Date of Award


Degree Type


Degree Name

Doctor of Philosophy


High signal-to-noise coude reticon spectra of five helium-strong stars have been obtained in two spectral regions ({dollar}\lambda\lambda{dollar}3980-4160 and {dollar}\lambda\lambda{dollar}4400-4580) at the Canada-France-Hawaii Telescope, Mauna Kea, Hawaii. Phase-resolved sets of spectra have been obtained for the magnetic-variable stars {dollar}\sigma{dollar} Ori E, HD 37776, and HD 64740. Additional observations have also been made of the sharp-lined helium-strong stars HD 58260 and {dollar}\delta{dollar} Ori C. These data, and additional Zeeman analyzer observations, have been used in an attempt to determine the magnetic field geometries and abundance distributions of the program stars. The models have been obtained by use of a line synthesis program that incorporates the effects of an assumed magnetic field and abundance distribution into the calculation of line profiles.;Helium line profile variations of HD 64740 may be modelled with two spots with anomalously high helium abundances near the magnetic poles of the star. Silicon, appears to have a large abundance in a high latitude band around the star. The surface magnetic field of HD 64740 is not strong enough to cause magnetic intensification effects in the Si III multiplet 2 line profiles.;The peculiar, broad helium line profiles of {dollar}\delta{dollar} Ori C suggest two possible helium abundance geometries for the star. A helium-rich spot near the visible magnetic pole of an otherwise extremely helium-poor star, and a vertical helium abundance gradient are both able to reproduce the observed profiles of the star. The second non-variable program star, HD 58260, also shows marginal evidence for such an abundance geometry.;The helium line profiles of {dollar}\sigma{dollar} Ori E are reproduced with two spots of large helium abundance ({dollar}\epsilon\sb{lcub}\rm He{rcub}{dollar} {dollar}>{dollar} 0.5) near the intersections of the magnetic and rotation equators of the star. Silicon, magnesium, nitrogen, and oxygen all appear to be overabundant in a high latitude band on the surface of the star. Observations of the variable H{dollar}\alpha{dollar} emission of the star are presented.;A theoretical model for the peculiar longitudinal magnetic field variations of HD 37776 is proposed. The model consists of colinear dipole, quadrupole, and octupole field components that result in a magnetic field that varies between 20 and 60 kG over the surface of the star. Helium and silicon abundance geometries consistent with equatorial and polar distributions of the respective elements support the field model, as do the pronounced differential magnetic intensification effects seen in the silicon lines.;Several suggestions for interesting future observational and theoretical work are discussed.



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