title: Observations of line profile variations in early-type stars
Creators: name: Wyllie, Thomas Henry Alan
Publisher: name: University of Glasgow
Year: 1976

description: During the past few years the literature has contained several reports concerning rapid line profile variations of some Be, Ap, Wolf- Rayet and other early-type stars. In some cases the time-scales of the variations have been reported to be as short as minutes or even, seconds and clearly, if such variations are intrinsic to the stars, the processes giving rise to them must be accounted for by any future theories regarding the stellar structures and atmospheres. However, some of the earlier observations are open to criticism and the reported variations may, in some cases, be the result of various-instrumental errors which have not been fully appreciated. The work reported in this thesis was undertaken in order to investigate further the variations reported for the HP line in some of these stars. A considerable amount of the work has been concerned with developing a novel stellar line profile scanner. By tilting a narrow band (~ 2 Å half-width) interference filter in a collimated beam, a moderate resolution wavelength-scanning monochromator is achieved. The tilt-scanning technique has been explored in some depth and it is believed that this work is the first intensive application of the technique to the measurement of stellar line profiles at this wavelength resolution. The indications are that the scanning range of one filter may be of the order of 200 Although there is a degree of distortion of the recorded profiles caused by the filter's transmittance decreasing and its pass-band broadening with tilt, the application for this study was concerned only with detecting line profile variations rather than making absolute measurements. However, over the smaller wavelength range normally scanned (~30 Å), the distortions of the recorded line profiles are sufficiently small as to be neglected. More important, however, are the changes of the filter passband, and hence of the recorded profiles, brought about by variations in the angular size and movement of the stellar seeing disc in the telescope's focal plane due to unsteady seeing conditions and telescope tracking errors. Both of these topics have been considered as sources of photometric noise and it is felt that some of the variations of the Be stars reported previously by others may be the result of similar effects, especially since the extent of the noise becomes greater as the spectral gradients increase. A special purpose double-beam photometer has been designed and built. One beam passes through a narrow band scanning filter while the other beam passes through a broader band (~50 Å) fixed interference filter .whose passband is centred on the continuum adjacent to the scanned line. This second beam acts as a reference and after subtracting sky and dark contributions, the ratio of the signal in the two beams is largely free of atmospheric noise. The scanning filter is moved automatically along a series of discrete wavelength points by a stepping motor and both the step-size and the number of spectral points can be set on the electronic control. Pulse counting photometry is employed; the signals in both beams are recorded simultaneously and, after preset integration times, are punched on paper tape together with a record of the filter position and the Universal Time. Wavelength/tilt calibration is performed by piping light from laboratory lamps into the photometer by a fibre optic tube. Observationally, the main emphasis of the work has been on measurements of the Hp line in Be stars, several of which have been found to show night- to-night profile variations. Although variations of this type have been reported previously for some of the stars observed (e.g. γ Cas, ζ Tau), it is believed that there are no previous reports of night-to-night variations -J for several of the others (α Ara, δ Cen, μ Cen, ζ1 Sco). No evidence of very rapid variability was found. Other observations were made of the Ap star ϵ UMa but these showed no signs of any of the variations reported earlier. The Volf-Rayet star 2 and spectroscopic binary γ2 Vel was also observed and in addition to the known variations of the Hβ line with phase, night-to-night variations were also indicated. The main advantages of the tilting-filter scanner over other line scanners with comparable resolution are its simplicity of design and its small physical size which, together with a high overall transmittance of the optical components, allows it to be used with small telescopes. Although the spectral purity of the recorded profiles is not as good as can be achieved with more sophisticated scanners, the technique clearly has a useful application for the detection of line profile variations.