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Florida State University Libraries Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School Constraining the Evolution of Massive StarsMojgan Aghakhanloo Follow this and additional works at the DigiNole: FSU's Digital Repository. For more information, please contact [email protected] FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCES CONSTRAINING THE EVOLUTION OF MASSIVE STARS By MOJGAN AGHAKHANLOO A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2020 Copyright © 2020 Mojgan Aghakhanloo. All Rights Reserved. Mojgan Aghakhanloo defended this dissertation on April 6, 2020. The members of the supervisory committee were: Jeremiah Murphy Professor Directing Dissertation Munir Humayun University Representative Kevin Huffenberger Committee Member Eric Hsiao Committee Member Harrison Prosper Committee Member The Graduate School has verified and approved the above-named committee members, and certifies that the dissertation has been approved in accordance with university requirements. ii I dedicate this thesis to my parents for their love and encouragement. I would not have made it this far without you. iii ACKNOWLEDGMENTS I would like to thank my advisor, Professor Jeremiah Murphy. I could not go through this journey without your endless support and guidance. I am very grateful for your scientific advice and knowledge and many insightful discussions that we had during these past six years. Thank you for making such a positive impact on my life. I would like to thank my PhD committee members, Professors Eric Hsiao, Kevin Huf- fenberger, Munir Humayun and Harrison Prosper. I will always cherish your guidance, encouragement and support. I would also like to thank all of my collaborators. In partic- ular, many thanks to Professors Nathan Smith and Keivan Stassun for providing amazing opportunities throughout my graduate education. This dissertation would not be possible without your contribution. I would like to thank the staff in the Physics Department, Jonathan Henry, Brian Wilcoxon, Dr. Felicia Youngblood, Shawana Elwood and Joe Ryan for all their help through- out these years. I have built up many friendships along the way and would not be where I am today without their love. In particular, a great thanks to Dr. Pegah Nasabian and Dr. Soheila Abrishami and Luz Jimenez Vela who have helped me with their valuable suggestions and motivated me during these years. Last but not least, many thanks to my lovely parents, Nosrat and Maloos and my partner Ramu for their constant support through every single challenge of my life. Thank you for encouraging me in all of my pursuits and inspiring me to follow my dreams. Words alone cannot express my gratitude. iv TABLE OF CONTENTS List of Tables . vi List of Figures . vii Abstract . ix 1 Background 1 1.1 Luminous Blue Variables . .1 1.2 Massive Stars and the Role of Binary Interaction . .6 1.3 Precise Distances to LBVs . .6 1.4 Westerlund 1 Cluster . .9 2 Modelling Luminous-Blue-Variable Isolation 12 2.1 Observations . 12 2.2 A Generic Model for the Spatial Distribution of the Stars in a Passive Dispersal Cluster . 16 2.3 Cluster Dissolution with Close Binary Interactions . 25 3 On the Gaia DR2 Distances for Galactic Luminous Blue Variables 38 3.1 Gaia Spacecraft . 38 3.2 Distances for LBVs in Gaia DR2 . 39 3.3 Notes on Individual LBVs . 57 3.4 Discussion . 68 4 Inferring the Parallax of Westerlund 1 from Gaia DR2 79 4.1 Method . 79 4.2 Parallax and Distance to Westerlund 1 . 92 4.3 Discussion . 93 5 Conclusions 104 Bibliography . 107 Biographical Sketch . 130 v LIST OF TABLES 2.1 List of LBVs and LBV candidates adapted from Smith and Tombleson [2015]. 14 2.2 The P -values for KS tests for the distributions of separation. 16 3.1 Previous literature distances for Galactic LBVs and candidate LBVs (in paren- theses). 42 3.2 Parameters from the Gaia DR2 and Bailer-Jones catalogs. 45 3.3 LBVs and LBV candidate (in parentheses) Gaia DR2 distances. 46 vi LIST OF FIGURES 2.1 Cumulative distributions for the projected separation to the nearest O star. 17 2.2 Normality test. 18 2.3 We propose a Monte Carlo model for the separations between O stars and LBVs by considering a random sample of dissolving clusters at random ages. 22 2.4 The mean (bottom panel) and std. deviation (top panel) distance to the nearest neighbour versus drift velocity. 24 2.5 Two simple spatial-distribution models for the derivation of our analytic scalings. 27 2.6 LBV isolation is inconsistent with the single-star model and passive dissolution of the cluster. 31 2.7 The relative isolation of LBVs is consistent with a binary merger in which the LBV is a rejuvenated star. 33 2.8 Merger model outline. 34 2.9 Kick model outline. 34 2.10 LBV dispersion velocity as a function of LBV age . 