Deviations from LTE in a stellar atmosphere.
Abstract
Deviations for LTE are investigated in an atmosphere of hydrogen atoms with one bound level, satisfying the equations of radiative, hydrostatic, and statistical equilibrium. The departure coefficient and the kinetic temperature as functions of the frequency dependence of the radiative cross section are studied analytically and numerically. Near the outer boundary of the atmosphere, the departure coefficient is smaller than unity when the radiative cross section grows with frequency faster than with the square of frequency; it exceeds unity otherwise. Far from the boundary the departure coefficient tends to exceed unity for any frequency dependence of the radiative cross section. Overpopulation always implies that the kinetic temperature in the statisticalequilibrium atmosphere is higher than the temperature in the corresponding LTE atmosphere. Upper and lower bounds on the kinetic temperature are given for an atmosphere with deviations from LTE only in the optically shallow layers when the emergent intensity can be described by a radiation temperature.
 Publication:

Journal of Quantitative Spectroscopy and Radiative Transfer
 Pub Date:
 April 1979
 DOI:
 10.1016/00224073(79)900657
 Bibcode:
 1979JQSRT..21..355K
 Keywords:

 Hydrogen Atoms;
 Radiative Transfer;
 Statistical Mechanics;
 Stellar Atmospheres;
 Thermodynamic Equilibrium;
 Kinetic Theory;
 Line Spectra;
 Lyman Spectra;
 Photoionization;
 Scattering Cross Sections;
 Temperature Effects;
 Astrophysics;
 Stellar Atmospheres:NonLTE Analyses