Ecology of Blue Straggler Stars Astrophysics and Space Science Library

EDITORIAL BOARD Chairman W. B. BURTON, National Radio Astronomy Observatory, Charlottesville, Virginia, U.S.A. ([email protected]); University of Leiden, The Netherlands ([email protected]) F. BERTOLA, University of Padua, Italy C. J. CESARSKY, Commission for Atomic Energy, Saclay, France P. EHRENFREUND, Leiden University, The Netherlands O. ENGVOLD, University of Oslo, Norway A. HECK, Strasbourg Astronomical Observatory, France E. P. J. VAN DEN HEUVEL, University of Amsterdam, The Netherlands V. M. KASPI, McGill University, Montreal, Canada J. M. E. KUIJPERS, University of Nijmegen, The Netherlands H. VAN DER LAAN, University of Utrecht, The Netherlands P. G. MURDIN, Institute of Astronomy, Cambridge, UK B. V. SO M OV, Astronomical Institute, Moscow State University, Russia R. A. SUNYAEV, Space Research Institute, Moscow, Russia

More information about this series at http://www.springer.com/series/5664 Henri M. J. Boffin • Giovanni Carraro • Giacomo Beccari Editors

Ecology of Blue Straggler Stars

123 Editors Henri M. J. Boffin Giacomo Beccari Giovanni Carraro ESO ESO Vitacura Garching bei MunchenR Santiago de Chile Germany Chile

ISSN 0067-0057 ISSN 2214-7985 (electronic) ISBN 978-3-662-44433-7 ISBN 978-3-662-44434-4 (eBook) DOI 10.1007/978-3-662-44434-4 Springer Heidelberg New York Dordrecht London

Library of Congress Control Number: 2014957414

© Springer-Verlag Berlin Heidelberg 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.

Cover illustration: The centre of the globular cluster Messier 4, as seen with the Hubble Space Telescope. Globular clusters are the first place where blue straggler stars were discovered and are still ideally suited to study them, even though we now have found these strange objects in many different places. Credit: ESA/Hubble & NASA

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Springer is part of Springer Science+Business Media (www.springer.com) Ah—but I was so much older then; I’m younger than that now Bob Dylan

Foreword

Don’t be fooled: even though the idea of this book arose from the Ecology of Blue Stragglers workshop which took place at the ESO headquarters in Santiago, Chile, this is not the proceedings of the conference. The editors, indeed, have from the start decided to produce the first ever textbook on this important subject. This book, therefore, will provide an invaluable resource to students and researchers interested in this rich, yet far from understood, stage in stellar evolution. The book begins with a deep review of the observational status of blue straggler stars (BSS) distributed in several chapters dealing with blue straggler stars (BSS) in clusters, nearby dwarf irregular galaxies, and the field. The picture that emerges from the observations confirms the earlier impressions that the BSS phenomenon is somehow related to binarity. Having firmly established the observational status of BSS, the book then goes on to summarise the essential aspects of the theory of BSS genesis, which seems intimately related to mass transfer between binary components, such that the evolution of both the mass-donor and mass-accreting stars is changed by the mass transfer process. Even more spectacularly, in order to reconcile the theory with the observations, it seems necessary to invoke two channels for the formation of BSS: mass transfer via the canonical Roche overflow, and a merger of the two components. This opens a completely new way of understanding stellar evolution that is not limited to low-mass stars, but seems to be ubiquitous across the entire Hertzsprung–Russell diagram. This has profound consequences in our understanding of stellar populations in galaxies, which are also described in this timely and comprehensive book.

