Interstellar dust charging in dense molecular clouds: cosmic ray effects

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.