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Galactic Archaeology with the Oldest Stars in the Milky Way

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Galactic Archaeology with the Oldest Stars in the Milky Way Galactic Archaeology with the oldest stars in the Milky Way Anke Arentsen Leibniz-Institut für Astrophysik Potsdam (AIP) Kumulative dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) in der Wissenschaftsdisziplin Astrophysik Eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät Institut für Physik und Astronomie der Universität Potsdam und das Leibniz-Institut für Astrophysik Potsdam (AIP) Potsdam, 16.09.2020 Betreuer: Dr. Else Starkenburg/Prof. Dr. Matthias Steinmetz 1. Gutachter: Dr. Else Starkenburg Leibniz-Institut für Astrophysik Potsdam 2. Gutachter: Prof. Dr. Matthias Steinmetz Leibniz-Institut für Astrophysik Potsdam/Universität Potsdam 3. Gutachter: Prof. Dr. Norbert Christlieb Zentrum für Astronomie der Universität Heidelberg/Landessternwarte Published online on the Publication Server of the University of Potsdam: https://doi.org/10.25932/publishup-47602 https://nbn-resolving.org/urn:nbn:de:kobv:517-opus4-476022 “... it is difficult to resist the impression that the evolution of „ the stellar universe proceeds at a slow majestic pace ...” — Sir A. S. Eddington (The internal constitution of the stars, 1920) Contents Summary/Zusammenfassung i 1 Introduction 1 1.1 What can we learn from metal-poor stars about the First Stars? . .4 1.1.1 Using observations to set constraints on the First Stars . .5 1.1.2 Models of CEMP star origins . .6 1.1.3 3D/NLTE effects on the carbon abundance . .7 1.1.4 Contribution of this thesis to the field . .9 1.2 Galactic archaeology with pristine stars . .9 1.2.1 How to find metal-poor stars . 10 1.2.2 Photometric surveys . 10 1.2.3 Recent metal-poor Milky Way results . 12 1.2.4 Contribution of this thesis to the field . 14 1.3 The Galactic bulge . 14 1.3.1 Evidence for a pseudo-bulge in the Milky Way . 15 1.3.2 Different metallicity populations in the bulge . 17 1.3.3 Contribution of this thesis to the field . 17 1.4 Low/medium-resolution spectroscopy . 18 1.4.1 Spectral analysis . 18 1.4.2 Contribution of this thesis to the field . 19 1.5 Thesis overview . 20 2 Binarity in CEMP-no stars 21 2.1 Introduction . 23 2.2 Data........................................ 24 2.2.1 Sample selection and observations . 24 2.2.2 Data reduction . 26 2.2.3 Radial velocity determination . 26 2.2.4 CEMP compilation . 29 2.3 Results . 30 2.3.1 Radial velocity database . 30 2.3.2 Radial velocity variation in the sample . 31 2.3.3 Orbit properties of the new binaries . 33 2.4 Properties of the CEMP-no radial velocity sample . 34 2.4.1 Orbit characteristics . 35 2.4.2 Enhancement in s-process elements . 36 2.4.3 Absolute carbon abundance . 37 2.4.4 Hertzsprung-Russell diagram . 38 2.5 Discussion . 40 2.5.1 CEMP-no stars and binary mass transfer . 40 vi 2.5.2 High fraction of binaries among intermediate- and high-carbon band CEMP-no stars . 41 2.5.3 HE 0107–5240 . 42 2.5.4 Magnesium . 43 2.6 Radial velocity outlook with Gaia ....................... 45 2.6.1 Method . 46 2.6.2 New CEMP binary candidates . 47 2.7 Conclusions . 48 2.8 Appendix . 50 2.8.1 Additional figures . 50 2.8.2 Tables . 52 3 Stellar atmospheric parameters for XSL 57 3.1 Introduction . 59 3.2 Data........................................ 61 3.3 Method . 62 3.3.1 Resolution matching . 65 3.3.2 Rest-frame reduction . 66 3.3.3 Multiplicative polynomial . 66 3.3.4 Fitting a spectrum . 67 3.4 Results . 67 3.4.1 Selection of the reliable solutions . 67 3.4.2 Error analysis . 68 3.4.3 Internal precision . 69 3.4.4 Systematics and total errors . 71 3.4.5 Combination of the UVB and VIS solutions: Adopted parame- ters and total error . 74 3.4.6 External precision: Comparisons with the general literature . 75 3.4.7 Comparison with previous ULySS determinations . 80 3.5 Summary..................................... 82 3.6 Appendix . 