Neutral helium lines and departures from LTE in hot stars

Abstract

Although the weakening of the singlet lines of neutral helium relative to the corresponding triplets along the spectral sequence in both directions from B3 has been known for many years, the explanation of the phenomenon is still uncertain. To assess the importance of departures from thermodynamic equilibrium among the populations of the bound states of He I in explaining the observations, we have calculated departures for several model helium atoms, including some with levels through n = 20, in model atmospheres of 20 000, 30 000 and 40 000 K. The coupled equations of radiative transfer and steady state were solved for all significant bound-bound transitions and for all bound-free continua. Results for various atomic models are presented and the influence of the model is discussed. It is possible, for these atmospheres, to separate the singlet and triplet levels into two effectively independent atoms. The regions of formation of the subordinate lines and resonance lines are found to be well separated. The ground state may be either over- or under-populated depending on the atmosphere. The level n = 2 is overpopulated in the region of the atmosphere where the cores of the subordinated lines are formed, but quickly goes to its equilibrium population deeper in the atmosphere. Higher levels are close to equilibrium throughout the region of line formation. The source functions for lines arising from n = 2 are therefore less than the Planck function in the outer atmosphere, but quickly return to it as one goes deeper. These lines will therefore have lower central intensities than predicted by LTE, and this should be detectable in observations with sufficiently high dispersion. Equivalent widths of subordinate lines will be very little changed from LTE values. Also, we find no differential non-LTE effect between singlets and triplets which can explain the observed behavior of the singlet/triplet ratio. In addition, our results indicate that if departures from equilibrium for helium are to be considered in constructing radiative equilibrium model atmospheres, one must solve the line transfer problem as well as the bound-free transfer problem.