A three-phase chemical model of hot cores: the formation of glycine
  A new chemical model is presented that simulates fully-coupled gas-phase,
grain-surface and bulk-ice chemistry in hot cores. Glycine (NH2CH2COOH), the
simplest amino acid, and related molecules such as glycinal, propionic acid and
propanal, are included in the chemical network. Glycine is found to form in
moderate abundance within and upon dust-grain ices via three radical-addition
mechanisms, with no single mechanism strongly dominant. Glycine production in
the ice occurs over temperatures ~40-120 K. Peak gas-phase glycine fractional
abundances lie in the range 8 x 10^{-11} - 8 x 10^{-9}, occuring at ~200 K, the
evaporation temperature of glycine. A gas-phase mechanism for glycine
production is tested and found insignificant, even under optimal conditions. A
new spectroscopic radiative-transfer model is used, allowing the translation
and comparison of the chemical-model results with observations of specific
sources. Comparison with the nearby hot-core source NGC 6334 IRS1 shows
excellent agreement with integrated line intensities of observed species,
including methyl formate. The results for glycine are consistent with the
current lack of a detection of this molecule toward other sources; the high
evaporation temperature of glycine renders the emission region extremely
compact. Glycine detection with ALMA is predicted to be highly plausible, for
bright, nearby sources with narrow emission lines. Photodissociation of water
and subsequent hydrogen-abstraction from organic molecules by OH, and NH2, are
crucial to the build-up of complex organic species in the ice. The inclusion of
alternative branches within the network of radical-addition reactions appears
important to the abundances of hot-core molecules; less favorable branching
ratios may remedy the anomalously high abundance of glycolaldehyde predicted by
this and previous models.