VLT Programme Scientist Jorge Melnick

vii

Preface

The existence of blue straggler stars (BSS), which appear younger, hotter, and more massive than their siblings, is at odds with a simple picture of stellar evolution, as such stars should have exhausted their nuclear fuel and evolved long ago to become cooling white dwarfs. As such, BSS could just be some quirks but in fact their understanding requires a deep knowledge of many different areas in astronomy, from stellar evolution through cluster dynamics, from chemical abundances to stellar populations. In November 2012, a workshop on this important topic took place at the ESO Chilean headquarters in Santiago. The many topics covered at this workshop were introduced by very comprehensive invited reviews, providing a unique and insightful view on the field. These reviews have now become chapters of the first ever book on BSS. The book starts with an introduction about stellar evolution that should be of use to students or researchers from different fields. The following chapter, by R. Cannon, covers the history of the field. It reminds us that the first discovery of blue straggler stars was made in 1953 by Alan Sandage in the globular cluster M3. The germ of the idea that BSS are due to binaries was planted by no-one else than Fred Hoyle, 2 years only after their discoveries, as well as by John Crawford the same year, and then further by William McCrea in 1964. The following chapters are dedicated to a review of the observational evidences, clearly demonstrating that BSS are ubiquitous: they exist in globular clusters (F. Ferraro and colleagues), open clusters (R. Mathieu and A. Geller), and nearby dwarf galaxies (Y. Momany). Field blue stragglers have also been identified from their anomalous kinematics and chemical abundances (G. Preston). R. Mathieu and A. Geller present the incredible fine study of the BSS population in the open cluster NGC 188, allowing his colleagues and him to do, as they call it, “micro-astronomics”, i.e. determining the binary frequency and orbital parameter’s (period, eccentricity and secondary mass) distributions. G. Preston shows what we know of BSS in the field, and the conclusion is that BSS in the field and in open clusters present a high binary fraction, with orbital periods of 1,000 days, small orbital eccentricities and typical secondary masses of 0.5 Mˇ. Whether this is

ix x Preface similar to the population of BSS in globular clusters is still unknown, but such properties are most likely the result of mass transfer from an AGB star, which is also responsible for the formation of peculiar red giants. The fact that most field BSS are chemically enriched in s-process elements—the exact signature of peculiar red giants!—is thus perhaps no surprise, even though much more work is required to connect the two categories of stars. F. Ferraro and his group present the challenges linked with observing these relatively faint stars in over-crowded stellar regions as the globular cluster cores. Blue straggler stars are a common population found in each globular cluster properly observed, with some clusters (e.g. M80) having more than 100 BSS identified! This allows for a comparative analysis of BSS in globular clusters with different physical parameters showing the deep connection that is emerging between the BSS properties and the parent cluster dynamical evolution. Y. Monamy takes us to even larger scales and presents the populations of BSS in dwarf galaxies, looking in details at their radial distribution and luminosity function. The following chapters are devoted to the theory and the interpretation of the data. Boffin reviews the mass transfer by wind scenario in binary systems, while N. Ivanova covers Roche lobe overflow. If the latter mass transfer scenario is most likely to occur in the very short period systems (with orbital period of a few days or less), the crucial question is to know in which conditions the mass transfer from an AGB star could be stable, so as to avoid the common-envelope phase, thought to cause a dramatic shrinkage of the orbit and so unable to explain the long orbital periods observed in open clusters. Further, H. Perets and M. Davies present the possible formation channels for BSS in their various environments, S. McMillan reviews the dynamical evolution of globular clusters and how BSS fit into the picture, while C. Knigge and A. Sills make detailed confrontations between theory and observations. In some cases, data exist that are so precise as to get detailed information on the characteristics of some blue straggler systems—mass, temperature and luminosity—so as to confront them with evolutionary tracks and mass transfer modelling. The conclusion is that it is still unclear how to produce BSS with their observed luminosities, much too low for their mass, or how one can even produce BSS with such high masses. Similarly, although it is generally assumed that two mechanisms are responsible for blue straggler stars—merger and mass transfer (both of which are operating in the same cluster)— the exact preponderance of one mechanism over the other is clearly depending on the cluster’s property, that is, the blue straggler’s ecology. The last chapter by Y. Xin and L. Deng shows the importance of BSS in stellar population synthesis, and how these can dramatically alter the observables of distant systems—their neglect could lead to large errors in the inferred properties of galaxies. Preface xi

The few good reviews about BSS date back from the 1990s and are completely outdated. No book existed on this important topic. We have now remedied to this situation and we hope this will bring even more researchers to this exciting field of study.

