Visible neutral helium lines in main sequence B-type stars: observations and NLTE calculations

Spectra in the visible range 410-710 nm have been obtained for a sample of main sequence B-type stars to determine the behavior of neutral helium lines with effective temperature and gravity. Equivalent widths have been compared with new calculations which combine the capabilities of LTE atmospheric models of including millions of lines contributing to the opacity and accurate NLTE treatment of the line formation. The latter take advantage of accurate atomic cross-sections. We find a satisfactory agreement between theory and observations with differences probably due to the microturbulence. We also investigate the effects of blending and find that the HeI412.1 nm is severely blended with oxygen lines for spectral types earlier than B3.