New in version 2.2 is the ability to have the gas density variable as a power law in
radius, i.e.
, where
and
are the density and
radius at the cloud illuminated face. NB Note that this creates the possibility
for calculations which do not end. This because the citeria for a model to end are either
that the input column density is reached, or that the electron fraction falls below xeemin.
If radexp has a value
-1 then the column density integral converges only logarithmically
at best, and the specified column may never be reached. If radexp
-2 the local ionization
parameter will increase with radius, and so the gas will not recombine and the xeemin
criterion will not be met.