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 sampl e of main sequence B-type stars to determine the behavior of neutral h elium lines with effective temperature and gravity. Equivalent widths have been compared with new calculations which combine the capabilitie s of LTE atmospheric models of including millions of lines contributin g to the opacity and accurate NLTE treatment of the line formation. Th e latter take advantage of accurate atomic cross-sections. We find a s atisfactory 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 oxy gen lines for spectral types earlier than B3.