Precision Diagnostics of Pulsation and Evolution in Helium-Rich Low-Mass Stars
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Precision Diagnostics of Pulsation and Evolution in Helium-Rich Low-Mass Stars A dissertation submitted to the University of Dublin for the degree of Doctor of Philosophy Pamela Martin, B.A. (Mod). Armagh Observatory and Planetarium & Trinity College Dublin 2019 School of Physics University of Dublin Trinity College ii Declaration I declare that this thesis has not been submitted as an exercise for a degree at this or any other university and it is entirely my own work. I agree to deposit this thesis in the University's open access institutional repository or allow the Library to do so on my behalf, subject to Irish Copyright Legislation and Trinity College Library conditions of use and acknowledgement. Name: Pamela Martin Signature: ........................................ Date: .......................... Abstract In this thesis we discuss many types of low mass stars linked by their un- usually low hydrogen atmospheric abundance, usually completely replaced with helium. We begin with the hot subdwarfs; subdwarf O and B (sdB, sdO) stars are low-mass core helium burning stars with extremely low-mass hydrogen envelopes. Their atmospheres are generally helium deficient; how- ever a minority have extremely helium-rich surfaces. An additional fraction have an intermediate surface-helium abundance, occasionally accompanied by peculiar abundances of other elements. LS IV−14◦116 is a non-radially pulsating slowly-rotating chemically-peculiar helium-rich sdB with a 4 dex surface excess of zirconium, yttrium, and strontium. The pulsations are un- expected and unexplained, as the star is 6 000K hotter than the blue edge of the hot subdwarf g-mode instability strip. The presence of pulsations offers a rare opportunity to study the structure of the photosphere. Using a time series of ultraviolet and visual spectra from the Very Large Tele- scope (VLT) we have measured high precision radial velocities in individual spectral lines thoughout the entire spectrum. We confirm that the presence of the photometric variability results in the physical motion of the photo- sphere, resolving the different layers proves difficult due to the horizontal nature of the propagation of a gravity wave. In the case of BX Cir, a short-period pulsating extreme helium supergiant, the absence of hydrogen opacity allows us to see deeper into the atmosphere than in any other star. High resolution spectra from the Southern African Large Telescope (SALT) has allowed us to apply the methodology developed for LS IV−14◦116 to resolve the motion in the atmosphere of BX Cir. Our aim was to resolve the motion of different layers within the photosphere, and the passage of the outward travelling waves. We have investigated the kinematics of three groups of hot subdwarf stars to determine whether they belong to similar or different Galactic popula- tions. We confirm that the majority of helium-deficient subdwarfs show a kinematic distribution similar to that of thick disk stars. Helium-rich sdBs show a more diverse kinematic distribution. Although the majority are probably disk stars, a minority show a much higher velocity dispersion consistent with membership of a Galactic halo population. Since the discovery of R Coronae Borealis in 1795, several classes of hydrogen- deficient stars have been identified (Jeffery, 2008) but, despite their great in- trinsic brightness, their members are so rare that virtually all lie at distances too great to measure directly from the ground. Reliable measurements have only become possible for a significant number following the second release of data from the astrometric space mission Gaia. Following this data release the new parallax measurements and more precise proper motions have allowed us to calculate the Galactic space velocities and orbits for over 200 hydrogen-deficient low-mass stars. Distances, and hence luminosities, are critical for explaining the origin and life histories of stars which possess so little surface hydrogen that, in several cases, its abundance cannot be measured. We discuss the formation and evolution of some of the rarest stars in the Galaxy. For my Mum. Acknowledgements First and foremost I would like to thank my supervisor, Simon, for always making time for me, inspiring me, providing a source of motivation and guidance when I have felt lost. I want to say thank you to everyone at Armagh Observatory and Planetar- ium, but especially to