Interstellar dust charging in dense molecular clouds: cosmic ray effects

TL;DR

This study investigates how cosmic rays influence dust grain charging in dense molecular clouds, revealing that CR-induced photoemission can promote dust coagulation by reducing Coulomb repulsion, with implications for astrophysical environments.

Contribution

It introduces the effects of cosmic ray-induced photoelectric emission on dust charging, a mechanism previously neglected, and explores its impact on dust coagulation in molecular clouds.

Findings

CR-induced photoemission significantly alters dust charge distribution.

Enhanced dust coagulation occurs in specific density regions due to reduced Coulomb repulsion.

The effects are likely relevant in various astrophysical environments beyond molecular clouds.

Abstract

The local cosmic-ray (CR) spectra are calculated for typical characteristic regions of a cold dense molecular cloud, to investigate two so far neglected mechanisms of dust charging: collection of suprathermal CR electrons and protons by grains, and photoelectric emission from grains due to the UV radiation generated by CRs. The two mechanisms add to the conventional charging by ambient plasma, produced in the cloud by CRs. We show that the CR-induced photoemission can dramatically modify the charge distribution function for submicron grains. We demonstrate the importance of the obtained results for dust coagulation: While the charging by ambient plasma alone leads to a strong Coulomb repulsion between grains and inhibits their further coagulation, the combination with the photoemission provides optimum conditions for the growth of large dust aggregates in a certain region of the cloud, corresponding to the densities $n(\mathrm{H_2})$ between $\sim10^4$ cm$^{-3}$ and $\sim10^6$ cm$^{-3}$. The charging effect of CR is of generic nature, and therefore is expected to operate not only in dense molecular clouds but also in the upper layers and the outer parts of protoplanetary discs.