STELLAR-WIND VARIABILITY IN IUE SPECTRA OF 68-CYGNI

We construct an empirical noise model for fully processed high-resolution, short-wavelength IUE spectra which takes crude account of the spatial variations of the camera response as well as the ripple-correction factors. We use this model to examine the velocity range over which statistically significant variability can be detected in the UV P Cygni profiles of the O7.5 III: star 68 Cyg, on timescales from days to years. 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 down to 0 +/- 50 km s(-1), in C IV. These differences may simply be due to the lower statistical sensitivity of the smaller subsets of data taken over shorter periods, rather than being an indication of long-term variations. The data therefore provide strong evidence on year timescales, 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 not 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 indicates that 68 Cyg has a highly structured wind containing shocks with velocity jumps of at least similar to 600 km s(-1).