the students for making the last four years enjoyable - especially the occasional pictionary afternoons. To my office mates, Rok and Conor, thank you for always keeping me updated on the most random facts that I will never remember. To my one and only, your support has been invaluable to me on this journey. I can't imagine how I would have survived it without you. You are part of my first memory here in Armagh and I cherish the fact I get to leave here with you by my side. Last, but not least, I would like to thank my Mum and Dad for always believing I could achieve anything I put my mind to. List of Publications 1. Martin P., Jeffery, C. S., Naslim, N., and Woolf, V. M. (2017). Kinematics of Subluminous O and B Stars by Surface Helium Abundance. MNRAS, 467:68-82. 2. Jeffery, C. S., Baran, A. S., Behara, N. T., Kvammen, A., Martin, P., Naslim, N., Ostensen, R. H., Preece, H. P., Reed, M. D., Telting, J. H., and Woolf, V. M. (2017). Discovery of a variable lead-rich hot subdwarf: UVO 0825+15. MNRAS, 465:3101-3124. 3. Martin, P. and Jeffery, C. S. (2017). LS IV−14◦116 : A Time-Resolved Spec- troscopic Study. Open Astronomy, 26:240-245 4. Martin, P. and Jeffery, C. S. in prep. First Gaia results for hydrogen-deficient supergiants and related stars. vii 0. LIST OF PUBLICATIONS viii Contents List of Publications vii List of Figures xiii List of Tables xix 1 Introduction1 1.1 The Hertzsprung-Russell diagram . .1 1.2 A Short Introduction to Stellar Evolution . .3 1.3 Basic Galactic Structure . .5 1.3.1 Radial Velocities . .6 1.3.2 Galpy . .7 1.4 Helium-Rich Low-Mass Stars . .8 1.4.1 Hot Subdwarf Stars . .9 1.4.2 Extreme Helium Supergiants . 10 1.4.3 R Coronae Borealis and Hydrogen Deficient Carbon stars . 11 1.4.4 [WC] stars . 11 1.4.5 PG1159 stars . 11 1.4.6 O(He) stars . 12 1.5 Evolution channels . 13 1.5.1 Hot-Flasher Scenario . 13 1.5.2 Merging of White Dwarfs . 14 1.5.3 Thermal Pulses . 16 1.6 Pulsation in sdBs . 17 1.6.1 LS IV−14◦116 ............................ 19 1.7 Pulsation in EHes . 20 1.8 Science Goals . 24 ix CONTENTS 2 Instrumentation and Data Reduction 27 2.1 Gaia ...................................... 27 2.1.1 Data Release 2 . 28 2.2 Echelle Spectrographs . 29 2.3 Very Large Telescope (VLT) . 32 2.3.1 Ultraviolet and Visual Echelle Spectrograph (UVES) . 33 2.3.2 Observations . 34 2.3.3 Data Reduction . 34 2.4 Southern African Large Telescope (SALT) . 36 2.4.1 Observations . 38 2.4.2 Data Reduction . 39 3 A Very Peculiar Intermediate-Helium sdB: LS IV−14◦116 41 3.1 LS IV−14◦116 : A History . 41 3.2 Observations & Methods . 43 3.2.1 Radial Velocity Curves . 43 3.3 Results . 44 3.3.1 Radial Velocity Curves . 44 3.3.2 Fourier Transforms . 46 3.3.3 Line Profile Variations . 48 3.3.4 Line Intensity . 52 3.4 Summary . 57 4 Resolving Photospheric Pulsations of the Extreme Helium Star BX Cir 59 4.1 Introduction . 59 4.1.1 Observations . 60 4.2 Phase folding . 61 4.3 Radial velocity measurements . 62 4.3.1 Errors . 65 4.4 Analysis of radial velocity curves . 66 4.5 Summary . 68 x CONTENTS 5 Kinematics of Subluminous O and B Stars by Surface Helium Abun- dance 73 5.1 Introduction . 73 5.2 Data . 76 5.2.1 Radial Velocities . 76 5.2.2 Abundances . 77 5.2.3 Proper Motions . 80 5.2.4 Distances and Reddening . 80 5.3 Kinematics . 81 5.3.1 Calculating Galactic Velocities . 81 5.3.2 Galactic velocities and velocity dispersions . 83 5.4 Galactic Orbits . 85 5.4.1 Calculating the orbits . 85 5.4.2 The orbits and orbit parameters . 88 5.5 Discussion . 89 5.6 Summary . 93 6 Kinematics of Hydrogen Deficient post-AGB stars with Gaia DR2 95 6.1 Introduction . 95 6.2 Data Sources . 97 6.2.1 Gaia .................................. 97 6.2.2 SDSS . 98 6.3 Helium{rich Hot subdwarfs . 100 6.3.1 Updated results from Chapter 5 . 100 6.3.2 Addition of new He-rich Hot subdwarfs . 102 6.3.3 Galactic and Orbital parameters . 102 6.3.4 Population Classification . 103 6.3.5 Selection Effects . 104 6.3.6 Discussion . 105 6.4 Helium-rich Giants . 110 6.4.1 Sample selection . 110 6.4.2 Galactic and orbital parameters . 110 6.4.3 Population classification . 113 6.4.4 Discussion . 114 6.5 Central Stars of Planetary Nebulae . 115 6.5.1 Sample selection . 115 xi CONTENTS 6.5.2 Galactic and Orbital parameters . 116 6.5.3 Population classification . 116 6.6 Summary and Discussion . 117 7 Evolution of Hydrogen Deficient post-AGB stars with Gaia DR2 121 7.1 Introduction . 121 7.2 Methods . 123 7.2.1 Masses, Radii and Luminosities . 123 7.2.1.1 Error propagation . 124 7.2.2 Bolometric correction . 125 7.3 Hot Subdwarfs . ..