Chemical history of molecules in circumstellar disks

TL;DR

This paper introduces a new 2D astrochemical model that traces the chemical evolution from molecular clouds through disk formation, revealing stratification and organic molecule formation mechanisms relevant for planet formation.

Contribution

The model uniquely incorporates 2D collapse dynamics to connect molecular cloud chemistry with disk composition, improving understanding of chemical inheritance in planet-forming regions.

Findings

Chemical stratification in disks due to different physical conditions.

Disk-envelope accretion shock has limited impact on disk material.

Complex organics form on grain surfaces at 20-40 K.

Abstract

The chemical composition of a protoplanetary disk is determined not only by in situ chemical processes during the disk phase, but also by the history of the gas and dust before it accreted from the natal envelope. In order to understand the disk's chemical composition at the time of planet formation, especially in the midplane, one has to go back in time and retrace the chemistry to the molecular cloud that collapsed to form the disk and the central star. Here we present a new astrochemical model that aims to do just that. The model follows the core collapse and disk formation in two dimensions, which turns out to be a critical upgrade over older collapse models. We predict chemical stratification in the disk due to different physical conditions encountered along different streamlines. We argue that the disk-envelope accretion shock does not play a significant role for the material in the disk at the end of the collapse phase. Finally, our model suggests that complex organic species are formed on the grain surfaces at temperatures of 20 to 40 K, rather than in the gas phase in the T>100 K hot corino.