STELLAR-WIND VARIABILITY IN IUE SPECTRA OF 68-CYGNI

We construct an empirical noise model for fully processed high-resolut ion, short-wavelength IUE spectra which takes crude account of the spa tial variations of the camera response as well as the ripple-correctio n factors. We use this model to examine the velocity range over which statistically significant variability can be detected in the UV P Cygn i profiles of the O7.5 III: star 68 Cyg, on timescales from days to ye ars. On short timescales, significant absorption-component variability at levels in excess of similar to 4% (rms) of the continuum level is not detected at velocities below \nu\ similar or equal to 800 km s(-1) (similar to 0.34 nu(infinity)), in Si IV. Over the full dataset (152 spectra spanning 5 years), variability at the 2% level is detected dow n to 0 +/- 50 km s(-1), in C IV. These differences may simply be due t o the lower statistical sensitivity of the smaller subsets of data tak en over shorter periods, rather than being an indication of long-term variations. The data therefore provide strong evidence on year timesca les, and tentative evidence on day timescales, for variability to the very base of the flow. The emission components of the major resonance- line P Cygni profiles are effectively constant, with rms variations no t exceeding 4% of the continuum level (with similar to 98% confidence) ; the C IV absorption trough is saturated over a velocity range of 530 km s(-1); and the width of the blue edge of the P Cygni profiles indi cates that 68 Cyg has a highly structured wind containing shocks with velocity jumps of at least similar to 600 km s(-1).