Condensation of Forsterite and Metallic Iron around Evolved and Young Stars

Introduction: Recent observation by infrared spectroscopy has revealed the presence of amorphous and/or crystalline silicate dusts around evolved stars or young star discs [e.g., 1-4]. The species, size, shape, composition, and orientation if they are crystalline, of the dusts control the optical properties of dusts, which further control the radiation field of discs. Thus, the formation and growth of dusts are fundamental processes for the evolution of circumstellar and young star discs. Condensation is a kinetic process that comprises nucleation and grain growth. In both atomistic processes, " sticking coefficient " plays a key role in the nucleation and growth rates, which linearly affects the rate. The coefficient is a number of atoms incorporated into the condensed phase relative to number of colliding atoms to surface of the condensed phase, which ranges from 0 to 1. Therefore, experimental determination of condensation coefficient is crucial for consideration on the evolution time scale of disks. Condensation coefficients: The condensation rate, J, is shown by the Hertz-Knudsen equation in the kinetic theory of gas molecules in general form of balance between forward and backward reactions, J = (α cond P-α evap P eq)/(2πmkT) 1/2 , where α cond is the condensation coefficient, P is the pressure, α evap is the evaporation coefficient, P eq is the equilibrium vapor pressure of the species concerned, m is the weight of gas molecule, k is the Boltzman constant, and T is the temperature. The rate also represents the evaporation coefficient when P eq >P. Except for vacuum (P=0), experiments give the net condensation or evaporation coefficients and the two coefficients are not obtained independently. Because of experimental difficulties, we have very limited data and only vacuum evaporation coefficients for forsterite, silicate melts with a few different compositions, and metallic iron have been obtained so far. In the model calculation, [5] assumed α of unity for all the species at all the conditions, whereas, [6-9] used constants of 0.1 for forsterite and enstatite and 1 for metallic iron by assuming that condensation coefficients are the same as evaporation coefficients. Experiments: We have previously investigated the evaporation coefficient of metallic iron at various ambient pressures from vacuum to equilibrium, and