  The outward migration of a pair of resonant-orbit planets, driven by tidal
interactions with a gas-dominated disk, is studied in the context of evolved
solar nebula models. The planets' masses, M1 and M2, correspond to those of
Jupiter and Saturn. Hydrodynamical calculations in two and three dimensions are
used to quantify the migration rates and analyze the conditions under which the
outward migration mechanism may operate. The planets are taken to be fully
formed after 1e6 and before 3e6 years. The orbital evolution of the planets in
an evolving disk is then calculated until the disk's gas is completely
dissipated. Orbital locking in the 3:2 mean motion resonance may lead to
outward migration under appropriate conditions of disk viscosity and
temperature. However, resonance locking does not necessarily result in outward
migration. This is the case, for example, if convergent migration leads to
locking in the 2:1 mean motion resonance, as post-formation disk conditions
seem to suggest. Accretion of gas on the planets may deactivate the outward
migration mechanism by raising the mass ratio M2/M1 and/or by reducing the
accretion rate toward the star, hence depleting the inner disk. For migrating
planets locked in the 3:2 mean motion resonance, there are stalling radii that
depend on disk viscosity and on stellar irradiation, when it determines the
disk's thermal balance. Planets locked in the 3:2 orbital resonance that start
moving outward from within 1-2 AU may reach beyond 5 AU only under favorable
conditions. However, within the explored space of disk parameters, only a small
fraction - less than a few per cent - of the models predict that the interior
planet reaches beyond 4 AU.
