Disk evaporation

For the PhD project of @pgriveaud we have Jupiter and Saturn in resonant configurations. We want to simulate the final phases of the disk introducing a simple evaporation recipe at time t_0:

\rho (t) = \rho_0 exp(t-t0)/T

with T a parameter of the order 100 orbits giving the "evaporation time scale".

The prescription is very simple and does not want to mimic the complicate disk evaporation, the idea is only to remove smoothly the gas , i.e. the effect of the disk potential on the planets, in order to have planetary orbits that can be used as initial condition for the dynamical phase after disk dispersal.