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 temperature (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 and intermediate photometric systems, hydrogen line profiles, continuum fluxes 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 abundance and Strömgren 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 luminosity classes. Generally the physical properties of a model (effective temperature T_eff and total particle pressure) in the line forming region change 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 visible HeI ( ~ -1 in depth mass variable units) is pushed down and enlarged in the atmosphere when increasing the He abundance. The colours, helium line profiles and hydrogen gamma profiles are tabulated and can be handled by simple Fortran software which is also provided. Finally, the third part of curve of growth for helium lines is better described by a slope of 0.36 +/-0.1 instead of the square root law for both luminosity classes (log g=4.0 and log g=3.5). Table~1 and the databank are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/