NLTE Radiative Transfer

One of our main goals is to understand the physics of stars, in particular, what physical processes govern energy transport in stellar atmospheres and shape their emergent radiation spectra, from UV to the Infra-Red.  

Spectrum tools

One of our major achievements over the past years is the currently largest database of NLTE abundance corrections for different chemical elements that can be accessed online at:

This database allows the user to interactively compute NLTE abundance corrections for over 10.000 spectral lines of O I, Mg I, Fe I, Si I, Co I, Ti I, Mn I on the fly, or even request grid calculations for large numbers of stars.

NLTE spectroscopy with mean 3D models

We have also devised the novel approach of <3D> NLTE spectroscopy, which allows to massively apply non-local thermodynamic equilibrium (NLTE) models with 3D hydrodynamics simulations of stellar convection in the determination of stellar parameters and abundances. The <3D> NLTE spectroscopy is a rapidly developing research direction and our ambition is to make the new <3D> NLTE modelling (the image to the left) a new standard in modern observational astrophysics, superseeding the classical 1D LTE spectra based on hydrostatic model atmosheres, such as the Kurucz models. 

We approach the goal by developing new software for massive calculations of <3D> NLTE spectral grids in collaboration with our colleagues at ANU (M. Asplund),  Aarhus (R. Collet), and Copenhagen (Z. Magic).

Atomic models

Our atomic models demand heavy amounts of quantum-mechanical and experimental atomic data. However, these models allow to determine chemical composition of stars with much higher accuracy compared to previous models, greatly increasing the number of useful diagnostic features in a spectrum.

With the NLTE models we can now be confident that we will get consistent and un-biased answers from the UV, optical, and IR observations. This has a direct impact on observing programs and on the design of new generation of astronomical surveys: we can now observe much fainter and distant objects, instead of trying to gain a larger coverage in wavelength at the expense of going deeper into space, which has been done so far. 

The image to the left shows a theoretical spectrum of Betelgeuse, the best-studied red supergiant with surface temperature of roughly 3500 degrees K. The enormous absorption in the optical part of the spectra is caused by molecular transitions, e.g. in the TiO molecule. In contrast, the near-IR and IR parts of the spectra are much cleaner and carry interesting information in the form of atomic spectral lines (the image to the right).