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Keywords:

  • stars: evolution;
  • stars: mass-loss;
  • stars: rotation;
  • ultraviolet: stars

ABSTRACT

To diagnose the time-variable structure in the fast winds of the central stars of planetary nebulae (CSPNe), we present an analysis of P Cygni line profiles in Far Ultraviolet Spectroscopic Explorer satellite far-ultraviolet (FUV) spectroscopic data. Archival spectra are retrieved to form time-series data sets for the H-rich CSPN NGC 6826, IC 418, IC 2149, IC 4593 and NGC 6543. Despite limitations due to the fragmented sampling of the time series, we demonstrate that in all five CSPN the UV resonance lines are variable primarily due to the occurrence of blueward migrating discrete absorption components (DACs). Empirical (Sobolev with Exact Integration) line-synthesis modelling is used to determine the range of fluctuations in radial optical depth, which are assigned to the temporal changes in large-scale wind structures. We argue that DACs are common in CSPN winds, and their empirical properties are akin to those of similar structures seen in the absorption troughs of massive OB stars. Constraints on PN central star rotation velocities are derived from the fast Fourier transform analysis of photospheric lines for our target stars. Favouring the causal role of corotating interaction regions, we explore connections between normalized DAC accelerations and rotation rates of PN central stars and O stars. The comparative properties suggest that the same physical mechanism is acting to generate large-scale structure in the line-driven winds in the two different settings.