Table 9
Summary of the chemical routes to the formation of the 3 isomers of C2 H4O2, and related reactions.
| # in text | Reaction | Type | Comments |
|---|---|---|---|
| MF | |||
| (4) | HCO + CH3O→CH3OCHO | gs | Efficient pathway for the formation of MF from (a) and (b). |
| (9) |  |
gp | Efficient for the formation of MF in cold environment (c). Not efficient to reproduce the observed abundance in G31 (b). |
 |
|||
| (10) | CH3OH + OH → CH3O + H2 | gp | Efficient for the formation of MF in cold environment (d,e). (e) Found react. (14) more efficient than (12). Not efficient to reproduce the observed abundance in G31 (b). |
| (11) | CH3O + CH3 → CH3OCH3 | ||
| (12) | CH3OCH3 + F∕Cl → CH3OCH2 + HF∕HCl | ||
| (13) | CH3OCH2 + O → CH3OCHO + H | ||
| (15) |  |
gp | Second most efficient reaction in cold environment (e), after (13). Not efficient in reproduce the observed abundance in G31 (b). |
 |
|||
| (19) | H2CO + H2CO → CH3OCHO | * | Proposed by (f), presents high energy barrier, and is more efficient on grain-surface but no calculation made to predict the abundances in star-forming regions |
| AA | |||
| (5) | CH3CO + OH → CH3COOH | gs | Main route on grain-surface, not efficient enough to reproduce the observed abundance in this work (a). |
| (16) | CH3CH2OH + OH∕Cl → CH2CH2OH + H2O∕HCl | gp | Predictions by (g) span in a broad range for GA, and may cover the abundance observed in this work for AA. Strong dependence on the rate coefficient of reaction (16) and on abundances of reactant OH and ethanol, which should be constrained to reduce uncertainties. |
| CH3CHOH + H2O∕HCl | |||
| (17) | CH3CHOH + O → CH3COOH + H | ||
| (19) | H2CO + H2CO → CH3OOCH | * | Proposed by (f), presents high energy barrier, and is more efficient on grain-surface but no calculation made to predict the abundances in star-forming regions |
| GA | |||
| (6) | HCO + CH2OH → CH2OHCHO | gs | Efficient to reproduce the abundance in this work together with (8) by (b) and in IRAS 16293-2422 A & B by (h). The estimate by (b) might change due to the new estimate of the distance to G31. |
| (7) | CH3OH + HCO → CH2OHCHO + H | gs | Efficient route for the formation of GA (i), not tested in (b) and (h). |
| (8) | 2HCO + 2H → CH2OHCHO | gs | Efficient to reproduce the abundance in this work together with (6) by (b) and possibly contributes in IRAS 16293-2422 A & B by (h). The estimate by (b) might change due to the new estimate of distance to G31. |
| (16) | CH3CH2OH + OH∕Cl → CH2CH2OH + H2O∕HCl | gp | Predictions by (g) span in a broad range, that cover the abundances observed in this work. Strong dependence on the rate coefficient of reaction (16) and on abundances of reactant OH and ethanol, which should be constrained to reduce uncertainties. |
| CH3CHOH + H2O∕HCl | |||
| (18) | CH2CH2OH + O → CH2OHCHO + H | ||
| (19) | H2CO + H2CO → CH3OCHO | * | Proposed by (f), presents high energy barrier, and is more efficient on grain-surface but no calculation made to predict the abundances in star-forming regions on grains surface. (j) Simulated the abundance in gas-phase in high-mass protostellar objects, and it is not efficient to reproduce G31 abundance. |
| Other reactions | |||
| (14) | CH3OCH3 + OH → CH3OCH2 + H2O | gp | More efficient than reaction (12) to produce CH3OCH2 (e) needed for reaction (13). |
Notes. gs stands for grain-surface reactions; gp stands for gas-phase reactions; * stands for both grain-surface and gas-phase reactions.
References. (a)Garrod (2013); (b)Coutens et al. (2018); (c)Vasyunin & Herbst (2013); (d)Balucani et al. (2015); (e)Vasyunin et al. (2017); (f)Ahmad et al. (2020) ; (g)Skouteris et al. (2018); (h)Rivilla et al. (2019); (i)Woods et al. (2012); (j)Silva et al. (2020).
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