TIME-SERIES OBSERVATIONS OF O STARS .3. IUE AND HST SPECTROSCOPY OF ZETA-OPHIUCHI AND IMPLICATIONS FOR THE PHOTOSPHERIC CONNECTION

Stellar-wind variability in the archetypal nonradially pulsating O star zeta Oph (O9.5 V) is discussed on the basis of new time-series IUE and HST spectroscopy and archival results. Time-variable ''discrete absorption components'' are first observed at high velocities (greater than or similar to 10(3) km s-1 congruent-to 0.8v(infinity)) and then migrate blueward; the recurrence time scale for the phenomenon is approximately 20 hr. This is the first record of this type of variability in a luminosity-class V star and provides support for the previously inferred ubiquity of such behavior across the O-star regime. The accelerations are slower than predicted by steady state wind models and are unlikely to represent the time-averaged velocity law of the outflow. The UV results are discussed in the context of simultaneous optical observations (Paper II); no evidence is found to suggest that nonradial pulsations have any direct role in determining the gross observational characteristics of discrete absorption components, on either long (years) or short (hours) time scales. The line-driven instability provides a promising mechanism to account for variability observed in the UV P Cygni profiles of zeta Oph, and, by extension, in all other O stars.