OuterSourceMass
Set this parameter to YES to have a
mass source at the outermost mesh ring. This can be useful if the disk
viscous or imposed drift (see ImposedDiskDrift)
is large. The disk can then be largely depleted at the outer boundary
if
no material is allowed to enter the mesh there. With an outer mass
source, the azimuthally average surface density of the outermost ring
is set to the value given by Sigma0 and SigmaSlope at any instant in time.