CHARGING OF DUST GRAINS IN ASTROPHYSICAL ENVIRONMENTS BY SECONDARY ELECTRON EMISSIONS

The dominant charging processes in various astrophysical environments are considered to be by photoelectric emissions with radiation from nearby stars and secondary electron emissions (SEE) by impact of electrons in a medium with sufficiently high energies. The charging of bulk materials with planar surfaces by SEE with electron impact appears to be well understood with theoretical expressions as well as by experimental techniques. However, the charging of submicron/micron-size dust grains by SEE with sufficiently high-energy electrons is a complex process, and is a function of electron energies, the electron current, and the grain size, and the charge or the surface potential. Development of viable theoretical models and acquisition of experimental data for charging properties of micron-size dust grains are still in the early stages. This paper focuses on SEE charging properties of individual micron-size dust grains by low-energy electron impact, obtained from laboratory measurements on an experimental facility based on an electrodynamic balance. The measurements of SEE yields of positively charged dust grains indicate the yields increase with decreasing grain size and the equilibrium surface potentials showing generally linear size dependence. These experimental results are generally in agreement with several independent experimental and analytical model studies in the literature, with the exception of a recently published paper which is in fundamental conflict with our studies as well as with several other experimental and theoretical studies. The sources and causes of this conflict are critically examined and discussed in this paper.