The earliest stage in the star formation cycle at which solid-state complex organic molecules (COMs) can form is the dense cloud stage. As the density rises past ~103 H2 molecules cm-3, external UV photons are increasingly blocked from entering the cloud which also leads to a drop in the cloud temperature to 10-20 K. Under such conditions, COMs are likely formed through a ‘non-energetic’ process, which is a radical-induced process that does not involve UV photons, electrons, and/or other ‘energetic’ particles such as cosmic rays. This way of synthesizing COMs has been shown to be quite effective from recent solid-state laboratory experiments. Simple sugar-related species (e.g., glycolaldehyde), sugar alcohols (e.g., ethylene glycol and glycerol), aldehydes (e.g., propanal), and primary alcohols (e.g., n-propanol) have been formed from molecule and radical recombination in the CO + H reaction network– a pathway that is observationally constrained to occur during the CO freeze-out stage of dense clouds. In this contribution, we show that COMs, and particularly alcohols, can already be formed before a majority of the CO freezes-out in the early period of the H2O-rich ice phase. By utilizing an ultrahigh vacuum apparatus in combination with computational investigations, we show that the isomers, n- and i-propanol (H3CCH2CH2OH and H3CCHOHCH3) and n- and i-propenol (H3CCH=CHOH and H3CCOH=CH2), can be formed in the ‘non-energetic’ reaction of propyne (H3CC≡CH) + OH at 10 K. Gas-phase H3CC≡CH has already been identified towards cold, dark and dense clouds, where it is expected to have an abundance &lt; 1% relative to H2O ice. The resulting 1:1 average abundance ratio of n-propanol:i-propanol found in the experiments is in agreement with the computationally-derived activation barriers and the finding that the geometric orientation between H3CC≡CH and OH strongly influences the branching ratio. It is additionally shown that propene and propane (H3CCH=CH2 and H3CCH2CH3) are formed from the hydrogenation of H3CC≡CH, with a H3CC≡CH: H3CCH=CH2: H3CCH2CH3 abundance ratio of 9:1:2, respectively. The alcohols analyzed in this study are expected to be partially formed at the interface of the H2O-rich ice and dust grain in dense clouds allowing preservation of alcohols and their extended counterparts containing H3C-(C≡C)n-H structures (e.g., fatty alcohols) upon delivery to the early Earth.

Combined laboratory and theoretical studies on the formation of alcohols in the H2O-rich ice phase of prestellar cores

2019 Fall National Meeting

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