Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Dr. T.A.A. Sigut

Abstract

A sample of 26 Be stars from the Magnetism in Massive Stars (MiMeS) spectroscopic survey are analyzed for their photospheric nitrogen abundances in an effort to detect rotational mixing in the Be stars. Be stars are massive stars, between 3 and 20 times the mass of the Sun, that are surrounded by a thin, equatorial disk of gas that produces emission lines in their optical and near-infrared spectra. Be stars are the most rapidly-rotating stellar population on the main sequence, where stars produce energy by core hydrogen burning. New, non-LTE line transfer calculations are performed for the Nii ion, the dominant ionization stage in the photospheres of the B stars, and an extensive error analysis is performed via Monte Carlo simulation to determine the achievable accuracy of nitrogen abundances among these stars. To analyze the measured Nii equivalent widths from the MiMeS survey, the effects of both gravitational darkening, due to the rapid rotation of the central B star, and the veiling effect of emission from the Be star circumstellar disk are considered. Both of these effects are found to be small for the sample population, affecting the final nitrogen abundances at the level ≈ ±0.1 dex. The final, average nitrogen abundance for the MiMeS sample, AMiMeS, N = 7.78, has a value in good agreement with the solar abundance, AN = 7.83, and recent, high-precision abundance measurements for main-sequence B stars. Nevertheless, the MiMeS sample standard deviation is over a factor of two larger than that of the observed nitrogen abundances in the atmospheres of B stars in the solar neighbourhood, with many low abundance objects and a few high abundance objects. No discernible trend of the nitrogen abundance can be seen with stellar gravity, log g (as a proxy for stellar age), or stellar equatorial velocity in the MiMeS survey. It is suggested that possible disk emission in the NII line transitions may explain the lower abundance objects, and this will be the subject of future work.


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