A Three-Phase Chemical Model of Hot Cores: The Formation of Glycine

Abstract

A new chemical model is presented that simulates fully coupled gas-phase, grain-surface, and bulk-ice chemistry in hot cores. Glycine (NH 2 CH 2 COOH), 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 × 10 −11 -8 × 10 −9 , occurring 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 NH 2 , are crucial to the buildup 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.

Complex molecule formation around massive young stellar objects

“…This has been explored extensively in chemical models of hot cores. For example, CH 3 CN can form on grains via CH 3 + CN at ∼30 K. It can also form from HCN in the gas-phase, when HCN evaporates at ∼40 K, through CH + 3 + HCN association followed by a recombination reaction 14 . This gas-phase CH 3 CN re-accretes onto the grains, and can then thermally evaporate around 90 K.…”

“…2 × 10 5 years for warm-up to 200 K) 14 . For CH 3 CN/CH 3 OH, the theoretical abundance ratio increase of two orders of magnitude with temperature up to 100 K agrees well with observations.…”

Solid state chemistry of nitrogen oxides – Part II: surface consumption of NO2

“…12,15 The use of two ultra-high vacuum (UHV) systems allows for complementary studies of selected interstellar relevant surface reactions. Therefore, a combination of experiments performed in these setups gives information on surface reactions occurring at different surface coverages (sub-monolayer vs. multilayer regime), on different substrates (gold, silicates, graphite, and ASW ice), and in different matrix environments (pure NO and NO 2 , NO in polar ice, and NO and NO 2 in apolar ice).…”

“…The cumulative outcome of recent observations, laboratory studies, and astrochemical models indicates that there is a strong interplay between the gas and the solid phase throughout the formation process of molecules in space. 1,2 Surface reaction mechanisms on cold dust grains initiate molecular chemistry starting with the formation of H 2 , and dominate the formation of complex organic molecules (COMs) in space. Indeed, interstellar grains provide surfaces on which gas-phase species can accrete, meet, and react, and to which they can donate the excess energy.…”

THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups

“…Although non-energetic surface reaction pathways leading to the formation of the more complex species studied in this work have not yet been investigated in the laboratory, several models indicate that those species should be formed in the solid phase and could participate in the formation of even more complex species. 11 For instance, Bisschop et al 52 studied the surface hydrogenation of acetaldehyde and found that it converts to ethanol (C 2 H 5 OH) or to CH 4 , H 2 CO, and CH 3 OH.…”