy C44H24 + dehydrogenates and photo-fragments through CH3 and CHCH2 unit losses; for higher laser energy isomerization takes place, yielding a regular PAH-like configuration, and both stepwise dehydrogenation and C2/C2H2 loss pathways are found. C36H12 + follows largely this latter fragmentation scheme upon irradiation. It is concluded that the photo-dissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows experimental evidence that photo-fragmentation of substituted PAHs may contribute to the formation in space of smaller species that are normally considered to form by merging atoms and molecules. more...

Context. Complex organic molecules (COMs) have been largely identified through their characteristic rotational transitions in the gas of interstellar and circumstellar regions. Although these species are formed in the icy mantles that cover dust grains, the most complex species that has been unambiguously identified in the solid-phase to date is methanol (CH3OH). With the upcoming launch of the James Webb Space Telescope (JWST), this situation may change. The higher sensitivity, spectral and spatial resolution of the JWST will allow for the probing of the chemical inventory of ices in star-forming regions. In order to identify features of solid-state molecules in astronomical spectra, laboratory infrared spectra of COMs within astronomically relevant conditions are required. This paper is part of a series of laboratory studies focusing on the infrared spectra of frozen COMs embedded in ice matrices. These reflect the environmental conditions in which COMs are thought to be found. Aims. This work is aimed at characterizing the infrared features of acetone mixed in ice matrices containing H2O, CO2, CO, CH4, and CH3OH for temperatures ranging between 15 K and 160 K. Changes in the band positions and shapes due to variations in the temperature, ice composition, and morphology are reported. This work also points out the IR features that are considered the best promising tracers when searching for interstellar acetone-containing ices. Methods. Acetone-containing ices were grown at 15 K under high-vacuum conditions and infrared (IR) spectra (500–4000 cm−1/20–2.5 μm, 0.5 cm−1 resolution) in transmission mode were recorded using a Fourier transform infrared spectrometer. Spectra of the ices at higher temperatures are acquired during the heating of the sample (at a rate of 25 K h−1) up to 160 K. The changes in the infrared features for varying conditions were analyzed. Results. A large set of IR spectra of acetone-containing ices is presented and made available as a basis for interpreting current and future infrared astronomical spectra. The peak position and full width at half maximum of selected acetone bands have been measured for different ice mixtures and temperatures. The bands that are best suitable for acetone identification in astronomical spectra are: the C=O stretch mode, around 1710.3 cm−1 (5.847 μm), that lies in the 1715–1695 cm−1 (5.83–5.90 μm) range in the mixed ices; the CH3 symmetric deformation, around 1363.4 cm−1 (7.335 μm) that lies in the 1353–1373 cm−1 (7.28–7.39 μm) range in the mixed ices; and the CCC asymmetric stretch, around 1228.4 cm−1 (8.141 μm), that lies in the 1224–1245 cm−1 (8.16–8.03 μm) range in the mixed ices. The CCC asymmetric stretch band also exhibits potential as a remote probe of the ice temperature and composition; this feature is the superposition of two components that respond differently to temperature and the presence of CH3OH. All the spectra are available through the Leiden Ice Database. more...

We report cosmic ray ionization rates toward ten reddened stars studied within the framework of the EDIBLES (ESO Diffuse Interstellar Bands Large Exploration Survey) program, using the VLT-UVES. For each sightline, between two and ten individual rotational lines of OH+ have been detected in its (0,0) and (1,0) A3Π − X3Σ− electronic band system. This allows constraining of OH+ column densities toward different objects. Results are also presented for 28 additional sightlines for which only one or rather weak signals are found. An analysis of these data makes it possible to derive the primary cosmic ray ionization rate ζp in the targeted diffuse interstellar clouds. For the ten selected targets, we obtain a range of values for ζp equal to (3.9–16.4) × 10−16 s−1. These values are higher than the numbers derived in previous detections of interstellar OH+ in the far-infrared/submillimeter-wave regions and in other near-ultraviolet studies. This difference is a result of using new OH+ oscillator strength values and a more complete picture of all relevant OH+ formation and destruction routes (including the effect of proton recombinations on PAHs), and the relatively high N(OH+) seen toward those ten targets. more...

Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C60+) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identification of interstellar C60+ requires a match between the wavelengths and the expected strengths of all absorption features detectable in the laboratory and in space. Here we present Hubble Space Telescope (HST) spectra of the region covering the C60+ 9348, 9365, 9428 and 9577 {\AA} absorption bands toward seven heavily-reddened stars. We focus in particular on searching for the weaker laboratory C60+ bands, the very presence of which has been a matter for recent debate. Using the novel STIS-scanning technique to obtain ultra-high signal-to-noise spectra without contamination from telluric absorption that afflicted previous ground-based observations, we obtained reliable detections of the (weak) 9365, 9428 {\AA} and (strong) 9577 {\AA} C60+ bands. The band wavelengths and strength ratios are sufficiently similar to those determined in the latest laboratory experiments that we consider this the first robust identification of the 9428 {\AA} band, and a conclusive confirmation of interstellar C60+., Comment: Published in ApJ Letters, April 2019, 875, L28 more...

