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Properties of Extragalactic Wolf-Rayet Stars

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Properties of Extragalactic Wolf-Rayet Stars Wolf-Rayet Populations in Local Group Metal Poor Galaxies Katie A. Tehrani Department of Physics & Astronomy The University of Sheffield A dissertation submitted in candidature for the degree of Doctor of Philosophy at the University of Sheffield 2019 Declaration I declare that, unless otherwise stated, the work presented in this thesis is my own. No part of this thesis has been accepted or is currently being submitted for any other qualification at the University of Sheffield or elsewhere. Much of the work presented here has already been published, such that the work in Chapter2 can be found in Tehrani, Crowther, & Archer I (2017, MNRAS:472:4618) and the work in Chapter5 can be found in Tehrani, Crowther, Bestenlehner, Littlefair, Pollock, Parker, & Schnurr (2019, MNRAS: 484:2692). iii Acknowledgements There have been highs and lows, and I would like to thank the people that have been there with me through it all. Firstly, to Paul. Without your unwavering calm and patience I imagine this story would end differently, but instead I look back on my four years of research with a smile. Thank you for your support, your reassurance, and the confidence you gave me to rise to the challenge. To Andy, who invested considerable time and effort sharing X-ray and statistics exper- tise, and who always made time for a discussion. To Joachim, & Stu for patiently wading through spectral models, or guiding me through radial velocity curve fitting and MCMC. And to Leisa and Pat for sharing their data, taking the time to explain unfamiliar concepts, and offering more help and resources when needed. I've learnt a lot. To Heloise & Martin who often offered a different perspective to my problems, and were experts at teasing out bugs in my codes. Together you kept spirits high and I'm so grateful I was fortunate enough to be completing this PhD with you. Also to Pablo, who despite my best efforts always managed to restore order to the chaos I had created on my computer. To Maman & Bobba. There were times when I felt overwhelmed but you kept me grounded and focussed, and always pushed me to do my best. I get my determination from you. And to Caron & Phil, who were always supportive and motivating, especially when I was nervous. To Joe, who often had to deal with the lows. You've been solid, never once letting me dwell on my anxieties and reminding me to enjoy the experience. You always understood. And finally to Bella, who patiently sat by me through it all. She's a good girl. v Summary Wolf-Rayet (WR) stars are curious beasts. Characterised by broad emission line spectra and strong stellar-winds driving huge mass-loss rates, these stars are rare and unusual. Understanding these stars has many implications for massive star evolution and stellar feedback, however numerous obstacles prevent progress, such as incomplete samples and gaps in our knowledge regarding their formation, binary status, stellar properties and final stages. Within this thesis we have focused on addressing some of these issues through studying the WR population residing in Local Group dwarf galaxies, specifically within IC10 and the 30 Doradus region of the LMC. Using Gemini/GMOS narrow-band optical imaging of IC10 we have completed a thorough search of the region for WR stars, identifying both previously confirmed and new candidates. Follow-up spectroscopy confirmed 3/11 new candidates with the remaining requiring further follow-up. We classified each star and used the spectra to search for evidence of binarity. We update the WC/WN ratio to 1.0±0.4, and using nebular emission lines we also revise the metallicity estimate, finding that IC10 is not as metal poor as previously considered. We also used X-ray data from the T-ReX Chandra X-ray visionary programme to study the 30 Doradus WR population. Our main focus here was to assess the binary status of these stars by looking for signatures of colliding winds. Our results identify 4 clear binary candidates; Mk34, R140a, R136c and Mk39. Using VFTS data we confirm Mk39 as an SB2. Finally, an optical follow-up campaign using VLT/UVES to monitor Mk34 revealed an SB2 system with clear radial velocity variations. We derive the orbital parameters and ob- 3 3 tain direct mass estimates for each component (MAsin (i)=65±7 M and MBsin (i)=60±7 M ). We also perform spectroscopic and evolutionary modelling, the results of which suggests Mk34 is the most massive binary system known to date. vii Contents 1 Introduction1 1.1 Introduction...................................2 1.2 Massive Stars..................................3 1.2.1 OB Stars................................3 1.2.2 WR Stars................................5 1.3 Classification of WR Stars...........................6 1.3.1 WN Sequence..............................7 1.3.2 WC/WO Sequence...........................8 1.3.3 Main Sequence WR Stars.......................8 1.4 Identification of WR Stars...........................9 1.4.1 Optical Observations.......................... 