Low-temperature Condensation of Carbon

TL;DR

This study investigates low-temperature carbon condensation processes in space, revealing that vaporized graphite forms high-temperature, partially graphitized dust, and that in dense regions, carbon atoms contribute to complex organic molecule formation without water involvement.

Contribution

It provides new insights into low-temperature carbon condensation mechanisms and the formation of complex organic molecules in the interstellar medium, highlighting the role of molecular hydrogen.

Findings

Vaporized graphite condenses into partially graphitized carbon at high temperatures.

Carbon atoms react with H$_2$ and CO to form complex organic molecules.

Water molecules do not directly participate in the formation of complex organics.

Abstract

Two different types of experiments were performed. In the first experiment, we studied the low-temperature condensation of vaporized graphite inside bulk liquid helium, while in the second experiment, we studied the condensation of single carbon atoms together with H$_2$, H$_2$O, and CO molecules inside helium nanodroplets. The condensation of vaporized graphite leads to the formation of partially graphitized carbon, which indicates high temperatures, supposedly higher than 1000{\deg}C, during condensation. Possible underlying processes responsible for the instant rise in temperature during condensation are discussed. This suggests that such processes cause the presence of partially graphitized carbon dust formed by low-temperature condensation in the diffuse interstellar medium. Alternatively, in the denser regions of the ISM, the condensation of carbon atoms together with the most abundant interstellar molecules (H$_2$, H$_2$O, and CO), leads to the formation of complex organic molecules (COMs) and finally organic polymers. Water molecules were found not to be involved directly in the reaction network leading to the formation of COMs. It was proposed that COMs are formed via the addition of carbon atoms to H$_2$ and CO molecules $(C + H_2 \rightarrow HCH, HCH + CO \rightarrow OCCH_2)$. Due to the involvement of molecular hydrogen, the formation of COMs by carbon addition reactions should be more efficient at high extinctions compared with the previously proposed reaction scheme with atomic hydrogen.