X-ray Chemistry and Radiative Transfer¶

Parameters¶

To activate X-ray chemistry and to configure the stellar X-ray spectrum, the following parameters are relevant:

------ X-rays ------
.true.      ! Xrays      : switch X-rays on?
1.E+30      ! Xray_Lum  [erg/s]  : X-ray luminosity[erg/s]
X-ray [keV]
1.E+7       ! Xray_Temp [K] : X-ray emission temperature [K]
0.1         ! Xray_Emin [keV] : optional: minimum photon energy to be considered as X-rays
62.0        ! Xray_Emax [keV] : optional: max. energy to be considered as X-rays 
1000        ! Xray_wlp   : optional: number of wl points used for the X-Ray spectrum
.false.     ! Xray_norm  : optional: normalization of the spectrum so that lum(0.3-10keV)==Xray_Lum 

The listed values correspond to the default values. in the code (except for Xrays which is .false. by default). The details of the implementation are described in Aresu+ (2011) and Meijerink+ (2012).

If you use StarSpectrum.in, ProDiMo will take what it finds concerning the X-ray luminosity. Therefore, the three parameters Xray_Lum, Xray_Emin, and Xray_Temp have no meaning if StarSpectrum.in is present. An exception is when StarSpectrum.in only contains Eddington fluxes longward of 0.3 Angstrom, then the X-rays are added anyway (see also Stellar properties). If there is no StarSpectrum.in or if X-rays are not included in StarSpectrum.in, then a Bremsstrahlung spectrum of the form

$$F(E)\propto \frac{1}{E} \times \exp\left(-\frac{E}{k\times\mathrm{Xray\_Temp}}\right)$$

is used and scaled to the requested Xray_Lum within the given limits of Xray_Eminto Xray_Emax (or 0.3-10 keV if Xray_norm=.true.).

Optional Parameters¶

The optional parameters Xray_Emax (in keV) and the number of wavelength points Xray_wlp (integer) can be used to control the spectrum and how many wl points are used to sample the spectrum (the default values are given above).

Including the EUV (Extreme-UV) range¶

By default the lower limit for the X-ray energy range is 0.1 keV. However by setting

0.0136      ! Xray_Emin    [keV]    : minimum energy considered for the X-ray chemistry. 

With this setting the EUV energy range (13.6 ev to 100 eV) is included in the X-ray chemistry treatment. This includes the cross-sections data and the radiation field of the star. For the latter simply the Spectrum described above is extended towards the EUV range. If one would like to have a different radiation field in the EUV one can use a custom StarSpectrum.in (see Stellar properties) for more details.

Chemistry¶

When X-rays are activated additional chemical reactions and Species (i.e. double ionised species) are required, and we recommend to also include Ne and Ar in the Elements.in. The required reactions are included in the default Reactions.in.csv and an example Species.in and Elements.in is included in the T Tauri example (see the examples Folder).

Secondary FUV reactions (experimental)¶

The chemical reactions caused by the X-Ray secondary FUV field (the same as for cosmic rays) can be activated by

.true.      ! Xray_FUVumist

In that case, the CP (cosmic ray photons) reactions are duplicated with a new reaction type XU. The reactions are exactly the same but instead of the cosmic ray ionisation rate, the X-Ray ionisation rate for $\mathrm{H_2}$ is used. The implementation follows Meijerink+ (2005).

X-ray radiative transfer (including scattering)¶

The default setting for X-rays and the X-ray chemistry do not include scattering of X-ray photons. This approximations is okay for many applications, however, if one wants to study the disk midplane, in particular with e.g. suppressed cosmic-ray ionization, X-ray scattering can become very important. For a proper treatment of such a scenario one can use the Xray Radiative Transfer treatment included in ProDiMo. We note including X-ray radiative transfer makes the model mor expensive as global iterations are required.

FIXME: Put all the X-ray stuff into one document.