Observations of Stellar Wind Instabilities and Variability on Small and Large Scales

I review various observations which suggest that the winds of hot stars are inhomogeneous because of instabilities in the wind flow. On large scales, local wind overdensities are indirectly detected in the form of excess in the infra-red (IR) and radio free-free continuum. The X-ray detection of a hot (T ∼ 10⁶) wind component suggests that the wind is pervaded with strong shocks. The small-scale density structure of the wind can be studied from observations of Line-Profile Variations (LPVs) in optical and UV spectral lines, which are formed close to the stellar surface. LPVs in lines of the P Cygni type consist of blue-edge variations in saturated profiles, and Discrete Absorption Components (DACs) and Periodic Absorption Modulations (PAMs) in unsaturated profiles. These LPVs are shown to be recurrent, and thought to result from instabilities propagating through the wind and generated at the stellar surface. LPVs in recombination lines appear as stochastic subpeaks, which suggest that wind instabilities have a clump-like, rather than shell-like, structure. The kinematics of LPVs in both line types is consistent with wind propagating shocks generated from radiative instabilities. This revised version was published online in July 2006 with corrections to the Cover Date.