Complex Chemistry in Star-forming Regions: An Expanded Gas-Grain Warm-up Chemical Model

By R. Garrod, S. W. Weaver, E. Herbst. 2008

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Gas-phase processes were long thought to be the key formation mechanisms for complex organic molecules in star-forming regions. However, recent experimental and theoretical evidence has cast doubt on the efficiency of such processes. Grain-surface chemistry is frequently invoked as a solution, but until now there have been no quantitative models taking into account both the high degree of chemical complexity and the evolving physical conditions of star-forming regions. Here, we introduce a new gas-grain chemical network, wherein a wide array of complex species may be formed by reactions involving radicals. The radicals we consider (H, OH, CO, HCO, CH3, CH3O, CH2OH, NH, and NH2) are produced primarily by cosmic ray-induced photodissociation of the granular ices formed during the colder, earlier stages of evolution. The gradual warm up of the hot core is crucial to the formation of complex molecules, allowing the more strongly bound radicals to become mobile on grain surfaces. This type of chemistry is capable of reproducing the high degree of complexity seen in Sgr B2(N), and can explain the observed abundances and temperatures of a variety of previously detected complex organic molecules, including structural isomers. Many other complex species are predicted by this model, and several of these species may be detectable in hot cores. Differences in the chemistry of high- and low-mass star formation are also addressed; greater chemical complexity is expected where evolution timescales are longer.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Organic Chemistry

View Paper: https://www.semanticscholar.org/paper/af2cf0910e10c0a3ea764ec023fcc9972b461b81

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Citations in the corpus

Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges (2010, background): “After the protostar turns on, the temperature of the cold material rises and the radicals produced by the photoprocessing diffuse rapidly enough to react to form more complex species, which are thermally released into the gas phase (Garrod and Herbst 2006; Garrod et al. 2008a).”
Observations of the Icy Universe (2015, background): “We speculate that it might be formed as the radicals, created by energetic processes and stored in the ices over the cloud and envelope life time, become more mobile and react at higher temperatures (Garrod et al., 2008; Vasyunina et al., 2014).”
A COLD COMPLEX CHEMISTRY TOWARD THE LOW-MASS PROTOSTAR B1-b: EVIDENCE FOR COMPLEX MOLECULE PRODUCTION IN ICES (2010, background): “While the laboratory results cannot be directly extrapolated to B1-b, a CO-rich ice is certainly needed to produce the observed low CH3OCH3(C2H5OH)/HCOOCH3 ratios; without CO in the ice, the most recent model predict ratios of >10, rather than the observed <0.5 (Garrod et al. 2008).”
Our astrochemical heritage (2012, background): “nd blue crosses: hot corinos and cores) and predicted (continuum lines) methyl formate normalized to methanol abundance as function of methanol abundance. The blue lines refer to models of hot cores ([172] and [254]) and red lines to models of hot corinos ([172] and [21]). Figure from [411], with pe”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “Formation of COMs in hot cores and corinos is reasonably well explained by the “warm-up scenario” (Garrod & Herbst 2006; Garrod et al. 2008), in which complex organic molecules first form via surface radical-radical reactions at 30– 40 K and then evaporate to the gas at higher temperatures reaching…”
The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “For completeness, we also supplement our reaction scheme with grain-surface association reactions extracted from the publicly available Ohio State University (OSU) network3 (Garrod et al. 2008).”
The UMIST database for astrochemistry 2012 (2012, background): “Our initial elemental abundances, which are listed in Table 3, are identical to those in Garrod et al. (2008), these are the low-metal abundances of Graedel et al. (1982), updated with more recent diffuse cloud values for He, C+, N and O. 5 See http://www.astrochemistry.net”
H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices – an extended view on complex organic molecule formation (2016, background): “Interactions between reactive radicals such as those produced here are actively discussed in the literature as a source for COM formation, both theoretically and experimentally (Hudson & Moore, 2000; Bennett et al. 2007; Garrod et al. 2008; Öberg et al. 2009; Woods et al. 2012; Garrod 2013).”
Increased complexity in interstellar chemistry: detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius B2(N) (2009, methodology): “Garrod et al. (2008) constructed a reaction network to account for the grain-surface formation of many complex molecules observed in hot cores.”
The chemical history of molecules in circumstellar disks - I. Ices (2009, methodology): “In turn, these two molecules form the basis of even larger organic species like methyl formate (HCOOCH3; Garrod & Herbst 2006; Garrod et al. 2008).”
Grain Surface Models and Data for Astrochemistry (2017, background): “While several of these classes of molecules may have viable gas-phase formation mechanisms, the larger species are thought to be formed primarily (or exclusively) on the surfaces of dust grains, or within/upon dust-grain ice mantles (Garrod and Herbst 2006; Garrod et al. 2008).”
2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules (2018, background): “A substantial amount of laboratory (e.g., Linnartz et al. 2015 and Fedoseev et al. 2015), quantum chemical (e.g., Woon 2002), and chemical modeling (e.g., Garrod et al. 2008) work suggests that these hydrogenation processes are efficient in the ISM, often even at low temperatures.”
A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, background): “…1.0 Hack et al. (1986) 94 NH2 + CH3OH→ NH3 + CH2OH 3280 1.0 Evans-Polanyi 95 NH2 + CH3OH→ NH3 + CH3O 3290 1.0 Evans-Polanyi 96 NH2 + CO→ NH2CO 2500 1.0 Garrod et al. (2008) 97 NH2 + H2CO→ NH3 + HCO 2360 1.0 Evans-Polanyi 98 NH2 + HCOOCH3 → NH3 + CH3OCO 3360 1.0 Evans-Polanyi 99 NH2 + HCOOCH3 →…”
Astrochemistry and compositions of planetary systems (2020, background): “recombine to produce more complex molecules (Garrod et al. 2008; Bennett et al. 2007; Öberg et al. 2009b; Chuang et al. 2017).”
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).”
Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores (2009, background): “Higher temperatures allow the radicals to diffuse rapidly enough to form large molecules such as methyl formate and dimethyl ether (Garrod et al. 2008; Öberg et al. 2009d).”
ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES (2011, methodology): “The basic model is that used by Garrod et al. (2008), and includes the grain-surface and gas-phase reactions used by those authors, as well those added to the network by Belloche et al. (2009).”