NLTE¶
NLTE [1] calculations are important to accuarately fit certain lines. PySME supports them using pre-computed grids of NLTE departure coefficients, which need to be created for every element. For common elements PySME provides grids via the LFS (see Large File Server). If any of these grids are used, please kindly take care to cite the papers describing the NLTE models and departure coefficient calculations.
If you want to provide your own NLTE grid files, they should be present in ~/.sme/nlte_grids.
NLTE calculations need to be specified for each element they are supposed to be used for individually using ‘sme.nlte.set_nlte(el, grid)’. Similarly they can be disabled for each element using ‘sme.nlte.remove_nlte(el)’, where sme is your SME structure. If no element is set to NLTE in the structure PySME will perform LTE calculations only.
Furthermore a long format linelist is required for NLTE calculations. If only a short format has been given, then the calculations will only be in LTE as well. (See Linelist)
The NLTE object has the following fields
- elements:
The elements for which NLTE has been activated
- grids:
The grid file that is used for each active element
- subgrid_size:
A small segment of the NLTE grid will be cached in memory to speed up calculations. This sets the size of that cache by defining the number of points in each axis (rabund, teff, logg, monh).
- flags:
After the synthesis all lines are flaged if they used NLTE
Grid interpolation¶
The grid has 6 dimensions.
- teff:
Effective Temperature
- logg:
Surface Gravity
- monh:
Overall Metallicity
- rabund:
relative abundance of that element
- depth:
optical depth in the atmosphere
- departure coefficients:
The NLTE departure coefficients describing how much it varies from the LTE calculation
We then perform linear interpolation to the stellar parameters we want to model. And we then perform a cubic spline fit to the depth scale of the model atmosphere we specified (See Atmosphere).
We then use the linelist to find only the relevant transitions in the grid, and pass the departure coefficients for each line to the C library.