Santiago de Chile, Chile Henri Boffin Santiago de Chile, Chile Giovanni Carraro Garching bei München, Germany Giacomo Beccari

Contents

1 Introduction to the Theory of Stellar Evolution ...... 1 Giacomo Beccari and Giovanni Carraro 1.1 ThePre-MainSequencePhase ...... 1 1.2 TheMainSequence ...... 3 1.3 The Combustion of Hydrogen in a Shell: The Sub andRedGiantBranches ...... 4 1.4 TheHelium-BurningintheCore:TheHorizontalBranch...... 8 1.5 TwoBurningShells:TheAGB...... 10 1.6 TheFinalStagesoftheEvolutionoftheStars...... 13 References...... 16 2 Blue Straggler Stars: Early Observations That Failed to Solve the Problem...... 17 Russell D. Cannon 2.1 Introduction...... 17 2.2 The Classical BlueStragglers ...... 19 2.2.1 GlobularClusters...... 19 2.2.2 TheOlderOpenClusters...... 20 2.2.3 Younger Open Clusters and BSSs in the Field ...... 23 2.3 EarlyIdeasontheOriginofBlueStragglers...... 24 2.3.1 BlueStragglersandAlgol-TypeEclipsingBinaries ...... 24 2.3.2 OtherPossibleExplanationsforBSSs...... 25 2.4 Expanding the Definition of BSSs, 1970–1990 ...... 26 Conclusions ...... 26 References...... 27 3 The Blue Stragglers of the Old Open Cluster NGC 188...... 29 Robert D. Mathieu and Aaron M. Geller 3.1 BlueStragglersinOpenClusters...... 29 3.1.1 TheOpenClusterNGC188...... 30 3.1.2 The WIYN Open Cluster Study and Radial Velocities ... 30

xiii xiv Contents

3.2 ObservationalFindingsfromtheBlueStragglersinNGC188 .... 32 3.2.1 BinaryFrequency...... 32 3.2.2 Orbital Period and Eccentricity Distributions ...... 32 3.2.3 Secondary-Star Mass Distribution ...... 34 3.2.4 DetectionofWhiteDwarfCompanions...... 35 3.2.5 Stellar Rotational Velocities...... 36 3.2.6 SpatialDistribution...... 38 3.2.7 Blue-StragglerMasses...... 39 3.2.8 InsightsfromTwoNotableBlueStragglerSystems ...... 42 3.2.9 Summary...... 44 3.3 Blue Straggler Formation Within an N -Body Model ofNGC188...... 45 3.3.1 The NGC 188 N -BodyModel...... 45 3.3.2 Formation Channels for Blue Stragglers intheNGC188Model...... 46 3.3.3 Implications for the Origins of the NGC 188 BlueStragglers...... 48 3.3.4 Outstanding Questions and Missing Pieces in the N -BodyModel ...... 53 3.3.5 Efficiency of Mass Transfer in the N -BodyModel...... 56 3.3.6 Summary of Findings from N -Body ModelingofNGC188...... 59 Conclusions ...... 61 References...... 62 4 Field Blue Stragglers and Related Mass Transfer Issues...... 65 George W. Preston 4.1 Introduction...... 65 4.1.1 Historical Developments in First Part oftheTwentiethCentury...... 65 4.2 IdentificationofBSS...... 67 4.2.1 TheMetal-PoorHalo...... 67 4.2.2 FBSoftheThick/ThinDisc...... 75 4.3 GroupPropertiesofMetal-PoorFBS ...... 75 4.3.1 Colour Boundaries...... 75 4.3.2 SpecificFrequencies...... 78 4.3.3 The Distinguishing Characteristics of FBS BinaryOrbits...... 79 4.4 GalacticDistribution...... 82 4.4.1 SmoothHaloField...... 82 4.4.2 TheGalacticBulge...... 83 4.4.3 HaloStreams...... 84 4.5 Metal-Rich A-Type Stars Above the Galactic Plane: AnotherInconvenientTruth...... 85 Contents xv

