  The evolution of disks around Young Stellar Objects (YSOs) is deeply affected
by the YSOs ultraviolet (UV) radiation field especially in the 500-1100 \AA\
spectral range. The two dominant processes are; the photo-dissociation of H_2
molecules in the Werner and Lyman bands, and the emission of photo-electrons
from dust grains when high energy photons are absorbed. Photo-electrons are an
important source of gas heating. Dust grain charging when exposed to various
possible UV fields in the YSOs environment is investigated. Numerical
simulation of the evolution of photo-electrons in the electric field created by
the charged dust grains are carried out to obtain a charging profile. From the
simulations it appears that the different spectra produce significant
quantitative and qualitative different charging processes. Both the UV
background and the Ae-Herbig star radiation field produce a relatively slow
charging of dust grains due to the low fraction of sufficiently energetic
photons. The radiation field of T Tauri Stars (TTSs) is harder due to the
release of magnetic energy in the dense magnetospheric environment. These
numerical results have been used to propose a new simple analytical model for
grain charging in the atmosphere of protostellar disks around TTSs susceptible
to be used in any disk modeling. It has been found that the yield decreases
exponentially with the dust charge and that two populations of photoelectrons
are produced. The high energy population is susceptible of dissociating the H_2
and ionizing some low ionization potential species, such as the Mg. These
results add an additional role to dust on the chemistry of the layers just
below the H_2 photoionization front. This photoelectic yield has been applied
to a simple evaluation of the dust charge in the atmospheres of accretion disks
and a simple evaluation of the dust charge in the atmospheres of accretion
disks.
