A non-LTE study of silicon line formation in early-type main-sequence atmospheres.

We have computed of 16 levels of Si iii-v and radiation fields in all connecting transitions; in particular the first six Si "`triplet levels, including the A4553 line, and the first six Si iv levels including A4089. The computations were done for four non-LTE H-He model atmospheres, provided by Auer and Mihalas, with = 25,000 , 30,000 , 35,000 , and 40,000 K, all with log g = 4. Estimates of corresponding MK types are 131.5 V, 130.5 V, 09 V, and 06. Solutions were obtained by iterating the linearized equations of radiative transfer and statistical equilibrium, except that for less important lines an approximate "equivalent two-level atom" treatment was used. Continuous opacities of C, N, 0, and Ne were included. All abundances were "solar" values. Comparison of the results with LTE calculations for the same models and with observations shows that (a) for all models but the hottest, for lines in the visual region, the line strengths obtained by non-LTE are significantly greater than the LTE 9nes; (b) for the 13 stars satisfactory agreement with the observations is attained; (c) for the 0 stars, although an improvement over LTE is shown, the predicted lines are still weaker than the observed ones. The observations can be matched by adopting a microturbulent velocity of 15 km 1, but the physical interpretation of this ad hoc parameter is questionable; (d) for the Si iv ultraviolet resonance line (using only Doppler broadening) our results are similar to these of LTE and do not match the observations. Subject headings: atmospheres, stellar - early-type stars - line formation