36 3.1 Distances by Bayesian inference, dBayes (Table 3.3) compared to: (top) literature distances, dLit, (Table 3.3), (middle) distances given by 1=$ (Table 3.2), and (bottom) the Bailer-Jones Bayesian distances, dBJ [Bailer-Jones et al., 2018] (Table 3.2). 52 3.2 The HR diagram showing only Galactic LBVs (filled circles) and Galactic LBV candidates (unfilled circles) with their luminosities scaled by the revised Gaia DR2 distances (dBayes)................................ 55 3.3 Same as Fig. 3.2, but including positions of LBVs based on both the old liter- ature distances (red) and those inferred from Gaia DR2 distances (black). 56 3.4 An HR diagram similar to Fig. 3.5, but showing only W243 based on the old distance of around 5 kpc (red) and using a distance of 3.2 0.4 kpc (black) based on the distance inferred for the whole Wd1 cluster from DR2± data [Aghakhanloo et al., 2020]. 66 vii 3.5 The HR diagram with LBVs (filled circles) and LBV candidates (unfilled cir- cles), adapted from a similar figure in Smith and Tombleson [2015] and Smith and Stassun [2017]. 78 4.1 Position of all Gaia stars within 10 arcmin of Westerlund 1. 80 4.2 Histograms of parallaxes (top left panel), parallax uncertainties (top right panel), astrometric excess noise (, bottom left panel), and astrometric ex- cess noise significance (D, bottom right panel) of all stars in the inner circle and the outer annulus. 82 4.3 Histogram of parallax over the uncertainty of all stars in the inner circle and the outer annulus. 83 4.4 Estimates for the true mean parallax for each ring. 84 4.5 Probabilistic graphical model for the Bayesian model. 87 4.6 Posterior distribution for the six-parameter model. 94 4.7 Bayesian inferred cluster parallax for each ring. 95 4.8 Same as Fig. 4.6, but we assume that the offset for the field-star distribution, $os is equal to the instrumental zero-point, $zp.................. 96 4.9 Histogram of scale factor x for all sources within 1 arcmin of centre of the cluster. 98 4.10 The HR diagram for evolved stars in Westerlund 1, including the LBV, W243. 102 viii ABSTRACT Massive stars play a crucial role in the universe. Yet, our understanding of massive stars remains incomplete due to their rarity, short lifetimes, complexity of binary interactions, and imprecise Galactic distances. An important challenge is to understand the physics and relative importance of steady and eruptive mass loss in the most massive stars. For example, the luminous blue variable (LBV) is one such poorly constrained class of eruptive stars. LBVs are the brightest blue irregular variable stars in any large star-forming galaxy. They can achieve the highest mass-loss rates of any known types of stars, and they exhibit a wide diversity of irregular and eruptive variability. In the single-star scenario, the hypothesis is that most stars above 30 solar mass pass through an LBV phase. However, the relative ∼ isolation of LBVs from O stars challenges this interpretation, and another hypothesis is emerging that the LBV phenomenon is the product of binary evolution. To test these hypotheses, we modeled the dissolution of young clusters and the separation between O stars and LBVs. We find that the single-star scenario is inconsistent with the observed LBV environments. If LBVs are single stars, then the lifetimes inferred from their luminosity and mass are far too short to be consistent with their isolation from O stars. This implies that LBVs are likely products of binary evolution. To further constrain these hypotheses, we must first infer the fundamental properties of LBVs such as luminosity, mass, and age. Ultimately, these depend upon accurate Galactic distances. Using Gaia parallaxes, we find that nearly half of the Galactic LBVs are significantly closer than previous literature estimates; these new distances lower their luminosities and their initial masses. We also infer a closer distance to the massive cluster, Westerlund 1, which hosts an LBV, 24 Wolf- Rayet stars, 6 yellow hypergiants, and a magnetar. Together these Gaia-based distances are more accurate (at least a factor of ten) and have consequences for the late-stage evolution of massive stars. ix CHAPTER 1 BACKGROUND Massive stars dramatically impact the evolution of galaxies. The high photon flux of mas- sive stars ionize galaxies and the Universe. Their winds and explosions influence the gas dynamics of entire galaxies, and their explosive deaths produce many of the elements in the universe heavier than hydrogen or helium. Therefore, it is important to understand their evolution. Stellar evolution theory predicts that massive stars, evolve rapidly through many dynamic phases, and how this evolution proceeds affects the massive star's fate. In particular, this evolution can affect whether or not they explode.
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