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A Variability Study of the Typical Red Supergiant Antares A

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Western University Scholarship@Western Electronic Thesis and Dissertation Repository 4-11-2013 12:00 AM A Variability Study of the Typical Red Supergiant Antares A Teznie J D Pugh The University of Western Ontario Supervisor Dr. D. F. Gray The University of Western Ontario Graduate Program in Astronomy A thesis submitted in partial fulfillment of the equirr ements for the degree in Doctor of Philosophy © Teznie J D Pugh 2013 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Physical Processes Commons, and the Stars, Interstellar Medium and the Galaxy Commons Recommended Citation Pugh, Teznie J D, "A Variability Study of the Typical Red Supergiant Antares A" (2013). Electronic Thesis and Dissertation Repository. 1190. https://ir.lib.uwo.ca/etd/1190 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. A VARIABILITY STUDY OF THE TYPICAL RED SUPERGIANT ANTARES A (Thesis format: Monograph) by Teznie Pugh Graduate Program in Astronomy A thesis submitted in partial fulfillment of the requirements for the degree of Doctorate of Philosophy The School of Graduate and Postdoctoral Studies The University of Western Ontario London, Ontario, Canada © Teznie Pugh 2013 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. Keywords stars: convection - stars: individual (α Sco A) - stars: oscillations - stars: variables: general - techniques: spectroscopic - techniques: radial velocities ii Acknowledgments I would, first of all, like to thank my supervisor, Dr. D. F. Gray for his assistance, for his guidance, for his support, and for his inspiration. I would also like to thank Michel DeBruyne for his technical support and training at the Elginfield Observatory, without his regular maintenance my observations would not have been possible. I acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. For their comments and assistance in the interpretation of the data I would like to thank Gregg Wade, Jaymie Matthews and our Astonomical Journal referee. For an explanation of the convective models of Betelgeuse I would like to thank Andrea Chiavassa. Discussions with Jan Cami, John Landstreet and David Stock have also been helpful. I am grateful for the funding I received through the Western Graduate Research Scholarship, for the financial support of my fianće Darren, and to the Canadian Astronomical Society for their travel grants. My acknowledgement would not be complete without thanking the people who have aided me in maintaining my sanity: my fianće Darren for providing emotional support and taking the lead with parenting so that I could concentrate on my research; my daycare provider Alison for being my daughter’s second mum; my family for not making a fuss about my immigration to Canada; and to all my friends for putting up with me through it all and making Canada so welcoming. iii Table of Contents Abstract .......................................................................................................................... ii Acknowledgments ......................................................................................................... iii Table of Contents .......................................................................................................... iv List of Tables............................................................................................................... viii List of Figures ............................................................................................................... ix Chapter 1 .........................................................................................................................1 1 Red Giants and Supergiants ........................................................................................1 1.1 Evolution History .................................................................................................3 1.1.1 Intermediate Mass Stars (2.5M⊙ to 10M⊙) ...............................................4 1.1.2 Low Mass Stars (0.5M⊙ to 2.3M⊙) ..........................................................5 1.1.3 High Mass Stars (>10M⊙-15M⊙) .............................................................7 1.2 Stellar Winds in the Upper HR Diagram ..............................................................8 1.2.1 Driving Mechanisms .............................................................................. 10 1.3 The Red Variables.............................................................................................. 11 Chapter 2 ....................................................................................................................... 14 2 Observations of Semiregular and Irregular Variables ................................................ 14 2.1 Photometry ........................................................................................................ 14 2.1.1 Photometric Power Spectra ..................................................................... 15 2.1.2 Long Secondary Periods and Multiperiodicity ........................................ 18 2.1.3 Period-Luminosity Relations and Mode of Pulsation .............................. 19 2.2 Temperature Variations ...................................................................................... 20 2.3 Radial Velocity .................................................................................................. 21 2.4 Interferometry .................................................................................................... 21 2.5 Polarization ........................................................................................................ 24 iv 2.6 Spectral Variability ............................................................................................ 24 2.7 Photospheric Excursions .................................................................................... 24 Chapter 3 ....................................................................................................................... 27 3 Mechanisms of Variability ........................................................................................ 27 3.1 Companion Stars ................................................................................................ 27 3.2 Surface Features ................................................................................................. 28 3.2.1 Starspots ................................................................................................. 28 3.2.2 Convection Cells .................................................................................... 30 3.2.3 Binary Heating & Ellipsoidal Variability ................................................ 32 3.3 Pulsation ............................................................................................................ 32 3.3.1 Radial Modes ......................................................................................... 33 3.3.2 Nonradial Modes .................................................................................... 34 Chapter 4 ....................................................................................................................... 38 4 Observations and Reductions ...................................................................................
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