Formic acid (HCOOH) and carbon dioxide (CO2) are simple species that have been detected in the interstellar medium. The solid-state formation pathways of these species under experimental conditions relevant to prestellar cores are primarily based off of weak infrared transitions of the HOCO complex and usually pertain to the H2O-rich ice phase, and therefore more experimental data are desired. In this article, we present a new and additional solid-state reaction pathway that can form HCOOH and CO2 ice at 10 K 'non-energetically' in the laboratory under conditions related to the "heavy" CO freeze-out stage in dense interstellar clouds, i.e., by the hydrogenation of an H2CO:O2 ice mixture. This pathway is used to piece together the HCOOH and CO2 formation routes when H2CO or CO reacts with H and OH radicals. Temperature programmed desorption - quadrupole mass spectrometry (TPD-QMS) is used to confirm the formation and pathways of newly synthesized ice species as well as to provide information on relative molecular abundances. Reflection absorption infrared spectroscopy (RAIRS) is additionally employed to characterize reaction products and determine relative molecular abundances. We find that for the conditions investigated in conjunction with theoretical results from the literature, H+HOCO and HCO+OH lead to the formation of HCOOH ice in our experiments. Which reaction is more dominant can be determined if the H+HOCO branching ratio is more constrained by computational simulations, as the HCOOH:CO2 abundance ratio is experimentally measured to be around 1.8:1. H+HOCO is more likely than OH+CO (without HOCO formation) to form CO2. Isotope experiments presented here further validate that H+HOCO is the dominant route for HCOOH ice formation in a CO-rich CO:O2 ice mixture that is hydrogenated. These data will help in the search and positive identification of HCOOH ice in prestellar cores., Comment: Accepted for publication in Astronomy and Astrophysics more...

Emission bands from polycyclic aromatic hydrocarbons (PAHs) dominate the mid-infrared spectra of a wide variety of astronomical sources, encompassing nearly all stages of stellar evolution. Despite their similarities, details in band positions and shapes have allowed a classification of PAH emission to be developed. It has been suggested that this classification is in turn associated with the degree of photoprocessing of PAHs. Over the past decade, a more complete picture of the PAH interstellar life-cycle has emerged, in which a wide range of PAH species are formed during the later stages of stellar evolution. After this they are photoprocessed, increasing the relative abundance of the more stable (typically larger and compact) PAHs. For this work we have tested the effect of the symmetry, size, and structure of PAHs on their fragmentation pattern and infrared spectra by combining experiments at the free electron laser for infrared experiments (FELIX) and quantum chemical computations. Applying this approach to the cations of four molecular species, perylene (C20H12), peropyrene (C26H14), ovalene (C32H14) and isoviolanthrene (C34H18), we find that a reduction of molecular symmetry causes the activation of vibrational modes in the 7–9 μm range. We show that the IR characteristics of less symmetric PAHs can help explain the broad band observed in the class D spectra, which are typically associated with a low degree of photoprocessing. Such large, nonsymmetrical irregular PAHs are currently largely missing from the NASA Ames PAH database. The band positions and shapes of the largest more symmetric PAH measured here, show the best resemblance with class A and B sources, representative of regions with high radiation fields and thus heavier photoprocessing. Furthermore, the dissociation patterns observed in the mass spectra hint to an enhanced stability of the carbon skeleton in more symmetric PAHs with respect to the irregular and less symmetric species, which tend to loose carbon containing units. Although not a direct proof, these findings are fully in line with the grandPAH hypothesis, which claims that symmetric large PAHs can survive as the radiation field increases, while their less symmetric counterparts are destroyed or converted to symmetric PAHs. more...

Methanol is formed via surface reactions on icy dust grains. Methanol is also detected in the gas-phase at temperatures below its thermal desorption temperature and at levels higher than can be explained by pure gas-phase chemistry. The process that controls the transition from solid state to gas-phase methanol in cold environments is not understood. The goal of this work is to investigate whether thermal CO desorption provides an indirect pathway for methanol to co-desorb at low temperatures. Mixed CH$_{3}$OH:CO/CH$_{4}$ ices were heated under UHV (ultra-high vacuum) conditions and ice contents are traced using RAIRS (reflection absorption IR spectroscopy), while desorbing species were detected mass spectrometrically. An updated gas-grain chemical network was used to test the impact of the results of these experiments. The physical model used is applicable for TW Hya, a protoplanetary disk in which cold gas-phase methanol has recently been detected. Methanol release together with thermal CO desorption is found to be an ineffective process in the experiments, resulting in an upper limit of $\leq$ 7.3 $\times$ 10$^{-7}$ CH3OH molecules per CO molecule over all ice mixtures considered. Chemical modelling based on the upper limits shows that co-desorption rates as low as 10$^{-6}$ CH$_{3}$OH molecules per CO molecule are high enough to release substantial amounts of methanol to the gas-phase at and around the location of the CO thermal desorption front in a protoplanetary disk. The impact of thermal co-desorption of CH3OH with CO as a grain-gas bridge mechanism is compared with that of UV induced photodesorption and chemisorption., Accepted for publication in Astronomy and Astrophysics. 10 pages, 8 figures more...

