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 sta r zeta Oph (O9.5 V) is discussed on the basis of new time-series IUE a nd HST spectroscopy and archival results. Time-variable ''discrete abs orption components'' are first observed at high velocities (greater th an or similar to 10(3) km s-1 congruent-to 0.8v(infinity)) and then mi grate blueward; the recurrence time scale for the phenomenon is approx imately 20 hr. This is the first record of this type of variability in a luminosity-class V star and provides support for the previously inf erred ubiquity of such behavior across the O-star regime. The accelera tions are slower than predicted by steady state wind models and are un likely to represent the time-averaged velocity law of the outflow. The UV results are discussed in the context of simultaneous optical obser vations (Paper II); no evidence is found to suggest that nonradial pul sations have any direct role in determining the gross observational ch aracteristics 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 C ygni profiles of zeta Oph, and, by extension, in all other O stars.