Departure from LTE

Asplund, M. (2005) New light on stellar abundance analyses departures from LTE and homogeneity. Annual Reviews of Astronomy and Astrophysics, 43, 481-530. A review of the determination of stellar abundances using the new three-dimensional modeling of the solar photophere. 

It is not clear that we know very well at all the ionizing spectral energy distribution of Wolf-Rayet stars, or of O stars for that matter. Stellar atmosphere models which incorporate stellar winds, departures from LTE and plane-parallel geometry, and realistic opacities are still in the development stage, and the effective temperature scale of O stars is still in flux (Martins, Schaerer, Hillier 2002). 

In real plasmas, departures from thermal equilibrium often occur. In the extreme case, as encountered in plasmas under reduced pressure, emission or absorption of radiation becomes so important that there is no longer a clear relation between the mean kinetic energies of the species and the excitation temperatures. The latter then lose the physical meaning of temperature. The absence of local thermal equilibrium in these non-LTE plasmas relates to the existence of high field gradients or a.c. fields only the electrons, not the heavy atoms and ions, can follow the field changes and fully take up the dissipated energies. Accord-... 

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