The formation of methanol (CH3OH) on icy grain mantles during the star formation cycle is mainly associated with the CO freeze-out stage. Yet there are reasons to believe that CH3OH also can form at an earlier period of interstellar ice evolution in CO-poor and H2O-rich ices. This work focuses on CH3OH formation in a H2O-rich interstellar ice environment following the OH-mediated H-abstraction in the reaction, CH4 + OH. Experimental conditions are systematically varied to constrain the CH3OH formation yield at astronomically relevant temperatures. CH4, O2, and hydrogen atoms are co-deposited in an ultrahigh vacuum chamber at 10-20 K. OH radicals are generated by the H + O2 surface reaction. Temperature programmed desorption - quadrupole mass spectrometry (TPD-QMS) is used to characterize CH3OH formation, and is complemented with reflection absorption infrared spectroscopy (RAIRS) for CH3OH characterization and quantitation. CH3OH formation is shown to be possible by the sequential surface reaction chain, CH4 + OH -> CH3 + H2O and CH3 + OH -> CH3OH at 10-20 K. This reaction is enhanced by tunneling, as noted in a recent theoretical investigation (Lamberts et al. 2017). The CH3OH formation yield via the CH4 + OH route versus the CO + H route is approximately 20 times smaller for the laboratory settings studied. The astronomical relevance of the new formation channel investigated here is discussed., Accepted for publication in Astronomy and Astrophysics more...

Context. In dense clouds, hydrogenation reactions on icy dust grains are key in the formation of molecules, like formaldehyde, methanol, and complex organic molecules (COMs). These species form through the sequential hydrogenation of CO ice. Although molecular hydrogen (H2) abundances can be four orders of magnitude higher than those of free H-atoms in dense clouds, H2 surface chemistry has been largely ignored; several laboratory studies show that H2 does not actively participate in non-energetic ice chemistry because of the high activation energies required. Aims. For the example of CO ice hydrogenation, we experimentally investigated the potential role of H2 molecules on the surface chemistry when energetic processing (i.e., UV photolysis) is involved. We test whether additional hydrogenation pathways become available upon UV irradiation of a CO:H2 ice mixture and whether this reaction mechanism also applies to other chemical systems. Methods. Ultra-high vacuum (UHV) experiments were performed at 8~20 K. A pre-deposited solid mixture of CO:H2 was irradiated with UV-photons. Reflection absorption infrared spectroscopy (RAIRS) was used as an in situ diagnostic tool. Single reaction steps and possible isotopic effects were studied by comparing results from CO:H2 and CO:D2 ice mixtures. Results. After UV-irradiation of a CO:H2 ice mixture, two photon-induced products, HCO and H2CO, are unambiguously detected. The proposed reaction mechanism involves electronically excited CO in the following reaction steps: CO + h��->CO*, CO* + H2->HCO + H where newly formed H-atoms are then available for further hydrogenation reactions. The HCO formation yields have a strong temperature dependence for the investigated regime, which is most likely linked to the H2 sticking coefficient. Finally, the astronomical relevance of this photo-induced reaction channel is discussed., 8 pages, 6 figures more...

Gas phase spectroscopic laboratory experiments for the buckminsterfullerene cation C60+ have resulted in accurate rest wavelengths for five C60+ transitions that have been compared with diffuse interstellar bands (DIBs) in the near infra-red. Detecting these in astronomical spectra is difficult because of the strong contamination of ground-based spectra by atmospheric water vapor, to the presence of weak and shallow stellar lines and/or blending with other weak DIBs. The detection of the two strong bands has been claimed by several teams, and the three additional and weaker bands have been detected in a few sources. Certain recent papers have argued against the identification of C60+ based on spectral analyses claiming (i) a large variation in the ratio of the equivalent widths of the 9632 and 9577 Å bands, (ii) a large redshift of the 9632 Å band for the Orion star HD 37022, and (iii) the non-detection of the weaker 9428 Å DIB. Here we address these three points: (i) We show that the model stellar line correction for the 9632 Å DIB overestimates the difference between the strengths of the lines in giant and dwarf star spectra, casting doubts on the conclusions about the ratio variability. (ii) Using high quality stellar spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES), recorded with the ESO/Paranal Ultraviolet Echelle Spectrograph (UVES) in about the same atmospheric conditions, we find no wavelength shift in the 9632 Å band toward HD 37022. (iii) Using EDIBLES spectra and data from the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at CFHT we show that the presence of a weak 9428 Å band cannot be ruled out, even in the same observations that a previous study claimed it was not present. more...

We present the first results of a pilot program to conduct an ALMA Band 10 spectral line survey of the high-mass star-forming region NGC 6334I. The observations were taken in exceptional weather conditions (0.19 mm precipitable water) with typical system temperatures $T_{\rm{sys}}$ $, Comment: Accepted in ApJ Letters more...

