The evolution of disks around Young Stellar Objects ( YSOs ) is deeply affected by the YSOs ultraviolet ( UV ) radiation field especially in the 500-1100 Å spectral range .
This 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 .
In this letter , 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 the charging profile of dust grains .
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 : a low energy population with mean kinetic energy E = 2.5 eV and a high energy population with E = 5.5 - 6 eV ; the energy dispersion within the populations is \sim 1.3 eV ( T \sim 1.5 \times 10 ^ { 4 } K ) .
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 ( \alpha -disks ) .