Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores

By R. Garrod, E. Herbst. 2006

628 citations recorded by the source corpus.

Aims. The production of saturated organic molecules in hot cores and corinos is not well understood. The standard approach is to assume that, as temperatures heat up during star formation, methanol and other species evaporate from grain surfaces and undergo a warm gas-phase chemistry at 100 K or greater to produce species such as methyl formate, dimethyl ether, and others. But a series of laboratory results shows that protonated ions, typical precursors to final products in ion-molecule schemes, tend to fragment upon dissociative recombination with electrons rather than just ejecting a hydrogen atom. Moreover, the specific proposed reaction to produce protonated methyl formate is now known not to occur at all. Methods. We utilize a gas-grain chemical network to probe the chemistry of the relatively ignored stage of hot core evolution during which the protostar switches on and the temperature of the surrounding gas and dust rises from 10 K to over 100 K. During this stage, surface chemistry involving heavy radicals becomes more important as surface hydrogen atoms tend to evaporate rather than react. Results. Our results show that complex species such as methyl formate, formic acid, and dimethyl ether can be produced in large abundance during the protostellar switch-on phase, but that both grain-surface and gas-phase processes help to produce most species. The longer the timescale for protostellar switch-on, the more important the surface processes.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/9b5235c7000114090986600a1a932b5fc5d902d1

How researchers see this paper

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In Astronomy and Astrophysics

1. R. Garrod
2. P. Caselli
3. E. Herbst
4. Cecilia Ceccarelli
5. E. Dishoeck
6. K. Oberg
7. E. Bergin
8. V. Wakelam

In Spectroscopy

1. R. Garrod
2. P. Caselli
3. Cecilia Ceccarelli
4. E. Herbst
5. E. Dishoeck
6. K. Oberg
7. V. Wakelam
8. E. Bergin

Citations in the corpus

• The UMIST database for astrochemistry 2012 (2012, background): “…(1973), Field et al. (1980), Forte et al. (1989), Freeman et al. (1979), Frost et al. (1993), Fukuzawa & Osamura (1997), Gannon et al. (2007), Garrod & Herbst (2006), Gredel et al. (1989), Gu et al. (2009), Hamberg et al. (2007), Hamberg et al. (2010), Hamberg et al. (2010), Harada & Herbst…”
• Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “Compilation done by Garrod & Herbst (2006) shows that the observed abundances of HCOOH in hot cores vary in the range [8.0×10−10 — 6.2×10−8] wrt. H.”
• The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks (2009, background): “…complex saturated molecules such as CH3OH (e.g., Charnley et al. 1995; Watanabe & Kouchi 2002), so the partioning of CO between the gas and grains is important for the overall chemical composition as well (e.g., Caselli et al. 1993; Rodgers & Charnley 2003; Doty et al. 2004; Garrod & Herbst 2006).”
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments (2009, methodology): “The scheme shares many features with previous ones considered by e.g. Garrod & Herbst (2006).”
• The chemical history of molecules in circumstellar disks - I. Ices (2009, methodology): “The radicals involved in the surface formation of HCOOCH3 (HCO and CH3O) are not mobile enough at lower temperatures and are not formed efficiently enough at higher temperatures.”
• Astrochemistry and compositions of planetary systems (2020, background): “The onset of this second generation of ice chemistry is powered by two factors: the build-up of radicals in cold ices, and diffusion of these radicals at slightly elevated ice temperatures (Garrod and Herbst 2006; Garrod et al. 2008; Garrod 2013).”
• A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, background): “Garrod & Herbst (2006) showed that grain-surface formation mechanisms could produce adequate quantities of methyl formate (MF) to explain observations; however, in some cases it was found (Garrod et al. 2008; Belloche et al. 2009) that the evaporation of MF at relatively low temperatures could…”
• Non-thermal desorption from interstellar dust grains via exothermic surface reactions (2007, methodology): “2001), including reactions with activation energy barriers, in the same way as Garrod & Herbst (2006); all surface atomic hydrogen reaction rates may be modified.”
• Complex Chemistry in Star-forming Regions: An Expanded Gas-Grain Warm-up Chemical Model (2008, methodology): “We concentrate on the T2 temperature profile adopted by Garrod & Herbst (2006), where T (t) = 10+ k · t2, measured in Kelvin, at time t. Gas and dust temperatures are assumed to be well coupled, due to the high densities and visual extinctions, so we assume TK = Tgrain at all times.”
• On the Formation of Glycolaldehyde (HCOCH2OH) and Methyl Formate (HCOOCH3) in Interstellar Ice Analogs (2007, background): “However, even these models are limited, as they are primarily based on the surface migration of radicals (e.g., Garrod & Herbst 2006) or of heavy atoms (Charnley & Rodgers 2006).”
• Testing grain-surface chemistry in massive hot-core regions (2007, background): “Indeed, in models by Garrod & Herbst (2006) where HCOOCH3 is formed in the solid state, many of the oxygen-bearing species have similar gas phase abundance variations throughout the chemical evolution and have constant abundance ratios.”
• Photodesorption of water ice - A molecular dynamics study (2008, background): “…not only in the desorption of ices but also in the cration of reactive photo-products such as energetic H atoms and radicals which can move through the ice and encounter other species leading to the formation of more complex molecules (e.g., d’Hendecourt et al. 1982; Garrod & Herbst 2006).”
• A COLD COMPLEX CHEMISTRY TOWARD THE LOW-MASS PROTOSTAR B1-b: EVIDENCE FOR COMPLEX MOLECULE PRODUCTION IN ICES (2010, background): “The focus is currently on an ice formation pathway (e.g. Garrod & Herbst 2006; Garrod et al. 2008) because of the failures of gas phase chemistry to explain the observed abundances of some of the most common complex molecules toward low-mass protostars, especially HCOOCH3.”
• 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).”
• Increased complexity in interstellar chemistry: detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius B2(N) (2009, methodology): “We employ the single-point physical model used by Garrod & Herbst (2006), in which the isothermal collapse of a diffuse medium, to a densitynH = 107 cm−3, is followed by a warm-up from 10 to 200 K. TheirT2 warm-up profile is assumed, in which the hot-core temperature has at2 dependence on the…”
• A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA) (2012, background): “See Garrod & Herbst (2006) for a discussion concerning the values of ED and Eb, which depend upon the surface.”
• ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES (2011, methodology): “We employ the grain-surface binding/desorption energies (Edes) and diffusion barriers (Edif) of Garrod & Herbst (2006) and those values added by Garrod et al. (2008), all of which are representative of a surface composed primarily of amorphous water-ice.”
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions (2010, methodology): “Several compilations of surface reactions have also been presented (e.g., Allen & Robinson, 1977; Tielens & Hagen, 1982; Hasegawa et al., 1992; Hasegawa & Herbst, 1993; Garrod & Herbst, 2006).”