The full cubic and semidiagonal quartic force fields of acetylene (C2H2), diacetylene (C4H2), triacetylene (C6H2), and tetraacetylene (C8H2) are determined using CCSD(T) (coupled cluster theory with single and double excitations and augmented by a perturbative treatment of triple excitations) in combination with the atomic natural orbital (ANO) basis sets. Application of second-order vibrational perturbation theory (VPT2) results in vibrational frequencies that agree well with the known fundamental and combination band experimental frequencies of acetylene, diacetylene, and triacetylene (average discrepancies are less than 10 cm−1). Furthermore, the predicted ground state rotational constants (B0) and vibration–rotation interaction constants (αi) are shown to be consistent with known experimental values. New vibrational frequencies and rotational parameters from the presented theoretical predictions are given for triacetylene and tetraacetylene, which can be used to aid laboratory and astronomical spectroscopic searches for characteristic transitions of these molecules. more...

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C60+ fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral-type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ~ 70000 -- 100000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305--1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided., 29 pages (including 14 pages appendix), 15 figures, accepted by A&A more...

In 2015, Campbell et al. presented spectroscopic laboratory gas phase data for the fullerene cation, ${{{\rm{C}}}_{60}}^{+}$, that coincide with the reported astronomical spectra of two diffuse interstellar band (DIB) features at 9633 and 9578 Å. In the following year, additional laboratory spectra were linked to three other and weaker DIBs at 9428, 9366, and 9349 Å. The laboratory data were obtained using wavelength-dependent photodissociation spectroscopy of small (up to three) He-tagged ${{{\rm{C}}}_{60}}^{+}\mbox{--}{\mathrm{He}}_{n}$ ion complexes, yielding rest wavelengths for the bare ${{{\rm{C}}}_{60}}^{+}$ cation by correcting for the He-induced wavelength shifts. Here we present an alternative approach to derive the rest wavelengths of the four most prominent ${{{\rm{C}}}_{60}}^{+}$ absorption features, using high-resolution laser dissociation spectroscopy of ${{{\rm{C}}}_{60}}^{+}$ embedded in ultracold He droplets. Accurate wavelengths of the bare fullerene cation are derived from linear wavelength shifts recorded for ${\mathrm{He}}_{n}$ ${{{\rm{C}}}_{60}}^{+}$ species with n up to 32. A careful analysis of all of the available data results in precise rest wavelengths (in air) for the four most prominent ${{{\rm{C}}}_{60}}^{+}$ bands: 9631.9(1) Å, 9576.7(1) Å, 9427.5(1) Å, and 9364.9(1) Å. The corresponding bandwidths have been derived and the relative band intensity ratios are discussed. more...

The surface formation of NH3 and its deuterated isotopologues – NH_2D, NHD_2, and ND_3 – is investigated at low temperatures through the simultaneous addition of hydrogen and deuterium atoms to nitrogen atoms in CO-rich interstellar ice analogues. The formation of all four ammonia isotopologues is only observed up to 15 K, and drops below the detection limit for higher temperatures. Differences between hydrogenation and deuteration yields result in a clear deviation from a statistical distribution in favour of deuterium enriched species. The data analysis suggests that this is due to a higher sticking probability of D atoms to the cold surface, a property that may generally apply to molecules that are formed in low temperature surface reactions. The results found here are used to interpret ammonia–deuterium fractionation as observed in pre-protostellar cores. more...

Solid state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed. more...

We have studied the photo-fragmentation of the HBC cation –C 42 H 18 + – a large PAH cation of potential astrophysical interest. HBC cation photo-fragment patterns are measured upon irradiation by an unfocused Nd:YAG laser (532 nm) for different experimental conditions, using quadrupole ion trap, time-of-flight mass spectrometry. Both stepwise dehydrogenation of C 42 H 18 + and C 2 /C 2 H 2 loss pathways are identified as relevant photodissociation routes. more...

In dense inter- and circumstellar clouds, molecules are maintained in the gas phase at temperatures well below their thermal accretion value, and the UV photodesorption of ice has been proposed as an efficient non-thermal desorption mechanism explaining this observation. The present work aims at studying the UV photodesorption yield of solid O2, a molecule considered as a starting point in the formation of water. For the infrared inactive O2 the standard spectroscopic procedure does not work, and a new experimental procedure is introduced that links quadrupole mass spectrometry of photodesorbed oxygen ice in the gas phase to a RAIR spectrum of a polar reference molecule in the solid state (in our case CO). The resulting photodesorption rate of pure O2 ice is found to be (6 ± 2) × 10 −4 molecules photon −1 upon irradiation with broad-band Lyα light. The main photodesorption product is O2; a smaller part photodesorbs as O-atoms. Within the accessible range, the photodesorption rate does not vary with ice temperature. Upon UV irradiation also ozone ice is formed and the present work allows in parallel to determine the photodesorption rate for O3 as (3 ± 1) × 10 −4 and (5 ± 2) × 10 −4 molecules photon −1 for 14 and 52 K, respectively. Ozone is found not to desorb directly, but via its fragments O2 and O; UV induced dissociation is concluded to be much more efficient than direct photodesorption. more...

