Chemical complexity in protoplanetary disks in the era of ALMA and Rosetta

TL;DR

This paper explores the chemical complexity in protoplanetary disks and comets, using observations from ALMA and Rosetta, combined with astrochemical modeling, to understand the origins of complex organic molecules.

Contribution

It presents a model showing that simple ices inherited from molecular clouds can develop complex molecules through cosmic-ray and X-ray processing, aligning with comet observations.

Findings

Cosmic-ray and X-ray processing can produce complex organics from simple ices.

Gas-phase CH3CN is abundant in warm disk regions, complicating its use as an ice reservoir tracer.

Model results agree with observed cometary molecules, supporting a disk origin hypothesis.

Abstract

Comets provide a unique insight into the molecular composition and complexity of the material in the primordial solar nebula. Recent results from the Rosetta mission, currently monitoring comet 67P/Churyumov-Gerasimenko in situ, and ALMA (the Atacama Large Millimeter/submillimeter Array), have demonstrated a tantalising link between the chemical complexity now confirmed in disks (via the detection of gas-phase CH3CN; Oberg et al. 2015) and that confirmed on the surface of 67P (Goesmann et al. 2015), raising questions concerning the chemical origin of such species (cloud or inheritance versus disk synthesis). Results from an astrochemical model of a protoplanetary disk are presented in which complex chemistry is included and in which it is assumed that simple ices only are inherited from the parent molecular cloud. The model results show good agreement with the abundances of several COMs observed on the surface of 67P with Philae/COSAC. Cosmic-ray and X-ray-induced photoprocessing of predominantly simple ices inherited by the protoplanetary disk is sufficient to generate a chemical complexity similar to that observed in comets. This indicates that the icy COMs detected on the surface of 67P may have a disk origin. The results also show that gas-phase CH3CN is abundant in the inner warm disk atmosphere where hot gas-phase chemistry dominates and potentially erases the ice chemical signature. Hence, CH3CN may not be an unambiguous tracer of the complex organic ice reservoir. However, a better understanding of the hot gas-phase chemistry of CH3CN is needed to confirm this preliminary conclusion.