The Fate of Formamide in a Fragmenting Protoplanetary Disk

David Quénard,Cassandra Hall,Ken Rice,Izaskun Jiménez-Serra,Duncan H Forgan,John D Ilee

doi:10.3847/1538-4357/aae4dd

Abstract

Recent high-sensitivity observations carried out with the Atacama Large Millimeter Array have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH3CN) and methanol (CH3OH) in relatively evolved protoplanetary discs. The behavior and abundance of COMs in earlier phases of disk evolution remain unclear. Here, we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disk with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH2CHO), a prebiotic species, in both the disk and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disks where the temperatures are <100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas phase in three fragments as a result of the high temperatures (≥200 K), but remains in the solid phase in one colder (≤150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core that is formed would inherit their formamide content directly from the protosolar nebula.

Highlights

The origin of the prebiotic content at the surface of the primitive Earth remains unclear
The total amount of formamide does not vary significantly with time, since the bulk of the formamide has already been produced during the protostellar phase
The formamide abundance in the gas phase or solid phase is mainly driven by the temperature, leading either to freeze-out or to desorption

Summary

Introduction

The origin of the prebiotic content at the surface of the primitive Earth remains unclear. It is postulated that a large quantity of prebiotic material might have been brought to Earth by comets and meteorites during its early formation (Caselli et al 2012) This hypothesis is supported by the detection of numerous prebiotically relevant molecular species in comets such as Hale-Bopp (Bockelée-Morvan et al 2000), Lemmon and Lovejoy (Biver et al 2014), and more recently 67P/ Churyumov–Gerasimenko (Goesmann et al 2015; Altwegg et al 2017). Among these species is formamide (NH2CHO), an important precursor of pre-genetic and pre-metabolic compounds such as nucleic acids, nucleobases, sugars, and amino acids (see Saladino et al 2012). A large proportion of these COMs are thought to remain frozen onto dust grains, making their detection in the gas phase challenging (see, e.g., Walsh et al 2014)

Methods

Results

Discussion

Conclusion