Inter- and circumstellar ices comprise different molecules accreted on cold dust particles. These icy dust grains provide a molecule reservoir where particles can interact and react. As the grain acts as a third body, capable of absorbing energy, icy surfaces in space have a catalytic effect. Chemical reactions are triggered by a number of possible processes; (i) irradiation by light, typically UV photons from the interstellar radiation field and Ly-α radiation emitted by excited hydrogen, but also X-rays, (ii) bombardment by particles, free atoms (most noticeably hydrogen, but also N, C, O and D-atoms), electrons, low energy ions and cosmic rays, and (iii) thermal processing. All these effects cause ices to (photo)desorb, induce fragmentation or ionization in the ice, and eventual recombination will make molecules to react and to form more and more complex species. The effects of this solid state astrochemistry are observed by astronomers; nearly 180 different molecules (not including isotopologues) have been unambiguously identified in the inter- and circumstellar medium, and the abundances of a substantial part of these species cannot be explained by gas phase reaction schemes only and must involve solid state chemistry. Icy dust grains in space experience different chemical stages. In the diffuse medium grains are barely covered by molecules, but upon gravitational collapse and darkening of the cloud, temperatures drop and dust grains start acting as micrometer sized cryopumps. More and more species accrete, until even the most volatile species are frozen. In parallel (non)energetic processing can take place, particularly during planet and star formation when radiation and particle fluxes are intense. The physical and chemical properties of ice clearly provide a snapshotroot to characterize the cosmological chemical evolution. In order to fully interpret the astronomical observations, therefore, dedicated laboratory experiments are needed that simulate dust grain formation and processing as well as ice mantle chemistry under astronomical conditions and in full control of the relevant parameters; ice morphology (i.e., structure), composition, temperature, UV and particle fluxes, etc., yielding parameters that can be used for astrochemical modeling and for comparison with the observations. This is the topic of the present manuscript. Laboratory experiments simulating the conditions in space are conducted for decades all over the world, but particularly in recent years new techniques have made it possible to study reactions involving inter- and circumstellar dust and ice analogues at an unprecedented level of detail. Whereas in the past “top-down scenarios” allowed to conclude on the importance of the solid state for the chemical enrichment of space, presently “bottom-up approaches” make it possible to fully quantify the involved reactions, and to provide information on processes at the molecular level. The recent progress in the field of “solid state laboratory astrophysics” is a consequence of the use of ultra high vacuum systems, of new radiation sources, such as synchrotrons and laser systems that allow extensions to wavelength domains that long have not been accessible, including the THz domain, and the use of highly sensitive gas phase detection techniques, explicitly applied to characterize the solid state such as fluorescence, luminescence, cavity ring-down spectroscopy and sophisticated mass spectrometric techniques. This paper presents an overview of the techniques being used in astrochemical laboratories worldwide, but it is incomplete in the sense that it summarizes the outcome of a 3-day workshop of the authors in November 2012 (at the Observatoire de Meudon in France), with several laboratories represented, but not all. The paper references earlier work, but it is incomplete with regard to latest developments of techniques used in laboratories not represented at the workshop. more...

Context. The number of identified complex organic molecules (COMs) in inter- and circumstellar gas-phase environments is steadily increasing. Recent laboratory studies show that many such species form on icy dust grains. At present only smaller molecular species have been directly identified in space in the solid state. Accurate spectroscopic laboratory data of frozen COMs, embedded in ice matrices containing ingredients related to their formation scheme, are still largely lacking. Aims. This work provides infrared reference spectra of acetaldehyde (CH$_3$CHO), ethanol (CH$_3$CH$_2$OH), and dimethyl ether (CH$_3$OCH$_3$) recorded in a variety of ice environments and for astronomically relevant temperatures, as needed to guide or interpret astronomical observations, specifically for upcoming James Webb Space Telescope observations. Methods. Fourier transform transmission spectroscopy (500-4000 cm$^{-1}$ / 20-2.5 $\mu$m, 1.0 cm$^{-1}$ resolution) was used to investigate solid acetaldehyde, ethanol and dimethyl ether, pure or mixed with water, CO, methanol, or CO:methanol. These species were deposited on a cryogenically cooled infrared transmissive window at 15~K. A heating ramp was applied, during which IR spectra were recorded until all ice constituents were thermally desorbed. Results. We present a large number of reference spectra that can be compared with astronomical data. Accurate band positions and band widths are provided for the studied ice mixtures and temperatures. Special efforts have been put into those bands of each molecule that are best suited for identification. For acetaldehyde the 7.427 and 5.803 $\mu$m bands are recommended, for ethanol the 11.36 and 7.240 $\mu$m bands are good candidates, and for dimethyl ether bands at 9.141 and 8.011 $\mu$m can be used. All spectra are publicly available in the Leiden Database for Ice., Comment: forthcoming paper in Astronomy & Astrophysics more...

