An Expanded Gas-Grain Model for Interstellar Glycine
  The study of the chemical evolution of glycine in the interstellar medium is
one of challenging topics in astrochemistry. Here, we present the chemical
modeling of glycine in hot cores using the state-of-the-art three-phase
chemical model NAUTILUS, which is focused on the latest glycine chemistry. For
the formation process of glycine on the grain surface, we obtained consistent
results with previous studies that glycine would be formed via the reactions of
COOH with CH$_2$NH$_2$. However, we will report three important findings
regarding the chemical evolution and the detectability of interstellar glycine.
First, with the experimentally obtained binding energy from the temperature
programmed thermal desorption (TPD) experiment, a large proportion of glycine
was destroyed through the grain surface reactions with NH or CH$_3$O radicals
before it fully evaporates. As a result, the formation process in the gas phase
is more important than thermal evaporation from grains. If this is the case,
NH$_2$OH and CH$_3$COOH rather than CH$_3$NH$_2$ and CH$_2$NH would be the
essential precursors to the gas phase glycine. Secondly, since the gas phase
glycine will be quickly destroyed by positive ions or radicals, early
evolutionary phase of the hot cores would be the preferable target for the
future glycine surveys. Thirdly, we suggest the possibility that the
suprathermal hydrogen atoms can strongly accelerate the formation of COOH
radicals from CO$_2$, resulting in the dramatic increase of formation rate of
glycine on grains. The efficiency of this process should be investigated in
detail by theoretical and experimental studies in the future.
