## Requirement

```
pip install numpy  
pip install astropy  
pip install matplotlib  
```

## Programs

**radiation_fields.py** :  
*Description* :  
This program calculates the scaling factor of the radiation field G0 relative 
to the data of an object or a set of objects radiating for given distances
A file containing my G0 values with the associated distance will then be created

*Input parameters* :  
filename : str,  
        name of the .txt containing intensity (in erg cm-2 s-1 nm-1 sr-1) and wavelength (in nm) data  
    parsec : float,  
        distance in parsec from the star  
    star_radius : float,  
        star radius, in unit of solar radius  
    ISRF : bool,  
        Interstellar Radiation Field; if true, then the filename is a file for ISRF  
        if false, the radiation field of a star is studied  
    RF_list: bool,  
        If false, the calculation of G0 is done for a given distance,
        if true, it is done for a hundred distances whose input distance 
        will correspond to a G0 of the order of 1e6   
        
For the interstellar medium, the parameters star_radius, parsec and RF_list
are no longer important; 
the part of the program executed will be independent of them

**three_levels_model.py** :  
*Description* :  
This program seeks to calculate how a neutral gas subjected to a radiation field 
is heated, involving PAHs in the ionization of the gas. The expressions used are based on the model with 3 energy levels of the molecule, going from the neutral molecule (charge state Z=0) to the dication (charge state Z=2) by ionization.  

*Input parameters* :  
     filename : str,   
            name of the file containing wavelength in nm and intensity of a star or of the interstellar medium  
        star_radius : float,   
            star radius, in unit of solar radius  
        t_gas : float,  
            gas temperature    
        n_e : float,   

            electron density in cm-3 (n_e = n_h * 1.6e-4, n_h : hydrogen density)  

        n_c : float,  
        number of carbon atoms in pah molecules (size of the PAH)      
        parsec : float,   
            distance in parsec from the star  
        fc_pah : float,   
            fraction of cosmic carbon locked in PAHs (default: 0.1)  
        ISRF : bool,   
            Interstellar Radiation Field; if true, then the filename is a file for ISRF
            if false, the radiation field of a star is studied (default: False)  
            

**four_levels_model.py** :  

*Description* :  

This program does the same thing and has the same parameters as the program three_levels_model.py, the difference being that we add expressions related to the 4 energy levels model. This model consists in taking into account the anions, i.e. the PAHs in a charge state Z = -1.

These last two programs use the parameters returned by radiations_fields.py in its calculations

## Useful files

2 files categories are usefull to run the programs :

1. First, cross-section data (in Mb) of each PAH for sizes ranging from minimum 24 C atoms to 66 C atoms with associated energy (in eV) for each charge state of the molecule.  

Here you can find :
    1. the anions file
    2. the neutrals file
    3. the cations file
    4. the dications file
    
2. Then you have the files containing the data of the stars or the interstellar medium for different approximations. Each file has a column with wavelengths (in nm) associated with an intensity (in erg cm-2 s-1 nm-1 sr-1).  
Here you can find :  
    1. ISRF file

    2. stars file

## Authors

The authors of the programs are O. Berné, C. Joblin, S. Foschino and F. Jalabert