84 4 PIGS I – kinematics of metal-poor stars in the Galactic bulge 87 4.1 Introduction . 89 4.2 Data........................................ 89 4.3 Results . 92 4.4 Discussion . 94 4.5 Appendix . 97 4.5.1 Analysis of the spectra . 97 4.5.2 Horizontal Branch stars . 98 4.5.3 Distances to our sample stars . 100 5 PIGS II – survey description 103 5.1 Introduction . 105 5.2 Photometric observations . 107 5.2.1 Data reduction and calibration . 108 5.2.2 Target selection for spectroscopic follow-up . 110 5.3 Spectroscopic follow-up observations . 112 5.3.1 Observing strategy . 113 5.3.2 Data reduction . 114 5.3.3 Radial velocities . 114 5.3.4 Spectral analysis with ULySS . 115 5.3.5 Spectral analysis with FERRE . 118 5.4 Stellar parameter results . 120 5.4.1 Metallicity distribution and Kiel diagram . 120 5.4.2 Total uncertainties . 123 5.4.3 Comparison with APOGEE . 124 5.5 Discussion of PIGS survey performance . 125 5.5.1 Metallicity distribution . 125 5.5.2 Survey success rates . 126 5.6 Summary and outlook . 128 6 PIGS III – CEMP stars in the inner Galaxy 131 6.1 Introduction . 133 6.2 The Pristine Inner Galaxy Survey (PIGS) . 135 6.2.1 Survey overview . 135 6.2.2 Stellar parameters and [C/Fe] using FERRE . 137 6.3 The Pristine filter and its effect on CEMP selection . 138 6.4 Stellar evolution on the giant branch . 142 6.4.1 [C/Fe] in the Kiel diagram . 142 6.4.2 Quantifying the carbon depletion . 143 6.5 CEMP stars in the inner Galaxy . 145 6.5.1 New CEMP stars . 145 6.5.2 What is the CEMP fraction? . 147 6.6 Discussion and conclusions . 148 6.6.1 Caveats and future tests . 148 6.6.2 Interpretations of the CEMP stars and fractions . 149 6.6.3 Conclusions . 151 7 Discussion and future perspectives 153 7.1 The future for PIGS and metal-poor bulge studies . 153 7.1.1 Dynamics of metal-poor bulge stars . 154 7.1.2 Additional spectroscopic efforts . 155 7.2 The most metal-poor stars and CEMP stars in the coming years . 158 7.2.1 Current and future samples of metal-poor stars . 158 7.2.2 Binary properties of (C)EMP stars . 159 7.3 Final remarks . 161 Acknowledgements 163 Bibliography 167 Summary The first stars in the Universe were born out of gas consisting of only the three lightest elements (hydrogen, helium and lithium) – no heaver elements (“metals”) were yet present. Metals were produced in subsequent generations of stars, which have chemically enriched the Universe over time. Low-mass stars that formed in the early Universe are therefore expected to be poor in metals. Some of these stars became members of the Milky Way system over the course of their lives, where we can currently observe them in detail. Metal-poor stars are extremely useful probes to learn about the First Stars and the conditions in the early Universe, and they are also important tools for Galactic Archaeology – the study of the formation and evolution of the Milky Way. Many of the stars with metallicities less than 1% of that of the Sun ([Fe/H] Ç 2.0) have unexpectedly high abundances of carbon. The fraction of such carbon- ¡ enhanced metal-poor (CEMP) stars increases for stars with even lower metallicities. This indicates that there may be an important connection between the CEMP stars and the First Stars. There are two main types of CEMP stars – those that formed as the result of an interaction with a binary companion where material rich in carbon and s-process elements was transferred to the star we see today (CEMP-s stars), and those that are not enhanced in s-process elements (CEMP-no stars). The latter are expected to represent the gas out of which they formed, and are typically the most useful for studying the First Stars. Chapter 2 of this thesis describes the investigation of the binary properties of CEMP-no stars. A difference is found between two groups of CEMP-no stars: the more carbon-rich CEMP-no stars appear to be in binary systems more often 15 14 (47Å14 %) than the rest of the CEMP-no stars (18Å9 %). This.
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