Behaviour of the HeI 587.6, 667.8, 706.5 and 728.1 nm lines in B-type stars - On the helium stratification in the atmosphere of magnetic helium peculiar stars

High resolution spectra of the HeI587.6, 667.8 706.5 and 728.1 nm lines have been obtained to test the prediction (notably by Vauclair et al. 1991) that helium abundance should decrease with depth in helium rich stars and increase in helium weak stars. A sample of B-type main sequence stars, with expected solar abundances and non stratified atmospheres, have also been observed in order to compare the behaviour of the selected lines with the chemically peculiar case and with theory. W e found significant discrepancies with theory for the lines HeI706.5. and 728.1 nm, and, in order to outline differences between 'normal' and 'peculiar' stars, we have adopted an empirical correction to the Lorentz broadening parameter in the Voigt profile, under the assumption of LTE. This parameter is derived from the imposition of a satisfactory fit with observations for the relation equivalent width vs. effective temperature for normal B stars. For helium rich stars we confirm Vauclair et al. (1991) predictions that helium abundance decreases with depth. However, we found that helium abundance decreases with depth in helium weak stars too, which contradicts Vauclair et al. (1991) predictions. For some peculiar stars, the inferred helium abundance is in disagreement with the peculiarity class reported in the General Catalogue of Ap and Am stars (Renson et al. 1991).