Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Dr. D. F. Gray

Abstract

Red giants and red supergiants have long been known to be variable. In the last 40 years many of the features of this variability have been associated with large convective cells. Unfortunately, due to the long timescales of these variations they are not well studied, with the exception of the bright M-class supergiant Betelgeuse (α Orionis, M2 Iab). Betelgeuse has been well studied both observationally and theoretically, and has many features that are well described by models of convection. It was these studies of Betelgeuse that provided the main motivation for this thesis. We ask if the dramatic motions seen in the atmospheres of Betelgeuse (Gray 2008 amongst others) are typical of red supergiants and if their variations can be described by convective motions as is the case for that enigmatic star.

Sixty-five spectra of the M-class supergiant Antares A (α Scorpii A, M1.5 Iab) were obtained at the Elginfield observatory from April 2008 until July 2010. These data were combined with historical radial velocity measures, Hipparcos photometry, and AAVSO photometry. From these data we determine four scales of variability: ~7140 days, 2167 days, ~1260 days, and ~100 days.

The longest of these periods is found from the AAVSO photometry and cannot be confirmed by any of the other data. A period of similar length has been reported in one previous study but no analysis was completed. A period of this length (~7140 days; 19 years) is consistent with suspected rotation rates for red supergiants, though it is also plausible that episodic dust ejection could cause such a variation. The 2167-day variation was found from historical and present radial velocity measures. Due to the shape of the radial velocity curve, and a phase shift compared to the temperature curve, we interpret this period as a pulsation or a long secondary period. The ~1260-day period found from both sets of photometry and the 100-day timescale found from our spectral analysis are both interpreted along with a handful of periods found in the literature as arising from convection.


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