Complex organic molecules (COMs) have been identified in different environments in star- forming regions. Laboratory studies show that COMs form in the solid state, on icy grains, typically following a non-energetic (atom-addition) or energetic (UV-photon absorption) trigger. So far, such studies have been largely performed for single processes. Here, we present the first work that quantitatively investigates both the relative importance and the cumulative effect of (non-)energetic processing. We focus on astronomically relevant CO:CH3OH = 4:1 ice analogues exposed to doses relevant for the collapse stage of dense clouds. Hydrogenation experiments result in the formation of methyl formate (MF HC(O)OCH3), glycolaldehyde (GA HC(O)CH2OH) and ethylene glycol (EG H2C(OH)CH2OH) at 14 K. The absolute abundances and the abundance fractions are found to be dependent on the H-atom/CO-CH3OH molecule ratios and on the overall deposition rate. In the case that ices are exposed to UV photons only, several different COMs are found. Typically, the abundance fractions are 0.2 for MF, 0.3 for GA and 0.5 for EG as opposed to the values found in pure hydrogenation experiments without UV in which MF is largely absent: 0.0, 0.2-0.6 and 0.8-0.4, respectively. In experiments where both are applied, overall COM abundances drop to about half of those found in the pure UV irradiation experiments, but the composition fractions are very similar. This implies COM ratios can be used as a diagnostic tool to derive the processing history of an ice. Solid-state branching ratios derived here for GA and EG compare well with observations, while the MF case cannot be explained by solid-state conditions investigated here., Comment: 14 pages, 8 figures, 1 table more...

Observational studies reveal that complex organic molecules (COMs) can be found in various objects associated with different star formation stages. The identification of COMs in prestellar cores, i.e., cold environments in which thermally induced chemistry can be excluded and radiolysis is limited by cosmic rays and cosmic ray induced UV-photons, is particularly important as this stage sets up the initial chemical composition from which ultimately stars and planets evolve. Recent laboratory results demonstrate that molecules as complex as glycolaldehyde and ethylene glycol are efficiently formed on icy dust grains via non-energetic atom addition reactions between accreting H atoms and CO molecules, a process that dominates surface chemistry during the 'CO-freeze out stage' in dense cores. In the present study we demonstrate that a similar mechanism results in the formation of the biologically relevant molecule glycerol - HOCH2CH(OH)CH2OH - a three-carbon bearing sugar alcohol necessary for the formation of membranes of modern living cells and organelles. Our experimental results are fully consistent with a suggested reaction scheme in which glycerol is formed along a chain of radical-radical and radical-molecule interactions between various reactive intermediates produced upon hydrogenation of CO ice or its hydrogenation products. The tentative identification of the chemically related simple sugar glyceraldehyde - HOCH2CH(OH)CHO - is discussed as well. These new laboratory findings indicate that the proposed reaction mechanism holds much potential to form even more complex sugar alcohols and simple sugars., Accepted for publication in The Astrophysical Journal more...

Triphenylene (C18H12) is a highly symmetric polycyclic aromatic hydrocarbon (PAH) molecule with a ‘fully-benzenoid’ electronic structure. This confers a high chemical stability compared with PAHs of similar size. Although numerous infrared and UV-visible experimental spectroscopic and theoretical studies of a wide range PAHs in an astrophysical context have been conducted, triphenylene and its radical cation have received almost no attention. There exists a huge body of spectroscopic evidence for neutral and ionised PAHs in astrophysical sources, obtained principally through detection of infrared emission features that are characteristic of PAHs as a chemical class. However, it has so far not proved possible to identify spectroscopically a single isolated PAH in space, although PAHs including triphenylene have been detected mass spectrometrically in meteorites. In this work we focus on recording laboratory electronic spectra of neutral and ionised triphenylene between 220 and 780 nm, trapped in H2O ice and solid argon at 12 K. The studies are motivated by the potential for spectroscopic astronomical detection of electronic absorption spectra of PAHs in ice mantles on interstellar grains as discussed by Linnartz (2014), and were undertaken also in a cold Ar matrix to provide guidance as to whether triphenylene (particularly in its singly positively ionised form) could be a viable candidate for any of the unidentified diffuse interstellar absorption bands. Based on the argon-matrix experimental results, comparison is made with previously unpublished astronomical spectra near 400 nm which contain broad interstellar absorption features consistent with the predictions from the laboratory matrix spectra, thus providing motivation for the recording of gas-phase electronic spectra of the internally cold triphenylene cation. more...

Water is the most abundant molecule found in interstellar icy mantles. In space it is thought to be efficiently formed on the surfaces of dust grains through successive hydrogenation of O, O2 and O3. The underlying physico-chemical mechanisms have been studied experimentally in the past decade and in this paper we extend this work theoretically, using Continuous-Time Random-Walk Monte Carlo simulations to disentangle the different processes at play during hydrogenation of molecular oxygen. CTRW-MC offers a kinetic approach to compare simulated surface abundances of different species to the experimental values. For this purpose, the results of four key experiments-sequential hydrogenation as well as co-deposition experiments at 15 and 25 K-are selected that serve as a reference throughout the modeling stage. The aim is to reproduce all four experiments with a single set of parameters. Input for the simulations consists of binding energies as well as reaction barriers (activation energies). In order to understand the influence of the parameters separately, we vary a single process rate at a time. Our main findings are: (i) The key reactions for the hydrogenation route starting from O2 are H + O2, H + HO2, OH + OH, H + H2O2, H + OH. (ii) The relatively high experimental abundance of H2O2 is due to its slow destruction. (iii) The large consumption of O2 at a temperature of 25 K is due to a high hydrogen diffusion rate. (iv) The diffusion of radicals plays an important role in the full reaction network. The resulting set of 'best fit' parameters is presented and discussed for use in future astrochemical modeling. more...

