INTERSTELLAR DUST CHARGING IN DENSE MOLECULAR CLOUDS: COSMIC RAY EFFECTS

Abstract

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