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Register a new userAdsorption of Organic Molecules on Onion-Like-Carbons: Insights on the Formation of Interstellar Hydrocarbons
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Using atomistic simulations, we characterize the adsorption process of organic molecules on carbon nanoparticles, both of which have been reported to be abundant in the interstellar medium (ISM). It is found that the aromatic organics are adsorbed more readily than the aliphatic ones. This selectivity would favor the formation of polycyclic aromatic hydrocarbons (PAHs) or fullerene-like structures in the ISM due to structural similarity. It is also observed in our simulations that the molecules form a monolayer over the nanoparticle surface before stacking up in aggregates. This suggests a possible layer-by-layer formation process of onion-like nanostructures in the ISM. These findings reveal the possible role of carbon nanoparticles as selective catalysts that could provide reaction substrates for the formation of interstellar PAHs, high-fullerenes and soots from gas-phase molecules.
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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.
Isolated dense molecular cores are investigated to study the onset of complex organic molecule formation in interstellar ice. Sampling three cores with ongoing formation of low-mass stars (B59, B335, and L483) and one starless core (L694-2) we sample lines of sight to nine background stars and five young stellar objects (YSOs; A_K ~0.5 - 4.7). Spectra of these stars from 2-5 $mu$m with NASAs Infrared Telescope Facility (IRTF) simultaneously display signatures from the cores of H$_2$O (3.0 $mu$m), CH$_3$OH (C-H stretching mode, 3.53 $mu$m) and CO (4.67 $mu$m) ices. The CO ice is traced by nine stars in which five show a long wavelength wing due to a mixture of CO with polar ice (CO$_r$), presumably CH$_3$OH. Two of these sight lines also show independent detections of CH$_3$OH. For these we find the ratio of the CH$_3$OH:CO$_r$ is 0.55$pm$0.06 and 0.73$pm$0.07 from L483 and L694-2, respectively. The detections of both CO and CH$_3$OH for the first time through lines of sight toward background stars observationally constrains the conversion of CO into CH$_3$OH ice. Along the lines of sight most of the CO exists in the gas phase and $leq$15% of the CO is frozen out. However, CH$_3$OH ice is abundant with respect to CO (~50%) and exists mainly as a CH$_3$OH-rich CO ice layer. Only a small fraction of the lines of sight contains CH$_3$OH ice, presumably that with the highest density. The high conversion of CO to CH$_3$OH can explain the abundances of CH$_3$OH ice found in later stage Class 1 low mass YSO envelopes (CH$_3$OH:CO$_r$ ~ 0.5-0.6). For high mass YSOs and one Class 0 YSO this ratio varies significantly implying local variations can affect the ice formation. The large CH$_3$OH ice abundance indicates that the formation of complex organic molecules is likely during the pre-stellar phase in cold environments without higher energy particle interactions (e.g. cosmic rays).
During the formation of stars, the accretion of the surrounding material toward the central object is thought to undergo strong luminosity outbursts, followed by long periods of relative quiescence, even at the early stages of star formation when the protostar is still embedded in a large envelope. We investigated the gas phase formation and the recondensation of the complex organic molecules (COMs) di-methyl ether and methyl formate, induced by sudden ice evaporation processes occurring during luminosity outbursts of different amplitudes in protostellar envelopes. For this purpose, we updated a gas phase chemical network forming complex organic molecules in which ammonia plays a key role. The model calculations presented here demonstrate that ion-molecule reactions alone could account for the observed presence of di-methyl ether and methyl formate in a large fraction of protostellar cores, without recourse to grain-surface chemistry, although they depend on uncertain ice abundances and gas phase reaction branching ratios. In spite of the short outburst timescales of about one hundred years, abundance ratios of the considered species with respect to methanol higher than 10 % are predicted during outbursts due to their low binding energies relative to water and methanol that delay their recondensation during the cooling. Although the current luminosity of most embedded protostars would be too low to produce these complex species in hot core regions that can be observable with current sub-millimetric interferometers, previous luminosity outburst events would induce a formation of COMs in extended regions of protostellar envelopes with sizes increasing by up to one order of magnitude.
We study whether polycyclic aromatic hydrocarbons (PAHs) can be a weighty source of small hydrocarbons in photo-dissociation regions (PDRs). We modeled the evolution of 20 specific PAH molecules in terms of dehydrogenation and destruction of the carbon skeleton under the physical conditions of two well-studied PDRs, the Orion Bar and the Horsehead nebula which represent prototypical examples of PDRs irradiated by high and low ultraviolet radiation field. PAHs are described as microcanonical systems. The acetylene molecule is considered as the main carbonaceous fragment of the PAH dissociation as it follows from laboratory experiments and theory. We estimated the rates of acetylene production in gas phase chemical reactions and compared them with the rates of the acetylene production through the PAH dissociation. It is found that the latter rates can be higher than the former rates in the Orion Bar at $A_{rm V}<1$ and also at $A_{rm V}>3.5$. In the Horsehead nebula, the chemical reactions provide more acetylene than the PAH dissociation. The produced acetylene participate in the reactions of the formation of small hydrocarbons (C$_2$H, C$_3$H, C$_3$H$^{+}$, C$_3$H$_2$, C$_4$H). Acetylene production via the PAH destruction may increase the abundances of small hydrocarbons produced in gas phase chemical reactions in the Orion Bar only at $A_{rm V}>3.5$. In the Horsehead nebula, the contribution of PAHs to the abundances of the small hydrocarbons is negligible. We conclude that the PAHs are not a major source of small hydrocarbons in both PDRs except some locations in the Orion Bar.
The formation of double and triple C-C bonds from the processing of pure c-C6H12 (cyclohexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly-charged, and energetic ions (219 MeV O^{7+} and 632 MeV Ni^{24+}) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ices analogs. The measurements were performed inside a high vacuum chamber at the heavy-ion accelerator GANIL (Grand Accelerateur National dIons Lourds) in Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometry at different ion fluences. Dissociation cross section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources indicates that the initial composition of grains in theses environments should contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several species containing double or triple bounds were identified in the radiochemical products, such as hexene, cyclohexene, benzene, OCN-, CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggest an alternative scenario for the production of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.
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