EVOLUTION OF SOLITARY DENSITY WAVES IN STELLAR WINDS OF EARLY-TYPE STARS - A SIMPLE EXPLANATION OF DISCRETE ABSORPTION COMPONENT BEHAVIOR

We model the evolution of a density shell propagating through the stel lar wind of an early-type star, in order to investigate the effects of such shells on UV P Cygni line profiles. Unlike previous treatments, we solve the mass, momentum, and energy conservation equations, using an explicit time-differencing scheme, and present a parametric study o f the density, velocity, and temperature response. Under the assumed c onditions, relatively large spatial scale, large-amplitude density she lls propagate as stable waves through the supersonic portion of the wi nd. Their dynamical behavior appears to mimic propagating ''solitary w aves,'' and they are found to accelerate at the same rate as the under lying steady state stellar wind (i.e., the shell rides the wind). Thes e hydrodynamically stable structures quantitatively reproduce the anom alous ''discrete absorption component'' (DAC) behavior observed in the winds of luminous early-type stars, as illustrated by comparisons of model predictions to an extensive IUE time series of spectra of zeta P uppis (O4f). From these comparisons, we find no conclusive evidence in dicative of DACs accelerating at a significantly slower rate than the underlying stellar wind, contrary to earlier reports. In addition, the se density shells are found to be consistent within the constraints se t by the IR observations. We conclude that the concept of propagating density shells should be seriously reconsidered as a possible explanat ion of the DAC phenomenon in early-type stars.