Interstellar dust charging in dense molecular clouds: cosmic ray effects
  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.
