St-Louis, Nicole; (1990) A study of variability in the ultraviolet spectra of galactic Wolf-Rayet stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The results of an archival search for ultraviolet spectroscopic variability of all galactic, single-lined Wolf-Rayet (WR) stars observed more than once with IUE are presented. At least 35% of stars are found to be variable; a large proportion considering that, in many cases, the available datasets are not ideally suited for this type of study. For each star, spectra have been co-added to form a mean spectrum with an improved signal-to-noise. The resulting spectra are presented as and atlas of high resolution IUE spectra for 28 galactic WR stars. A detailed study of the ultraviolet spectroscopic variability for the WR stars HD 192163 (WR 136) and HD 50896 (WR 6) has been conducted, using high resolution IUE spectra. For HD 192163, significant variability is found in the C IV λ1550, He II λ1640 and N IV λ1718 P Cygni profiles, over a period of ~1 day. Enhanced absorption is observed at velocities exceeding the usual maximum wind outflow velocity. Weak variability is also detected in the emission components of the P Cygni profiles on a similar timescale as the variability found in the absorption components. In the case of HD 50896, variations are observed in the absorption and emission components of the N V λ1240, C IV λ1550, He II λ1640 and N IV λ1718 P Cygni profiles as well as for a series of Fe V and Fe VI lines. As for HD 192163, the changes in the absorption components occur at velocities in excess of the terminal velocity of the wind. As a result of the excellent time resolution of the dataset for this stax, it has been established that the absorption component variability takes place on a timescale of ~1 day and has a similar recurrence timescale. The variations found in the emission components have a smaller amplitude and occur on a much longer timescale, indicating that the mechanism causing the changes is not occurring on the scale of the wind but is much more localised. For both stars, the amplitude and character of the P Cygni profile variability is found to alter with epoch. In each case, it is concluded that the observed variations cannot be interpreted as effects of a compact companion orbiting in the dense stellar wind, and therefore, the ultraviolet variability does not support previous claims that these stars are WR+ compact binary systems. Instead, the ultraviolet spectroscopic variability of HD 192163 and HD 50896 are considered to be intrinsic to the WR stellar winds. Models incorporating radioactively driven wind instabilities are found to, at least qualitatively, account for the variations observed at the highest outflow velocities. The line of sight towards HD 192163 and other stars in Cygnus has been investigated. High-velocity components have been detected in HI-type and highly-ionised species towards 10 out of 13 stars in the sample and are interpreted as arising in an expanding supershell enveloping the Cyg OBI association. A study of the phase-dependent ultraviolet variability observed in high resolution Copernicus and IUE spectra of the well-known WR+O spectroscopic binary γ Velorum is presented. Changes in the P Cygni profiles of resonance and low-excitation transitions are confirmed cis being partly caused by selective eclipses of the O star continuum light by the WR wind. The appearance of a high velocity wing in the absorption component of N V λ1240, Si IV λ1396 and C IV λ1550, at phases when the O star companion is in front of the WR star, is attributed to the formation of a region of shocked gas, following the collision between the two stellar winds. A broad absorption in the eclipse spectrum between ~1410-1910 Å is found to be due to a large number of Fe IV transitions and reveals that the density distribution of this ion in the wind of the WR star is asymmetric. A similar pattern of ultraviolet line profile variability is found in the binary systems V444 Cygni (WN5 + O5) and CV Serpentis (WC8 + O9).

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