Properties of He-rich stars III. Model atmospheres and diagnostic tools

We present a study of helium rich atmospheres in the interval He/H 0.1-1.0 as derived from fully consistent model computations for effective temperatu re (from 15000K up to 32000K) and surface gravity (log g=3.5 and 4.0). Also the basic diagnostic tools are re-examined, particularly broad-band UBV an d intermediate photometric systems, hydrogen line profiles, continuum fluxe s and helium equivalent widths. Photometric systems were found to be affected by the helium abundance (He/H number ratio): the Geneva [U-B] index by 0.025 mag. per 0.4 of He abundanc e and Stromgren u - v index and c1 index by 0.04 mag. in the most sensitive cases. The hydrogen gamma profile is also affected by up to 7% at FWHM for both lu minosity classes. Generally the physical properties of a model (effective t emperature T-eff and total par tide pressure) in the line forming region ch ange with the He abundance most strongly in the hottest models and for the subgiant class (log g=3.5). In particular, the line forming region of visib le HeI (similar to -1 in depth mass variable units) is pushed down and enla rged in the atmosphere when increasing the He abundance. The colours, heliu m line profiles and hydrogen gamma profiles are tabulated and can be handle d by simple Fortran software which is also provided. Finally, the third part of curve of growth for helium lines is better descr ibed by a slope of 0.36 +/-0.1 instead of the square root law for both lumi nosity classes (log g=4.0 and log g=3.5).