The proper modeling of stellar atmospheres is the backbone of any quantitative spectroscopic analysis.
Hybrid LTE/non-LTE approach
To analyze spectra of early-type stars close or below the main sequence, in particular those of spectral type B, we make use of the so-called hybrid (or ADS) approach, which is based on the idea that non-local thermodynamic equilibrium (non-LTE) effects are important for the details of radiative transfer but negligible for the atmospheric structure. The following codes are applied for this purpose:
- ATLAS12: Kurucz’ code computes a plane-parallel, homogeneous, hydrostatic, and line-blanketed atmospheric structure in LTE.
- DETAIL (extended and updated by K. Butler): Based on the fixed ATLAS12 atmosphere, the coupled radiative transfer and statistical equilibrium equations are solved to obtain the radiation field and the occupation number densities in non-LTE.
- SURFACE (extended and updated by K. Butler): Using the atmospheric structure from ATLAS12, the occupation numbers from DETAIL, and a considerably finer frequency grid as well as more detailed line profiles than doable with the DETAIL code, SURFACE computes the final synthetic spectrum which can be compared to observations.
The hybrid approach is one of the most sophisticated in terms of atomic input physics and, at the same time, one of the most efficient in terms of computing time because numerical resources are focused on dealing with very complex model atoms rather than on numerically expensive non-LTE effects on the atmospheric structure.