Hydrogenation of accreting C-atoms and CO molecules -- simulating ketene
  and acetaldehyde formation under dark and translucent cloud conditions
  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 co-deposited with H$_2$O molecules on a 10 K surface mimicking the
ongoing formation of an "H$_2$O-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 SURFRESIDE$^3$, an UHV
cryogenic setup. Formation of ketene (CH$_2$CO) in the solid state is observed
"in situ" by means of reflection absorption IR spectroscopy. C$^1$$^8$O and D
isotope labelled experiments are performed to further validate the formation of
ketene. Data analysis supports that CH$_2$CO is formed through C-atom addition
to a CO-molecule, followed by successive hydrogenation transferring the formed
:CCO into ketene. Efficient formation of ketene is in line with the absence of
an activation barrier in C+CO reaction reported in the literature. We also
discuss and provide experimental evidence for the formation of acetaldehyde
(CH$_3$CHO) and possible formation of ethanol (CH$_3$CH$_2$OH), two COM
derivatives of CH$_2$CO hydrogenation. The underlying reaction network is
presented and the astrochemical implications of the derived pathways are
discussed.
