  We propose the possibility of a new phenomenon affecting the settling of dust
grains at the terrestrial region in early protoplanetary disks. Sinking dust
grains evaporate in a hot inner region during the early stage of disk
evolution, and the effects of condensation and evaporation on vertical dust
settling can be significant. A 1-D dust settling model considering both
physical and chemical aspects is presented in this paper. Modeling results show
that dust grains evaporate as they descend into the hotter interior and form a
"condensation front," above which dust-composing major elements, Mg, Si, and
Fe, accumulate, creating a large temperature gradient. Repeated evaporation at
the front inhibits grain growth, and small grain sizes elevate the opacity away
from the mid-plane. Self-consistent calculations including radiative heat
transfer and condensation theory suggest that the mid-disk temperature could be
high enough for silicates to remain evaporated longer than previous estimates.
The formation of a condensation front leads to contrasting settling behaviors
between highly refractory elements, such as Al and Ca, and moderately
refractory elements, such as Mg, Si, and Fe, suggesting that elemental
abundance in planetesimals may not be a simple function of volatility.
