PAH heating/cooling and PAH chemistry

TODO: merge PAH pages into one single page PAH_heating_and_chemistry, PAH_size, PAH_chemical_network

By default, ProDiMo calculates the PAH heating rate (photoeffect) and recombination cooling rate by formula of Bakes & Tielens (1994) ApJ 427, 822. Since these rates are calculated in interstellar radiation condistions, there is a color correction involved BT94 (Bastiaan Jonkheid private communication) which is, however, only valid for blackbody irradiation Bnu(Teff).

Change these default settings in Parameter.in as

 0.1        ! fPAH      [-]           : PAH abundance with respect to ISM
 .false.    ! colour_correct_from_RT  : PAH colour correction from RT
 .false.    ! PAH_from_RT             : PAH heating from RT

We have added a module to calculate this color correction explicitly, in the radiation field provided by ProDiMo's dust RT, by using the neutral cross sections of the typical PAH molecule circumcoronene (NC=54, NH=18) from Draine & Li (2007 ApJ 657, 810). Activate this treatment with colour_correct_from_RT=.true.

There is a further option PAH_from_RT=.true.. If you switch this on, the fractional charge of the PAHs (PAH-, PAH, PAH+, PAH++, PAH+++) is calculated consistently with the chemistry. You need to copy Elements.in, Species.in, and Reactions.in (or better Reactions_ChargeExchange.in) from ProDiMo/input_examples/smallPAH_chemistry to include these additional specimen and to tell ProDiMo the ISM abundance of PAHs. This Reactions.in includes the collisional and photo-ionisation reactions, and the recombination rates considered. It's not a full PAH chemistry, but only a small reaction network to calculate the charging of circumcoronene. The total PAH abundance is still uniformly given by the initial abundance value specified in Species.in (10-6.52), multiplied by fPAH.

If PAH_from_RT=.true., the PAH heating rate will be calculated accordingly, using the cross-sections and resultant abundances of charged circumcoronene. The recombination cooling will also use the results of PAH "chemsitry" to compute the cooling rate.