4.6 Abundance Issues ...... 86 4.6.1 Lithium...... 87 4.6.2 Alpha/Fe ...... 88 4.6.3 TheNeutron-CaptureElements...... 89 4.6.4 How Many Evolved Mass-Transfer Binaries AreThere?...... 90 4.7 PulsatingBlueStragglers ...... 91 4.7.1 Non-variable Stars in the SX Phe Instability Strip ...... 91 4.7.2 RRLYR-02792, Archetype of a New Kind ofMass-TransferPulsator...... 93 4.8 OddsandEnds...... 93 4.8.1 MassTransferinHierarchicalTriples ...... 93 4.8.2 Heresy...... 94 References...... 95 5 Blue Straggler Stars in Globular Clusters: A Powerful Tool to Probe the Internal Dynamical Evolution of Stellar Systems...... 99 Francesco R. Ferraro, Barbara Lanzoni, Emanuele Dalessandro, Alessio Mucciarelli, and Loredana Lovisi 5.1 Introduction...... 99 5.2 BSSSpecificFrequencyandPrimordialBinaryFraction...... 100 5.3 TheUltravioletRoutetoStudyBSS ...... 103 5.4 The Discovery of the Double BSS Sequence ...... 106 5.5 TheBSSRadialDistribution ...... 109 5.6 Setting the Dynamical Clock for Stellar Systems...... 111 5.7 SearchingfortheBSSProgeny:EvolvedBSSs ...... 116 5.8 ChemicalandKinematicalPropertiesofBSSs ...... 119 References...... 125 6 The Blue Straggler Population in Dwarf Galaxies ...... 129 Yazan Momany 6.1 Introduction...... 129 6.1.1 BlueStragglersinGlobularClusters ...... 129 6.1.2 TheImportanceofDwarfGalaxies...... 130 6.1.3 DwarfGalaxiesvsGlobularClusters...... 131 6.2 BSSIdentificationinDwarfGalaxies...... 132 6.2.1 TheBSSIdentificationintheGalacticHalo ...... 134 6.3 BSSSpecificFrequencyinDwarfGalaxies ...... 136 6.3.1 TheDwarfGalaxiesSample ...... 136 BSS 6.3.2 The FHB MV Anti-correlation...... 137 6.3.3 TheSignificanceoftheAnti-correlation ...... 141 6.4 The BSS Radial Distribution and Luminosity Function inDwarfGalaxies...... 143 6.4.1 RadialDistribution ...... 143 6.4.2 LuminosityFunction ...... 144 xvi Contents

