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K.K. Tanaka (Tokyo Inst. Tech./U. of Colorado), H. Tanaka (Tokyo Inst. Tech./SwRI), K. Nakazawa (Tokyo Inst. Tech.)
As a promising scenario, tiny refractory metal nuggets found in primitive meteorites, called `Fremdlinges', are believed to be the first condensate in the protoplanetary disk ( e.g., Sylvester et al. 1990, Geochim.~Cosmochim.~Acta. 54. 3491-3508). We investigated the non-equilibrium condensation of refractory metal elements in relation to the origin of Fremdlinges. In this study, instead of the classical nucleation theory, we adopted the semi-phenomenological model, which achieves good agreements with the experiments (Dillmann & Meier, 1991, J.~Chem.~Phys. 94. 3872-3884). In the semi-phenomenological model, the parameter appearing in the expression of the nucleation rate is determined by the second virial coefficient of vapor. In this study, we derived the alternative useful expression which depends on the chemical potential of a dimer since the data of the second virial coefficient is meager for the refractory material.
From the numerical simulations, we found that the condensation temperatures, Tc, of the refractory metal elements are considerably low comparing with the equilibrium condensation temperature, Te. The value of Tc depends largely on the characteristic time of the vapor cooling, \tauT. We found that \Delta T(=Te-Tc) is larger than 300 K for the atoms with large abundance such as Fe, and Ni, while larger than 700 K for the atoms with small abundance, Re, Os, and W in the range of \tauT < 1 \times 105yr at the total pressure of 1 \times 10-5atm. As a result, not only the condensation temperature but also the sequence of the condensation is different from the equilibrium theory. Our results show that it is possible to make Fremdlinges only for the large value of \tauT.