Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach

Abstract

We present the results of chemical modeling of complex organic molecules (COMs) 2 under conditions typical for prestellar cores. We utilize an advanced gas-grain astrochemical model with updated gas-phase chemistry, with a multilayer approach to ice-surface chemistry and an up-to-date treatment of reactive desorption based on recent experiments of Minissale et al. (2016b). With the chemical model, radial profiles of molecules including COMs are calculated for the case of the prototypical prestellar core L1544 at the timescales when the modeled depletion factor of CO becomes equal to that observed. We find that COMs can be formed efficiently in L1544 up to the fractional abundances of 10(-10) wrt. total hydrogen nuclei. Abundances of many COMs such as CH 3 OCH 3 , HCOOCH 3 , and others peak at similar radial distances of 2000-4000 AU. Gas-phase abundances of COMs depend on the efficiency of reactive desorption, which in turn depends on the composition of the outer monolayers of icy mantles. In prestellar cores, the outer monolayers of mantles likely include large fractions of CO and its hydrogenation products, which may increase the efficiency of reactive desorption according to Minissale et al. (2016b), and makes the formation of COMs efficient under conditions typical for prestellar cores, although this assumption is yet to be confirmed experimentally. The hydroxyl radical (OH) appears to play an important role in gas-phase chemistry of COMs, which makes it deserving further detailed studies.

Prevalence of Complex Organic Molecules in Starless and Prestellar Cores within the Taurus Molecular Cloud

“…This reaction requires methanol to have already been chemically desorbed into the gas phase. The chemical link between CH 3 OH and CH 3 CHO is supported by the models of Vasyunin et al 2017, which show strong similarity in radial abundance profiles of both species (see their Figure 8).…”

“…These radicals have been observed in the gas-phase toward prestellar cores (Bacmann & Faure 2016). New models support the idea that precursor molecules of COMs first form on icy surfaces of interstellar grains, and then get ejected into the gas via reactive desorption (Vasyunin & Herbst 2013, Balucani, et al 2015, Vasyunin et al 2017. Unlike other processes, chemical desorption links the solid and gas-phase without immediate interaction with any external agents such as photons, electrons, or other energetic particles; and in this process the newly formed molecule possesses an energy surplus that allows it to evaporate (Minissale et al 2016).…”

Seeds Of Life In Space (SOLIS): The Organic Composition Diversity at 300–1000 au Scale in Solar-type Star-forming Regions*

“…amidogen are the products of hydrogenation on the grain surfaces of CO and N, respectively, occurring during the cold prestellar phase (e.g., Vasyunin et al 2017). The SOLIS observations of the distribution of the formamide abundance compared with other iCOMs can distinguish between the two theories, as they have different formation and destruction timescales, as we show in Codella et al (2017).…”

“…Yet despite the great importance of iCOMs, their routes of formation and destruction are still largely debated (e.g., Herbst & van Dishoeck 2009;Caselli & Ceccarelli 2012, Vasyunin & Herbst 2013Balucani et al 2015;Enrique-Romero et al 2016;Vasyunin et al 2017). The reasons for this impasse are multiple.…”

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

“…Rubin et al (1971);Cazaux et al Kahane et al (2013); Mendoza et al (2014); López-Sepulcre et al (2015); Belloche et al (2017); Ligterink et al (2017); McGuire et al (2018)), in the circumstellar envelopes of AGB stars (Cernicharo et al (2000)), shocked regions (Arce et al (2008); Codella et al (2017); Lefloch et al (2017)) and even in external galaxies (Muller et al (2013)). Despite their presence has been known for decades, how iCOMs are synthesized is still an open question and under debate (Herbst & van Dishoeck (2009) ;Caselli & Ceccarelli (2012); Woods et al (2013); Balucani et al (2015); Butscher et al (2017); Butscher et al (2019); Fedoseev et al (2015); Enrique-Romero et al (2016); Enrique-Romero et al (2019); Gal et al (2017); Rivilla et al (2017); Vasyunin et al (2017); Rimola et al (2018); Lamberts et al (2019)). Two different paradigms have been proposed: i) on the surfaces of grains (either during the cold prestellar or warmer collapse phase (e.g.…”