Photoelectric charging of dust grains in the environment of young stellar objects

Abstract

The evolution of disks around young stellar objects (YSOs) is deeply affected by the YSOs' ultraviolet (UV) radiation field especially in the 500-1100 angstrom spectral range. The two dominant processes are: the photodissociation of H(2) molecules in the Werner and Lyman bands, and the emission of photoelectrons from dust grains when high energy photons are absorbed. Photoelectrons are an important source of gas heating. In this paper, dust grain charging when exposed to various possible UV fields in the YSOs' environment is investigated. Numerical simulations of the evolution of photoelectrons 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 differences in the 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 similar to 1.3 eV (T similar to 1.5 x 10(4) K). The high energy population is susceptible of dissociating the H2 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).