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The evolution of disks around Young Stellar Objects (YSOs) is deeply affectedby the YSOs ultraviolet (UV) radiation field especially in the 500-1100 \AA\spectral range. The two dominant processes are; the photo-dissociation of H_2molecules in the Werner and Lyman bands, and the emission of photo-electronsfrom dust grains when high energy photons are absorbed. Photo-electrons are animportant source of gas heating. Dust grain charging when exposed to variouspossible UV fields in the YSOs environment is investigated. Numericalsimulation of the evolution of photo-electrons in the electric field created bythe charged dust grains are carried out to obtain a charging profile. From thesimulations it appears that the different spectra produce significantquantitative and qualitative different charging processes. Both the UVbackground and the Ae-Herbig star radiation field produce a relatively slowcharging of dust grains due to the low fraction of sufficiently energeticphotons. The radiation field of T Tauri Stars (TTSs) is harder due to therelease of magnetic energy in the dense magnetospheric environment. Thesenumerical results have been used to propose a new simple analytical model forgrain charging in the atmosphere of protostellar disks around TTSs susceptibleto be used in any disk modeling. It has been found that the yield decreasesexponentially with the dust charge and that two populations of photoelectronsare produced. The high energy population is susceptible of dissociating the H_2and ionizing some low ionization potential species, such as the Mg. Theseresults add an additional role to dust on the chemistry of the layers justbelow the H_2 photoionization front. This photoelectic yield has been appliedto a simple evaluation of the dust charge in the atmospheres of accretion disksand a simple evaluation of the dust charge in the atmospheres of accretiondisks.

Photoelectric charging of dust grains in the environment of Young Stellar Objects