Electronic spectra of C$_6$H are measured in the $18\,950-21\,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2\Pi$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $\nu_6$ mode associated with the C-C bond vibration and the $\nu_4$ and $\nu_3$ modes associated with C$\equiv$C triple bonds. For the two lowest $\nu_{11}$ and $\nu_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $\mu^2\Sigma$($\nu_{11}$) and both $\mu^2\Sigma$($\nu_{10}$) and $\kappa^2\Sigma$($\nu_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $\mu^2\Sigma$($\nu_9$) and $\mu^2\Sigma$($\nu_8$) levels. In the excitation region below the first non-degenerate vibration ($\nu_6$), some $^2\Pi-^{2}\Pi$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2\Pi-^{2}\Pi$ transition found above the $\nu_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2\Pi$ state of C$_6$H is derived and presented., Comment: Accepted for publication in The Journal of Physical Chemistry A (26 July 2016) more...

Infrared emission features assigned to gas phase polycyclic aromatic hydrocarbons (PAHs) are observed in space along many lines of sight. In regions where interstellar ices are present, these emissions are largely quenched. It is here that PAHs form agglomerates covered by ice or freeze out on to dust grains, together with volatile species such as H2O, CO, CO2 and NH3. Upon exposure to the Lyα-dominated interstellar radiation field, PAHs are expected to participate in photo-induced chemical reactions, explicitly also involving the surrounding ice matrix. In this paper, a systematic laboratory-based study is presented for the solid-state behaviour of the PAHs pyrene and benzo[ghi]perylene upon Lyα irradiation in ammonia and mixed NH3:H2O astronomical ice analogues. The results are compared to recently published work focusing on a pure water ice environment. It is found that the ice matrix acts as an ‘electronic solid-state switch’ in which the relative amount of water and ammonia determines whether positively or negatively charged PAHs form. In pure water ice, cations are generated through direct ionization, whereas in pure ammonia ice, anions form through electron donation from ammonia-related photoproducts. The solid-state process controlling this latter channel involves electron transfer, rather than acid–base type proton transfer. In the mixed ice, the resulting products depend on the mixing ratio. The astronomical consequences of these laboratory findings are discussed. more...

Ice desorption affects the evolution of the gas-phase chemistry during the protostellar stage, and also determines the ice composition of comets forming in circumstellar disks. From observations, most volatile species, including CO2 , are found in H2 O-dominated ices. In this study, the desorption of CO2 mixed in H2 O ice and the impact of ice thickness, mixture ratio and heating rate are experimentally determined. The results are used to parametrize an extended three-phase model (gas, ice surface and ice mantle) which describes ice mixture desorption using rate equations and a minimum number of free parameters. The model can be used to predict the evolution in thickness and concentration of volatile-rich H2 O ice during infall of icy grains around protostars. more...

With this paper we provide a solution to a disagreement between astronomical- and laboratorybased CO-ice spectroscopic data. In observations towards icy sources, the CO-ice stretching band comprises a prominent and broad ‘red component’ around 2136.5 cm −1 . This feature is generally attributed to solid CO mixed in a hydrogen-bonded environment like H2O, but, as far as we are aware, laboratory spectra have not been able to fully reproduce this feature. Water-containing CO ice cannot reproduce the observed band position and bandwidth without simultaneously producing a shoulder at 2152 cm −1 (4.647µm). This band, believed to originate from the interaction of dangling-OH bonds with CO, is not observed in astronomical spectra. Fraser et al. suggested that the 2152 cm −1 feature is suppressed in astronomical ices by blocking of dangling-OH bonds by other species such as CO2 and CH3OH. In the present paper, we test this hypothesis by a systematic spectroscopic study of different H2O:CO:CO2 mixtures. It is shown that even though the 2152 cm −1 band is suppressed in low-temperature spectra, the width and peak position of the red component cannot be reproduced at the same time. An ice mixture containing only CO and CH3OH, however, does reproduce the spectra at low temperatures, both in terms of peak position and width of the red component, and the 2152 cm −1 band does not appear. This indicates that CO may reside in water-poor (rather than water-rich) ice in space. The astrophysical implications are discussed. more...

The surface formation of CO 2 at low temperatures through the reaction CO + OH and direct dissociation of the resulting HO-CO complex is shown by hydrogenation of a CO:O 2 ice mixture. Such a binary ice is not fully representative for an interstellar ice, but the hydrogenation of O 2 ice produces OH radicals, which allows the investigation of the interstellar relevant CO + OH solid state reaction under fully controlled laboratory conditions. Similar recent astrophysical ice studies have focused on the investigation of isolated surface reaction schemes, starting from the hydrogenation of pure ices, like solid CO or O 2 . For such ices, no CO 2 formation is observed upon H-atom exposure. The hydrogenation of binary ice mixtures presented here allows to investigate for the first time the influence of the presence of other species in the ice on the pure ice reaction shemes. Mixtures of CO:O 2 are deposited on a substrate in an ultra high vacuum setup at low temperatures (15 and 20 K) and subsequently hydrogenated. The ice is monitored by means of Reflection Absorption InfraRed Spectroscopy (RAIRS). Results show that solid CO 2 is formed in all studied CO:O 2 mixtures under our laboratory conditions. Within the experimental uncertainties no dependency on ice temperature or composition is observed. The laboratory results show a correlation between the formation of CO 2 and H 2 O, which is consistent with the astronomical observation of solid CO 2 in water-rich environments. The results also show that the contemporary presence of CO and O 2 molecules in the ice influences the final product yields of the separate CO + H (H 2 CO, CH 3 OH) and O 2 + H (H 2 O 2 and H 2 O) channels, even though the formation rates are not significantly affected. more...

