X-ray Radiative Transfer

The X-ray radiative transfer can be activated in addition to the X-ray chemistry. The X-Ray radiation field is then calculated during the dust radiative transfer part in the same way as for the other wavelengths. The X-ray RT requires global iterations and is therefore more computationally expensive (see Sect. Comments on Performance).

To use the X-ray RT within ProDiMo, you need to install the xraylib library first and recompile the code, see Installation of xraylib.

More details on the ProDiMo X-ray RT module can be found in Rab+ 2018. Please cite this paper if you use X-ray RT for your modelling.

To enable the X-Ray RT, simply set:

.true.     ! XrayRT     : do Xray radiative transfer (including scattering) ?

In addition to the parameters for the X-ray chemistry.

Warning: For the moment, the X-ray RT does not work with Rphoto_bandint enabled, so please set it to .false. if you want to use this module.

Other parameters

There are a couple of other (technical) parameters that allow for some more control of the X-ray RT mode, but are optional

20      ! NXray       : Number of wl bands used for the X-Ray regime <912 A  (including EUV)
4       ! XrayRT_opac : which variant of the X-Ray opacities should be used
.true.  ! XrayRT_chem : couple Xray_RT to chemistry (if the X-ray RT radiation field is not used for the chemistry)

The values listed above are the default values (i.e., if one only sets XrayRT). The parameter XrayRT_chem is mostly for testing purposes (should always be true if X-ray RT is used).

X-ray cross-sections/opacities

Gas opacities

There are different options for the X-Ray gas opacities (XrayRT_opac):

     1 ..... the same as used by Aresu (only absorption)
     2 ..... the same absorption opacities as Aresu + scattering
              opacities for these species from xraylib
     3 ..... the same as 2 but with pseudo anisotropic scattering
     4 ..... absorption cs and scattering cs from xraylib
     5 ..... absorption cs from Aresu scattering only for H and H2
     6 ..... the same as 1 but set the cs for the EUV bands to zero
     10 .... use Verner1996 for the absorption cs
             (already the sigma_* are filled with this -> also
             the cs for the implementation of Aresu are affected)

Most of them are only intended for testing. The default value is 4; to select different gas opacities, use the parameter XrayRT_opac.

4          ! XrayRT_opac     : which variant of the X-ray Opacities should be used

Dust opacities

To include dust opacities for the X-Ray regime, a new dust opacity list file dust_opacity_list2X.txt has to be used. This can be done by setting the parameter dust_opacity_list_file. Further, the parameter XrayRT_dust_opac has to be set to true.

dust_opacity_list2X.txt   ! dust_opacity_list_file  : the dust opacity list file including X-ray opacities
.true.                    ! XrayRT_dust_opac        : activate dust opacities in the X-ray wavelength regime

There are X-ray dust opacities available for most dust species defined in dust_opacity_list2.txt except for the Oss_Draine species. However, we note that those have been constructed in an approximate way, and there is no guarantee that they are correct (only the AstroSilicates have been verified).

TODO: more details on the opacities, and how they have been created (citation to Draine etc. ).

EUV

The X-ray implementation also allows to include the EUV (Extreme-UV) energy range, for details see X-ray chemistry. If one wants to include the EUV with X-ray RT one should use:

3       ! XrayRT_opac : recommended setting for including EUV in the X-ray RT

as those opacities are better suited for the EUV range (there is a check in the code anyway).

X-Ray Background field

To enable an X-Ray background radiation field (which extends the background field already used in ProDiMo), the parameter Xray_Bg_flux has to be set to a value greater than zero.

1.E-5      ! Xray_Bg_Flux [erg cm^-2 s^-1] : total background flux for the X-Ray regime
4.E7       ! Xray_Bg_Temp [K] Xray Emission Temperature for the Background (default: 4.E7)

X-Ray emission is then added to the incident intensities (for the spectral shape same method as for the star is used, parameter Xray_Bg_Temp). Typical values for the flux can be found in Adams+ 2012. The default value for Xray_Bg_Temp produces a spectrum with a typical hardness for T Tauri stars. For more details, see Rab+ 2018.

X-Ray dust heating

The dust heating by X-Rays can be enabled with the parameter Xray_dust:

.true.      ! XrayRT_dust

However, for the moment, it is assumed that all of the absorbed energy of an X-Ray photon is used for dust heating (see Dwek & Smith 1996). For the moment, this is only an experimental feature, so this option should not be used for real models.

Comments on Performance

If X-Ray radiative transfer is enabled, more computational time is needed for the radiative transfer part (depends on the number of wavelength bands used for X-Rays). Further, global iterations are needed (at least 3) if a fixed disk structure is used. This is necessary as the gas opacities are changed by the chemistry. For a hydrostatic model, the global iterations are used anyway.

An efficient way to run an X-ray RT model is to first run a model without X-ray RT to e.g. calculate the dust temperature and UV-radiation field. Using the restart option of ProDiMo, it is then possible to run the radiative transfer only for the X-ray regime by setting the radtrans option to .false.. This also works if one runs a ProDiMo model on top of an MCFOST or MCMax model.