Formation of Complex Organic Molecules in Cold Interstellar Environments through Nondiffusive Grain-surface and Ice-mantle Chemistry

Abstract

A prevailing theory for the interstellar production of complex organic molecules (COMs) involves formation on warm dust-grain surfaces, via the diffusion and reaction of radicals produced through grain-surface photodissociation of stable molecules. However, some gas-phase O-bearing COMs, notably acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and dimethyl ether (CH3OCH3), are now observed at very low temperatures, challenging the warm scenario. Here, we introduce a selection of new nondiffusive mechanisms into an astrochemical model to account for the failure of the standard diffusive picture and to provide a more generalized scenario of COM formation on interstellar grains. New generic rate formulations are provided for cases where (i) radicals are formed by reactions occurring close to another reactant, producing an immediate follow-on reaction; (ii) radicals are formed in an excited state, allowing them to overcome activation barriers to react with nearby stable molecules; and (iii) radicals are formed through photodissociation close to a reaction partner, followed by immediate reaction. Each process occurs without thermal diffusion of large radicals. The new mechanisms significantly enhance cold COM abundances, successfully reproducing key observational results for prestellar core L1544. H abstraction from grain-surface COMs, followed by recombination, plays a crucial role in amplifying chemical desorption into the gas phase. The ultraviolet-induced chemistry produces significant COM abundances in the bulk ices, which are retained on the grains and may persist to later stages. O2 is also formed strongly in the mantle though photolysis, suggesting cometary O2 could indeed be interstellar.

Photolysis of acetonitrile in a water-rich ice as a source of complex organic molecules: CH3CN and H2O:CH3CN ices

“…The reaction type discussed above, of an excited radical with a neutral species, has been recently demonstrated to be efficient also at low temperatures for gas phase CH 3 OH + OH and CH 3 CN + CN (Shannon et al 2013;Sleiman et al 2016). Hence, this type of reactions should be considered in gas-grain astrochemical models, which is already the case in recent work by Jin & Garrod (2020).…”

O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: Detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3

“…There is as of yet no consensus on how HCOOCH 3 and CH 3 OCH 3 are formed in cold sources. Models in which the synthesis relies on chemical desorption and gasphase radiative associations usually require a chemical desorption efficiency as high as 10% (Vasyunin & Herbst 2013;Balucani et al 2015;Chang & Herbst 2016), which can be relaxed if Eley-Rideal processes (Ruaud et al 2015), radiation chemistry (Shingledecker et al 2018), or non-diffusive grainsurface processes (Jin & Garrod 2020) are considered. These models can account for abundances relative to H 2 of around 10 −10 for HCOOCH 3 and/or CH 3 OCH 3 under certain assumptions, although they rely on still poorly constrained chemical and physical processes.…”

“…In the last decade, these molecules have also been observed in a few cold sources, such as the dense cores B1-b (Öberg et al 2010;Cernicharo et al 2012) and L483 (Agúndez et al 2019), the dark cloud Barnard 5 (Taquet et al 2017), the pre-stellar cores L1689B (Bacmann et al 2012) and L1544 (Jiménez-Serra et al 2016), and the starless core TMC-1 (Soma et al 2018). The low temperatures in these environments inhibit thermal desorption, and how these molecules are formed, whether in the gas phase or on grain surfaces followed by some nonthermal desorption process, is still an active subject of debate (Vasyunin & Herbst 2013;Ruaud et al 2015;Balucani et al 2015;Chang & Herbst 2016;Vasyunin et al 2017;Shingledecker et al 2018;Jin & Garrod 2020).…”

Efficiency of non-thermal desorptions in cold-core conditions

“…3.1 and 3.2) are not sensitive to this chemistry. We note that our model predictions for COMs cannot be compared to Jin & Garrod (2020) because we have used a different physical model, which is much less dense than theirs. Their chemical model includes new non-diffusive surface mechanisms to enhance the production of COMs and does not include the chemistry of Van Der Waals complexes from Ruaud et al (2015).…”