Author ORCID Identifier
Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Red supergiants (RSGs) are cool, luminous stars with radii that can exceed 1000 R⊙. Indeed, such is their size that nearly every advance in stellar imaging has used the closest RSG, Betelgeuse, as a test case! These objects represent a late stage in the evolution of some massive stars, and, via their mass-loss and eventual demise in supernovae, they play an important role in the chemical evolution of the Universe. Moreover, their high luminosities have made them an object of interest for astronomers studying nearby galaxies. As a result of their increasingly broad use in astronomy, a solid understanding of RSGs and in the limitations of models of these objects is important.
One of the biggest challenges in modeling red supergiants is convection. In RSGs, granules and convection cells are quite large relative to the size of the star---with granules roughly 0.10-0.30 R*. and convection cells at least 0.50 R*. This results in large surface features that can be studied using optical interferometry, but which can also corrupt measurements of parallax and other stellar parameters. Increasingly, there exist models of RSGs which take into account this behavior, but it is important to constrain these models with actual observations.
In this dissertation, we present a long-term study of surface features on RSGs using the Michigan InfraRed Combiner (MIRC/MIRC-X after 2016) at the Center for High Angular Resolution (CHARA) Astronomy Array on Mt. Wilson. Images resulting from these data are among the highest resolution obtained for any star (apart from the Sun). Fitting to model spectra, we derive Teff= 3989±117 K and log(g)=0.29±0.26 for the RSG AZ Cyg and Teff= 3650±50 K and log(g)=0.30±0.26 for the RSG SU Per. We also determine radii for 17 RSGs including AZ Cyg and SU Per. We reconstruct images of AZ Cyg from 2011, 2012, 2014, 2015 and 2016, and reconstruct images of SU Per from 2015 and two months in 2016. In both cases, we find evidence of long lived (>1 year) features roughly 0.50 R* in size and short lived (<1 year) features roughly 0.10 R* in size. We compare these observations to predictions from 2D and 3D models. We also discuss future directions for studying RSGs using optical interferometric imaging.
Norris, Ryan P., "Seeing Stars Like Never Before: A Long-term Interferometric Imaging Survey of Red Supergiants." Dissertation, Georgia State University, 2019.
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