Abstract
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.
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 nonenergetic 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.
The astronomical gas-phase detection of simple species and small organic molecules in cold pre-stellar cores, with abundances as high as ∼10−8-10−9 nH, contradicts the generally accepted idea that at ...10 K, such species should be fully frozen out on grain surfaces. A physical or chemical mechanism that results in a net transfer from solid-state species into the gas phase offers a possible explanation. Reactive desorption, i.e., desorption following the exothermic formation of a species, is one of the options that has been proposed. In astronomical models, the fraction of molecules desorbed through this process is handled as a free parameter, as experimental studies quantifying the impact of exothermicity on desorption efficiencies are largely lacking. In this work, we present a detailed laboratory study with the goal of deriving an upper limit for the reactive desorption efficiency of species involved in the CO-H2CO-CH3OH solid-state hydrogenation reaction chain. The limit for the overall reactive desorption fraction is derived by precisely investigating the solid-state elemental carbon budget, using reflection absorption infrared spectroscopy and the calibrated solid-state band-strength values for CO, H2CO and CH3OH. We find that for temperatures in the range of 10 to 14 K, an upper limit of 0.24 0.02 for the overall elemental carbon loss upon CO conversion into CH3OH. This corresponds with an effective reaction desorption fraction of ≤0.07 per hydrogenation step, or ≤0.02 per H-atom induced reaction, assuming that H-atom addition and abstraction reactions equally contribute to the overall reactive desorption fraction along the hydrogenation sequence. The astronomical relevance of this finding is discussed.
Context. 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. Aims. 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. Methods. 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. Results. 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, A&A, 599, A132). 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.
Context. Complex organic molecules (COMs) have been identified toward high- and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early ...stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C2HnO, such as acetaldehyde (CH3CHO), vinyl alcohol (CH2CHOH), ketene (CH2CO), and ethanol (CH3CH2OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts. Aims. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H2O ice under translucent cloud conditions (1 ≤ AV ≤ 5 and nH ~ 103 cm−3). We focus on the interactions of C2H2 with H-atoms and OH-radicals, which are produced along the H2O formation sequence on grain surfaces at 10 K. Methods. Ultra-high vacuum experiments were performed to study the surface chemistry observed during C2H2 + O2 + H codeposition, where O2 was used for the in situ generation of OH-radicals. These C2H2 experiments were extended by a set of similar experiments involving acetaldehyde (CH3CHO) – an abundant product of C2H2 + O2 + H codeposition. Reflection absorption infrared spectroscopy was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer was used as a complementary analytical tool. The IR and QMS spectral assignments were further confirmed in isotope labeling experiments using 18O2. Results. The investigated 10 K surface chemistry of C2H2 with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C2H4) and ethane (C2H6), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C2H2, acting as a molecular backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides so far an experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed.
Context. 1-propanol (CH3CH2CH2OH) is a three carbon-bearing representative of the 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. Aims. 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. Methods. Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) was used to identify the newly formed propanal and 1-propanol. Reflection absorption infrared spectroscopy (RAIRS) was 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. Results. Here it is shown that recombination of HCO radicals formed upon CO hydrogenation with radicals formed via 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 to the formation of larger complex organic molecules. The results are compared 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.
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 + hν→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. Moreover, the derived formation cross section reflects a cumulative reaction rate that mainly determined by both the H-atom diffusion rate and initial concentration of H2 at 8–20 K and that is largely determined by the H2 sticking coefficient. Finally, the astronomical relevance of this photo-induced reaction channel is discussed.
Context. Complex organic molecules (COMs) have been detected ubiquitously in protostellar systems. However, at shorter wavelengths (~0.8 mm), it is generally more difficult to detect larger molecules ...than at longer wavelengths (~3 mm) because of the increase in millimeter dust opacity, line confusion, and unfavorable partition function. Aims. We aim to search for large molecules (more than eight atoms) in the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 spectrum of IRAS 16293-2422 B. In particular, the goal is to quantify the usability of ALMA Band 3 for molecular line surveys in comparison to similar studies at shorter wavelengths. Methods. We used deep ALMA Band 3 observations of IRAS 16293-2422 B to search for more than 70 molecules and identified as many lines as possible in the spectrum. The spectral settings were set to specifically target three-carbon species such as i- and n-propanol and glycerol, the next step after glycolaldehyde and ethylene glycol in the hydrogenation of CO. We then derived the column densities and excitation temperatures of the detected species and compared the ratios with respect to methanol between Band 3 (~3 mm) and Band 7 (~1 mm, Protostellar Interferometric Line Survey) observations of this source to examine the effect of the dust optical depth. Results. We identified lines of 31 molecules including many oxygen-bearing COMs such as CH 3 OH, CH 2 OHCHO, CH 3 CH 2 OH, and c-C 2 H 4 O and a few nitrogen- and sulfur-bearing ones such as HOCH 2 CN and CH 3 SH. The largest detected molecules are gGg-(CH 2 OH) 2 and CH 3 COCH 3 . We did not detect glycerol or i- and n-propanol, but we do provide upper limits for them which are in line with previous laboratory and observational studies. The line density in Band 3 is only ~2.5 times lower in frequency space than in Band 7. From the detected lines in Band 3 at a ≳ 6σ level, ~25–30% of them could not be identified indicating the need for more laboratory data of rotational spectra. We find similar column densities and column density ratios of COMs (within a factor ~2) between Band 3 and Band 7. Conclusions. The effect of the dust optical depth for IRAS 16293-2422 B at an off-source location on column densities and column density ratios is minimal. Moreover, for warm protostars, long wavelength spectra (~3 mm) are not only crowded and complex, but they also take significantly longer integration times than shorter wavelength observations (~0.8 mm) to reach the same sensitivity limit. The 3 mm search has not yet resulted in the detection of larger and more complex molecules in warm sources. A full deep ALMA Band 2–3 (i.e., ~3–4 mm wavelengths) survey is needed to assess whether low frequency data have the potential to reveal more complex molecules in warm sources.
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