Pollution of P Cygni line profiles by Fe IV and Fe V photospheric absorption in the ultraviolet spectrum of O-type stars

It is reported that the main pollution in the spectral ranges 1270-1500 and 1500-1860 A is caused by photospheric absorption lines of FeV and FeIV, respectively. Making use of the first order moment W1 of a P Cygni line profile, it is shown that the effect of this photospheric pollution may lead to systematic errors, exceeding typically 50 percent in the determination of mass loss rates. It is also emphasized that the importance to correct the observed P Cygni line profiles for this pollution when studying the dependence of the physical parameters, terminal velocity, etc. of the stellar winds upon the various classes and types of O stars.