Complex organic molecules along the accretion flow in isolated and externally irradiated protoplanetary disks

Abstract

The birth environment of the Sun will have influenced the physical and chemical structure of the pre-solar nebula, including the attainable chemical complexity reached in the disk, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network which includes gas-phase reactions, gas-grain interactions, and thermal grain-surface chemistry. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major chemical alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs can efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot efficiently synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated disks with the latter composed of more simple ices.

Formation of Glycerol through Hydrogenation of CO Ice under Prestellar Core Conditions

“…Gasphase reactions are not efficient enough to explain many of the observed COM abundances, and it is generally accepted that such species form on the surfaces of icy dust grains by merging smaller species to larger and larger constituents. This idea, supported by numerous laboratory results and astrochemical simulations, indicates that surface reactions, triggered by accreting atoms, impacting cosmic rays and VUV photons, as well as by thermal processing, provide efficient pathways toward molecular complexity (Charnley & Rodgers 2008;Garrod et al 2008;Vasyunin & Herbst 2013b;Walsh et al 2014aWalsh et al , 2014bLinnartz et al 2015;Öberg 2016).…”

“…It rather complements these, showing that COMs of astrobiological importance can be formed as early as the cold dark cloud stage, during the CO freeze-out stage, and well before formation of the protostar and heating of the dust occurs. COMs produced on icy dust grains in this early stage of star formation can, in turn, be locked on the grain surface and participate in active energetic processing during later stages, or even be released into the gas phase by shocks, heat, or photodesorption (Charnley & Rodgers 2008;Caselli & Ceccarelli 2012;Walsh et al 2014aWalsh et al , 2014bJiménez-Serra et al 2016). Their efficient release into the gas phase by heat and photodesorption is unlikely, however, due to the low volatility of such complex species and a tendency of even the simplest COMs (e.g., CH 3 OH) to photodissociate and desorb as fragments, rather than as an intact molecule (Bertin et al 2016).…”

Methanol along the path from envelope to protoplanetary disc

“…More recently, Walsh et al (2014a) modelled the abundances of complex organic molecules along multiple streamlines in a disc. The authors found that methanol was preserved along the midplane, prior to crossing of its snowline.…”

A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules

“…However, even these gasgrain networks could not replicate the observed abundances of COMs, implying that key production pathways to COMs are still elusive. The crucial drawback of existing models is the assumption of an ice mantle of which only the first few monolayers participate in an active chemistry via diffusion-limited thermal surface reactions (11). This conjecture limits the validity of preceding gas-grain reaction networks because laboratory studies provided compelling evidence that an interaction of ionizing radiation in the form of cosmic rays and their secondary electrons with interstellar ices forms COMs as complex as sugars (12), amino acids (13), and dipeptides (14).…”