We present a theoretical investigation of non-equilibrium condensation of refractory metals in the primordial solar nebula, in relation to the origin of "Fremdlinges" included in CAIs. To describe the nucleation process of grains from vapor, we adopted a semi-phenomenological model modified from the classical nucleation theory by the introduction of the second virial coefficient of vapor. This model achieves excellent agreement with nucleation rate experiment. However, the second virial coefficients are unknown for a vapor of refractory metals. To overcome this, we express the nucleation rate by the use of the chemical potential of dimers instead of the second virial coefficient. On the basis of this new nucleation theory, we have performed numerical simulations of non-equilibrium condensation of refractory metals and find that their condensation temperatures, Tc, decrease considerably in comparison with equilibrium condensation. Even if the characteristic cooling time scale is as large as 1 × 105 years, the decrease in Tc is from 200 to 400 K for rare elements such as W, Re, and Os. This remarkable non-equilibrium behavior mainly stems from the low total pressure in the primordial solar nebula. From our new model we also obtain the typical size of grains formed in condensation. We find that the cooling time should be ≳1 × 105 years for sub-micron-sized or larger refractory metal nuggets to form.
- Solar nebula
- Thermal histories