Colley, Stuart Richard; (2002) Observational studies of wind and photospheric variability in three early type stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Analyses of extensive time-series optical spectroscopy of three early-type stars, HD 64760 (B0.5 la), HD 151804 (O8 Iaf), and HD 152408 (08: Iafpe) are presented. Observations of the two O stars are based primarily on a time-series data set of high resolution echelle spectra spanning 21 nights. These datasets are supplemented by older observations with a longer time span, but poorer temporal and spectral coverage. The observations of HD 64760 are based principally on a 6-night longslit time-series data set from Mount Stromlo, and are supplemented with echelle spectra from the AAT and ESO. The optical data sets allow the photosphere and innermost wind region to be probed, to look for a possible 'photospheric connection' to wind variability. In HD 64760, the well known 1.2 and 2.4 d periods found in the UV wind lines (Prinja et al., 1995; Fullerton et al, 1997) are detected in the photospheric spectra. The results presented here indicate that stellar surface structure is maintained in the wind transition zone, and into the main wind region sampled by UV data. The discovery of shorter periods at 20.0 h and 13.7 h are also reported. These are attributed to non-radial pulsation in the modes l = -m = 3, and l = - m = 4 respectively. The investigation of the two Of stars focuses on the wind lines of the Balmer series and He 1, and the weak metal absorption and emission lines (e. g. He II λ4542, O III λ5592, and C III λ5696) which probe the near-photosphere region. Greyscale plots of the time-series spectra demonstrate substantial organised structure in the inner wind region. The wind activity is characterised by localised absorption and emission features that evolve blueward and redward over ~ 4 days. Interestingly, the acceleration and maximum velocity reached by the red-ward features is approximately half that of the blueward features. The discrepancy between the maximum velocities reached arises from cancellation effects, whilst the difference between the acceleration of the blue and red features (if interpreted in the context of CIR models) may indicate different velocity fields at the leading and trailing edge of the CIR. The repetitive nature of the wind activity is suggestive of rotational modulation of the wind, although it is difficult to uniquely establish a link to stellar rotation given the average values of ve sin i of the O stars. Analysis of the weak metal lines reveals significant radial velocity shifts. In the metal emission lines, the radial velocity shifts sometimes betray the start of a strong wind event, whilst Fourier analysis of the metal absorption lines reveals periodic radial velocity shifts (P=1.5, 1.9 d) that may result from pulsation in the radial fundamental mode.

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