The production of formic acid (HCOOH) in cold and hot regions of the interstellar medium is not well understood. Recent gas-phase experiments and gas-grain models hint at a solid-state production process at low temperatures. Several surface reaction schemes have been proposed in the past decades, even though experimental evidence for their efficiency was largely lacking. The aim of this work is to give the first experimental evidence for an efficient solid-state reaction scheme providing a way to form HCOOH under astronomical conditions. Several surface reaction channels have been tested under fully controlled experimental conditions by using a state-of-the-art ultrahigh vacuum set-up through co-deposition of H atoms and CO:O2 mixtures with 4:1, 1:1 and 1:4 ratios. During deposition spectral changes in the ice are monitored by means of a Fourier transform infrared (FTIR) spectrometer in reflection absorption infraRed (RAIR) mode. After co-deposition a temperature programmed desorption (TPD) experiment is performed and gas-phase molecules are detected by a quadrupole mass spectrometer (QMS). Formation of HCOOH is observed at low temperatures mainly through hydrogenation of the HO–CO complex, while reactions with the HCO radical as intermediate are found to be inefficient. The HO–CO complex channel, which was previously not considered as an important HCOOH formation route, can explain the presence of HCOOH in dense cold clouds, at the beginning of the warm-up phase of a protostar, and, therefore, is likely to be astrochemically relevant. more...

PAHs constitute a major component of the interstellar medium carbon budget, locking up to 10--20% of the elemental carbon. Sequential fragmentation induced by energetic photons leads to the formation of new species, including fullerenes. However, the exact chemical routes involved in this process remain largely unexplored. In this work, we focus on the first photofragmentation steps, which involve the dehydrogenation of these molecules. For this, we consider a multidisciplinary approach, taking into account the results from experiments, DFT calculations, and modeling using dedicated Monte-Carlo simulations. By considering the simplest isomerization pathways --- i.e., hydrogen roaming along the edges of the molecule --- we are able to characterize the most likely photodissociation pathways for the molecules studied here. These comprise nine PAHs with clearly different structural properties. The formation of aliphatic-like side groups is found to be critical in the first fragmentation step and, furthermore, sets the balance of the competition between H- and H2-loss. We show that the presence of trio hydrogens, especially in combination with bay regions in small PAHs plays an important part in the experimentally established variations in the odd-to-even H-atom loss ratios. In addition, we find that, as PAH size increases, H2 formation becomes dominant, and sequential hydrogen loss only plays a marginal role. We also find disagreements between experiments and calculations for large, solo containing PAHs, which need to be accounted for. In order to match theoretical and experimental results, we have modified the energy barriers and restricted the H-hopping to tertiary atoms. The formation of H2 in large PAHs upon irradiation appears to be the dominant fragmentation channel, suggesting an efficient formation path for molecular hydrogen in PDRs., Comment: 17 pages, 13 figures, accepted for publication in A&A more...

The so-called C2-DIBs are a class of very weak bands that fall in the blue part of the optical spectrum and are associated with high column densities of the C2 molecule. DIB profile structures constrain potential molecular carriers, but their measurement requires high S/N and spectra and the use of sightlines without Doppler splitting, as typical for a single-cloud situation. Spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) conducted at the VLT (ESO/Paranal) were explored to identify single-cloud and high C2 column sightlines, extract the corresponding C2-DIBs and study their strengths and profiles, and to investigate in detail any sub-structures. The target selection was made based on profile-fitting of the Sodium doublets and the detection of C2 lines. The C2 (2-0) Phillips system was fitted using a physical model of the host cloud. C2 column densities, temperatures as well as gas densities were derived for each sightline. 18 known C2-DIBs and 8 strong non-C2 DIBs were extracted towards 8 targets, comprising 7 single-cloud and one multi-cloud line-of-sights. Correlational studies revealed a tight association of the former group with the C2 columns, whereas the non-C2 DIBs are primarily correlated with reddening. We report three new weak diffuse band candidates. We show for the first time that at least 14 C2-DIBs exhibit spectral sub-structures which are consistent with unresolved rotational branches of molecular carriers. The variability of their peak separations among the bands for a given sightline implies that their carriers are different molecules with quite different sizes. We also illustrate how profiles of the same DIB vary among targets and as a function of physical parameters and provide tables defining the sub-structures to be compared with future models and experimental results., 19 pages, 15 figures, accepted for publication in A&A more...