10 1.4.2 IR Observations............................. 15 1.4.3 X-ray Observations........................... 16 1.5 Massive Star Evolution............................. 20 1.5.1 Single Star Origin of WR Stars.................... 20 1.5.2 Binary Origin of WR Stars....................... 24 1.5.3 Supernovae............................... 27 1.6 Properties of WR Stars............................. 29 1.6.1 Deriving Stellar Properties....................... 29 1.6.2 Milky Way Environment........................ 33 1.6.3 Low Metallicity Environment..................... 36 1.7 This Thesis................................... 39 2 The Wolf-Rayet Population of IC10 41 2.1 Introduction to IC10.............................. 42 2.2 Observations................................... 43 2.2.1 Imaging................................. 43 ix 2.2.2 Spectroscopy.............................. 54 2.3 Nebular Results................................. 56 2.3.1 Nebular Extinction........................... 61 2.3.2 Metallicity................................ 61 2.3.3 Star Formation Rate.......................... 65 2.4 Stellar Results.................................. 68 2.4.1 New Wolf-Rayet Stars......................... 68 2.4.2 Binary Fraction............................. 70 2.4.3 Stellar Extinction............................ 71 2.4.4 WR Line Luminosities......................... 76 2.5 Discussion.................................... 78 2.5.1 WR Content............................... 78 2.5.2 WC/WN Ratio............................. 80 2.6 Summary.................................... 81 3 The X-ray Properties of Wolf-Rayet Stars in 30 Doradus 83 3.1 Introduction................................... 84 3.1.1 Chandra X-ray Observatory...................... 85 3.2 T-ReX Survey.................................. 89 3.3 T-ReX WR Sample............................... 90 3.4 X-ray Variability................................ 97 3.4.1 Chi-squared (χ2) - Gaussian Variability Statistic........... 99 3.4.2 C - Poisson Variability Statistic.................... 100 3.4.3 Median Absolute Deviation...................... 101 3.5 Spectra..................................... 101 3.6 XSPEC..................................... 108 3.6.1 1T Model................................ 108 3.6.2 2T Models................................ 114 3.6.3 X-ray Luminosity............................ 117 3.7 Individual Sources............................... 118 3.7.1 R130 (053749.06-690508.1, BAT99 92)................ 119 3.7.2 R135 (053833.62-690450.4, BAT99 95, VFTS 402).......... 119 3.7.3 VFTS 427 (053836.40-690657.5, BAT99 96, Mk53).......... 119 3.7.4 Mk51 (053838.82-690649.6, BAT99 97, VFTS 457).......... 120 3.7.5 Mk49 (053839.15-690621.2, BAT99 98)................ 120 3.7.6 Mk39 (053840.22-690559.8, BAT99 99, VFTS 482).......... 120 x 3.7.7 R134 (053840.54-690557.1, BAT99 100, VFTS 1001)......... 120 3.7.8 R140a1/a2 (053841.59-690513.4, BAT99 101/102, VFTS 507).... 121 3.7.9 R140b (053841.62-690515.1, BAT99 103, VFTS 509)......... 121 3.7.10 R136a1/a2 (053842.38-690602.8, BAT99 108/109).......... 122 3.7.11 R136a3/a6 (053842.32-690603.4, BAT99 106)............. 122 3.7.12 R136c (053842.90-690604.9, BAT99 112, VFTS 1025)........ 122 3.7.13 Mk30 (053843.09-690546.9, BAT99 113, VFTS 542)......... 123 3.7.14 Mk35 (053843.20-690614.4, BAT99 114, VFTS 545)......... 123 3.7.15 Mk34 (053844.25-690605.9, BAT99 116)............... 123 3.7.16 R146 (053847.48-690025.1, BAT99 117, VFTS 617)......... 124 3.7.17 R144 (053853.36-690200.9, BAT99 118)................ 124 3.7.18 VFTS 682 (053855.52-690426.7).................... 125 3.7.19 R145 (053857.06-690605.6, BAT99 119)................ 125 3.7.20 R147 (053911.27-690201.2, BAT99 122, VFTS 758)......... 125 3.7.21 R136b (BAT99 111).......................... 125 3.8 Other T-ReX Early Type Sources....................... 126 3.9 Conclusion.................................... 127 4 The Binary Nature of Wolf-Rayet Stars in 30 Doradus 129 4.1 Introduction................................... 130 4.2 WR stars in 30 Dor............................... 130 4.2.1 R130................................... 130 4.2.2 R135................................... 131 4.2.3 VFTS 427................................ 133 4.2.4 Mk51................................... 133 4.2.5 Mk49................................... 134 4.2.6 Mk39................................... 134 4.2.7 R134................................... 140 4.2.8 R140a1/a2................................ 140 4.2.9 R140b.................................. 141 4.2.10 R136a.................................. 143 4.2.11 R136c.................................
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