UES and IUE observations of the O9.5V star HD 93521 : non-radial pulsatutions, wind and distance.

We have obtained time-series spectroscopy of the rapidly rotating O9.5 V star HD 93521 during commissioning of the Utrecht Echelle Spectrograph (UES) on the William Herschel Telescope. The He I lines show 'wiggles' at the approximately 1% level, which move systematically from blue to red across the profile; by contrast, the He II lines do not show detectable variations. We interpret these characteristics as indicative of sectorial-mode non-radial pulsations coupled with significant gravity darkening, and estimate P = 1.8 hours, l = -m approximately equal to 9, k less than 0.3. The H-alpha and He I lambda 5876 lines show emission wings, and line-profile variability which may be associated with circumstellar material. If that inference is correct, and if (as is plausible) the material corotates, then the star's rotation period may be estimated as P_rot approximately equal to 35 hr. We also examine IUE observations, which show no detectable variations in the wind on pulsation timescales. We show that the data provide direct observational evidence for a strong equator-to-pole asymmetry in the outflow velocity, confirming Massa's interpretation of the wind as being rotationally distorted; the star may possess a 'wind-compressed disc'. We estimate the equatorial and polar terminal velocities; a steady-state wind model predicts the observed maximum (approximately polar) velocity well. Reviewing the spectroscopic determination of stellar distances, taking rotation fully into account, we show that previous suggestions that HD 93521 may be a Population II star are in error. We present a self-consistent, illustrative set of stellar parameters which are in complete accord with the notion that HD 93521 is a rapidly rotating, but otherwise quite normal, Population I star at approximately 2 kpc.