Chemical modeling of aminoketene, ethanolamine, and glycine production in interstellar ices

TL;DR

This work demonstrates that aminoketene NH2CHCO can form efficiently on interstellar grain surfaces at very low temperatures through barrierless reactions of atomic carbon with NH3 and subsequent CO coupling, acting as a precursor to ethanolamine NH2CH2CH2OH and glycine NH2CH2COOH. By extending the MAGICKAL three-phase chemical network to include NH2CHCO and its products and by exploring hot-core evolution and shock conditions, the authors quantify solid- and gas-phase abundances, revealing peak NH2CHCO solid-phase abundances around $2\times10^{-10}$ $n_H$ and substantial grain-surface production of ethanolamine. The shock models show rapid mantle sputtering that releases these species into the gas phase, with lifetimes sensitive to the cosmic-ray ionization rate, offering a plausible route to detect amine-bearing COMs in regions like G+0.693. Overall, the study supports a scenario where grain-surface carbon chemistry provides a robust pathway to prebiotic molecules, with shocks enhancing detectability and CRIR setting the survival timescales in the ISM.

Abstract

Icy interstellar dust grains are a source of complex organic molecule (COM) production, although their formation mechanisms are debated. Laboratory experiments show that atomic C deposited onto interstellar ice analogs can react with solid-phase NH3 to form a CHNH2 radical, a possible precursor to COMs, including aminoketene (NH2CHCO). We used astrochemical kinetics models to explore the role of the reaction of atomic C with NH3 and subsequent reaction with CO in the formation of NH2CHCO and other COMs. We applied the three-phase chemical model MAGICKAL to hot molecular core conditions from the cold-collapse through to the hot-core stage. The chemical network was extended to include NH2CHCO and a range of associated gas-phase, grain-surface, and bulk-ice products and reactions. We also approximated conditions in a shocked cloud, including sputtering of ice mantles. NH2CHCO is formed on grains at low temperatures (~10 K) with a peak solid-phase abundance of ~2x10^-10 nH. Its formation is driven by nondiffusive reactions, in particular the Eley-Rideal reaction of C with surface NH3, followed by immediate reaction with CO. Surface hydrogenation of NH2CHCO produces ethanolamine with a significant abundance of ~8x10^-8 nH. In the gas-phase, although ethanolamine reaches a modest abundance peak immediately following its desorption from grains under hot-core conditions, it is destroyed more rapidly due to its high proton affinity. Molecular survival is much higher in the shocked regions, where these species seem most likely to be detected. NH2CHCO is produced efficiently on simulated interstellar grain surfaces, acting subsequently as an important precursor to more complex organics, including ethanolamine and glycine. Ion-molecule gas-phase destruction of NH3-bearing COMs is less efficient in shocked lower-density regions, in contrast to hot cores, enhancing their abundances and lifetimes.

Figures (8)

• Figure 1: Energy level diagram of the C + NH$_3$ + CO reaction. The reaction proceeds from left to right. It starts with a triplet state; the intersystem crossings to the singlet states are marked with dashed arrows. S and TS stand for singlet and transition states. Red lines indicate the most probably pathway.
• Figure 2: Physical parameters produced by the parametric shock model, assuming a 20 km s$^{-1}$ shock speed, initial gas density $n_{\mathrm{H}} = 2 \times 10^4$ cm$^{-3}$, and a grain size $a = 0.1$$\mu$m. The difference between the speeds of the neutral gas and the dust grains is important in determining the rate of ice sputtering in the chemical model. The shock treatment is based on the methods of JS2008 and Miura2017.
• Figure 3: Fractional abundances of NH2CHCO and relevant products with respect to total hydrogen. The results correspond to the Basic setup, with the collapse stage shown on the left and the warm-up stage shown on the right. Solid lines indicate gas-phase abundances; dotted lines of the same color indicate the species on the grain (surface and bulk ice combined). The thicker bar in the upper axis of the left panel indicates that the dust temperature remains at 8 K once that value is reached.
• Figure 4: Left: Net rates of change in the aggregate (gas, surface and bulk phases) abundances of aminoketene (NH2CHCO; upper) and ethanolamine (NH2CH2CH2OH; lower) during the collapse and warm-up stages of the Basic model. Net gain is shown in green, and net loss is shown in blue. The vertical dotted line indicates the start of the warm-up stage. The vertical dashed lines indicate the onset and end-point of water desorption. From left to right, the background color indicates the temperature of the dust: >10 K, 10--50 K, 50--100 K, 100--200 K, 200--400 K. Right: Integrated net rates of change over each temperature range. Positive (formation) and negative (destruction) rates are integrated independently and are both normalized to the total integrated formation rate.
• Figure 5: Fractional abundances of NH2CHCO with respect to total hydrogen for four different setups, with the collapse stage on the left and the warm-up stage on the right. Solid lines indicate gas-phase abundances; dotted lines of the same color indicate the species on the grain (surface and bulk ice).