Abstract
Abstract Simple and complex organic molecules (COMs) are observed along different phases of star and planet formation and have been successfully identified in prestellar environments such as dark and translucent clouds. Yet the picture of organic molecule formation at those earliest stages of star formation is not complete and an important reason is the lack of specific laboratory experiments that simulate carbon atom addition reactions on icy surfaces of interstellar grains. Here we present experiments in which CO molecules as well as C and H atoms are codeposited with H2O molecules on a 10 K surface mimicking the ongoing formation of an “H2O-rich” ice mantle. To simulate the effect of impacting C atoms and resulting surface reactions with ice components, a specialized C-atom beam source is used, implemented on SURFRESIDE3, an ultra-high vacuum cryogenic setup. Formation of ketene (CH2CO) in the solid state is observed in situ by means of reflection absorption IR spectroscopy. C18O and D isotope labeled experiments are performed to further validate the formation of ketene. Data analysis supports that CH2CO is formed through C-atom addition to a CO molecule, followed by successive hydrogenation transferring the formed :CCO into ketene. Efficient formation of ketene is in line with the absence of an activation barrier in C+CO reaction reported in the literature. We also discuss and provide experimental evidence for the formation of acetaldehyde (CH3CHO) and possible formation of ethanol (CH3CH2OH), two COM derivatives of CH2CO hydrogenation. The underlying reaction network is presented and the astrochemical implications of the derived pathways are discussed.
Highlights
Astronomical gas-phase observations show that a variety of simple and complex organic molecules (COMs) exist in translucent and dense clouds as well as dark cores (Matthews et al 1985; Turner et al 1999; Bacmann et al 2012; Soma et al 2018)
The topic of the present study is to experimentally investigate the formation of ketene following the third suggested mechanism as well as the formation of two of its proposed derivatives, acetaldehyde and ethanol, under solid state conditions resembling the early phases of dark clouds or translucent clouds in which atom addition reactions play a key role
An unambiguous detection of newly formed CH2CO becomes possible by performing a temperature programmed desorption (TPD) experiment during RAIRS, as COMs described with formula (CO) (~30 K) and CH2CO (~100 K) have very distinct desorption temperatures
Summary
Astronomical gas-phase observations show that a variety of simple and complex organic molecules (COMs) exist in translucent and dense clouds as well as dark cores (Matthews et al 1985; Turner et al 1999; Bacmann et al 2012; Soma et al 2018). Following astronomical detections of COMs in dark and translucent clouds, recent laboratory experiments and computational efforts attempted to move the onset of efficient COM formation to an earlier stage, prior to or along with the formation of the H2O-rich ice layer in the interstellar medium (Hudson & Loeffler 2013, Qasim et al 2019; Chuang et al 2020, 2021, Ioppolo et al 2021) In these regions, atoms such as H, O, C, and N freeze-out onto bare dust grains (van de Hulst 1946; Boogert et al 2015), where they can react to form COMs along with the formation of already identified H2O, CO2, CH4, and NH3. The paper is completed with a discussion on the astronomical implications, which incorporates the newly verified reaction pathways into a larger astrochemically relevant reaction network
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