A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE

By R. Garrod. 2013

254 citations recorded by the source corpus.

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 × 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 NH2, 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.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/3abb204e8ffd24eaef0f8f0bcc101010c3258a35

How researchers see this paper

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In Astronomy and Astrophysics

1. R. Garrod
2. E. Herbst
3. V. Wakelam
4. P. Caselli
5. K. Oberg
6. M. Ruaud
7. E. Bergin
8. H. Cuppen

In Atmospheric Science

1. R. Garrod
2. V. Wakelam
3. P. Caselli
4. E. Herbst
5. M. Ruaud
6. H. Cuppen
7. F. Hersant
8. S. Ioppolo

In Spectroscopy

1. R. Garrod
2. P. Caselli
3. V. Wakelam
4. E. Herbst
5. E. Bergin
6. K. Oberg
7. M. Ruaud
8. C. Walsh

Citations in the corpus

• Experimental evidence for glycolaldehyde and ethylene glycol formation by surface hydrogenation of CO molecules under dense molecular cloud conditions (2015, background): “Despite theoretical studies and laboratory-based work that show that glycine as well as several other amino acids should form in space (Muñoz Caro et al. 2002; Blagojevic, Petrie & Bohme 2003; Congiu et al. 2012; Garrod 2013), unambiguous detections are still lacking (Snyder et al. 2005).”
• Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, methodology): “…to mantle rates are then built to match a total rate identical to the mantle to surface one, ksswap(i) = ∑ j k m swap(j)nm(j) ns,tot [s−1], Unlike Garrod (2013), we set all the species (except H, H2, C, N and O) with E m diff(i)<E m diff(H2O) to the diffusion barrier of H2O considering that…”
• Grain Surface Models and Data for Astrochemistry (2017, background): “The latter assumption comes from the fact that radicals are commonly assumed to become mobile around 30 K (e.g., Garrod 2013; Butscher et al. 2015).”
• Astrochemistry and compositions of planetary systems (2020, background): “One important feature of most contemporary rate equation models is the treatment of ice surfaces separate from ice layers (Hasegawa and Herbst 1993), and so called three-phase models (ice mantle, ice surface and gas-phase) are now standard (Vasyunin and Herbst 2013; Garrod 2013).”
• Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “Garrod (2013) defines the rate of diffusion of a species from bulk to surface with his Eq.”
• Exploring Molecular Complexity with ALMA (EMoCA): Deuterated complex organic molecules in Sagittarius B2(N2) (2015, background): “Unlike all other complex organic molecules, the formation of methyl cyanide is dominated by gas phase processes in our own models (e.g., Garrod et al. 2008; Garrod 2013).”
• Complex organic molecules in protoplanetary disks (2013, background): “Recently, Garrod (2013) investigated the formation of glycine in hot cores via grain-surface radical-radical reactions, i.e., an extension to the reaction scheme used here, incorporating the ice chemistry proposed in Woon (2002) to describe the formation of glycine in UV-irradiated ices.”