Condensation front migration in a protoplanetary nebula

Condensation-front dynamics is investigated in the solar nebula. A quasi-steady model of the evolving nebula is combined with equilibrium vapor pressure curves to determine evolutionary sublimation fronts for water and ammonia. A simple one-dimensional model includes local viscous and luminescent heating, allowing analytical solutions. The study is extended to two-dimensional protoplanetary disks including grain heating in the disk photosphere and vertical radiative transfer. Both approaches show the fronts migrating inward from the far nebula to final positions in the inner nebula during a period of 10(7) yr. The center-plane sublimation fronts are shown to evolve in a similarmanner, but the two-dimensional model predicts a more complex two-branched structure. One branch forms an elongated gaseous region below the density scale height, and the second branch places a sublimation front in the low-density region near the disk photosphere. Along the center plane, the fronts initially propagate much faster than the nebular viscous accretion velocity, but eventually the accreting gas and dust overtake the slowing condensation front.