Variability of the HeI line profiles in the spectrum of the hot O9.5V star HD 93521

CCD spectra acquired with the PFES echelle spectrograph on the 6 m telescope of the Special Astrophysical Observatory (Russian Academy of Sciences) were used to study short-term variations in the HeI-line profiles in the spectrum of HD 93521. For all the lines, the variability pattern relative to the mean profile is the same, and can be described as a sinusoidal wave passing through the profiles, from the blue to the red wings. The variability amplitudes and time scales are different for different HeI lines. We studied variations of the radial velocities at the level of 0.5 R 0 of the line residual intensity, for the absorption bisector and the blue and red halves of the absorption profile. The variation time scales and amplitudes for the line halves differ from one HeI line to another, and show good correlations with the line central depths. Going from the weak to the strong lines, the time scale of the radial-velocity variations measured for both halves of the absorption profile increases, and the amplitude decreases. The time scale of the radial-velocity variations for weak lines is, on average, twice the time scale for strong HeI lines. A variable absorption feature was detected in the profiles of strong HeI lines, which moves across the profile synchronously with the star’s axial rotation. Generally, the observed line variations are probably due to nonradial photospheric pulsations, together with the influence of the stellar wind on the profiles of the strong lines.