nonLTE

Non-LTE, or non-Local Thermodynamic Equilibrium, describes conditions in which the populations of atomic or molecular energy levels, and the ionization balance, do not follow the Boltzmann and Saha distributions set by a single local temperature. In many astrophysical environments—hot, low-density stellar atmospheres, planetary nebulae, H II regions, and the interstellar medium—the radiation field decouples from the local kinetic temperature, so radiative processes compete with or dominate collisional processes.

Under LTE, collisions efficiently thermalize level populations and the radiation field approximates a Planck function at the local temperature. In non-LTE, this is not the case; level populations and ionization can be driven by the ambient radiation field and by nonthermal processes, leading to departures from Boltzmann-Saha distributions. These departures affect how spectral lines form and how energy is distributed among levels.

Modeling non-LTE requires solving the coupled statistical equilibrium equations for detailed atomic or molecular level populations together with the radiative transfer equation for the radiation field. Departure coefficients describe how actual populations differ from LTE values. Because radiative and collisional transition rates across many levels must be included, non-LTE calculations are computationally demanding and rely on accurate atomic and molecular data such as transition probabilities, collisional cross sections, and photoionization cross sections.

The non-LTE approach has important implications for interpreting spectra: it changes line strengths and shapes, alters inferred abundances and temperatures, and affects ionization balances. It is particularly important in hot stars, nebular environments, and other low-density plasmas where radiation dominates. Practical non-LTE modeling uses specialized codes and extensive atomic data to produce reliable synthetic spectra for comparison with observations.