6.5 VariableBSSinDwarfGalaxies...... 145 6.6 TheProgenyofBSS...... 147 Conclusions ...... 149 References...... 150 7 Mass Transfer by Stellar Wind ...... 153 Henri M.J. Boffin 7.1 Stars in Couple ...... 153 7.2 WindMassTransfer...... 157 7.2.1 The Bondi–Hoyle–Lyttleton Model ...... 157 7.3 WindAccretioninBinarySystems...... 160 7.3.1 ChemicalPollution ...... 163 7.3.2 OrbitalParametersEvolution ...... 164 7.3.3 Spin-UpofAccretor...... 165 7.3.4 Angular Momentum Loss ...... 165 7.4 TheZooofPeculiarStars...... 168 7.4.1 BariumandRelatedStars ...... 168 7.4.2 SymbioticStarsandtheCaseofSSLep ...... 171 7.4.3 TheFellowshipoftheRing ...... 173 7.4.4 Evidence for Wind Accretion Before Common-EnvelopeEvolution...... 175 References...... 176 8 Binary Evolution: Roche Lobe Overflow and Blue Stragglers ...... 179 Natalia Ivanova 8.1 Introduction...... 179 8.2 Stability of the Mass Transfer: The Global Condition...... 181 8.3 Roche Lobe Response...... 182 8.4 Donor’s Response ...... 183 8.4.1 Timescales ...... 183 8.4.2 Envelope’sStructure...... 185 8.5 The Donor’s Response and the Consequences for the Mass Transfer Stability ...... 189 8.5.1 Initial Stability ...... 189 8.5.2 Stability of the Ensuing Mass Transfer ...... 190 8.5.3 StableorNotStable?...... 191 8.5.4 Three-DimensionalProblem...... 192 8.6 The Accretor’s Response and Consequences for Mass Transfer Stability ...... 193 8.6.1 The Stream’s Angular Momentum...... 193 8.6.2 The Accretor’s Response ...... 194 8.6.3 Donor’s Entropy and the Accretor’s Response ...... 195 8.7 HowWellDoWeUnderstandStableMassTransfer?...... 195 Contents xvii

8.8 RLOFandBlueStragglersFormation...... 197 8.8.1 CaseA andEarlyCaseB...... 197 8.8.2 LateCaseB/CaseC ...... 199 8.8.3 RoleofGlobularClusterDynamicsontheRLOF...... 200 References...... 201 9 Formation Channels for Blue Straggler Stars ...... 203 Melvyn B. Davies 9.1 Introduction...... 203 9.2 Stellar Collisions ...... 204 9.3 Post-Collision Evolution ...... 209 9.4 Encounters Involving Binary Stars ...... 210 9.5 MakingBlueStragglersviaBinaryEvolution...... 214 9.6 Comparing Primordial and Collisional Formation RatesinClusters...... 217 References...... 222 10 Dynamical Processes in Globular Clusters ...... 225 Stephen L.W. McMillan 10.1 Introduction...... 225 10.2 Virial Equilibrium ...... 226 10.2.1 TheVirialTheorem...... 226 10.2.2 LengthandTimeScales...... 227 10.3 Relaxation ...... 228 10.3.1 Two-BodyScattering...... 229 10.3.2 Strong Encounters ...... 230 10.3.3 Distant Encounters...... 230 10.3.4 ComparisonofTimescales...... 232 10.3.5 ClusterDynamicalEvolution ...... 233 10.3.6 InternalHeating...... 235 10.4 Multiple Stellar Populations ...... 238 10.5 ModelingStarClusters...... 239 10.5.1 Continuum Methods ...... 240 10.5.2 Monte Carlo Methods ...... 241 10.5.3 N -Body Methods ...... 242 10.5.4 Hardware Acceleration ...... 243 10.5.5 TheKitchenSink ...... 244 10.5.6 TheAMUSESoftwareFramework...... 245 References...... 246 11 The Multiple Origin of Blue Straggler Stars: Theory vs. Observations ...... 251 Hagai B. Perets 11.1 Introduction...... 251 11.2 TheObservedPropertiesofBSSs ...... 252 11.2.1 PhysicalProperties ...... 252 11.2.2 Population Characteristics...... 254 xviii Contents

11.3 ModelsforBlueStragglerStarFormation...... 256 11.3.1 Collisions in Dense Clusters ...... 256 11.3.2 BinaryEvolution...... 258 11.3.3 TripleEvolution ...... 261 11.4 LongTermDynamicalEvolutionofBSSsinClusters ...... 264 11.4.1 MassSegregationinClusters ...... 264 11.4.2 DynamicalEvolutionofBSSsBinaries ...... 265 11.5 BlueStragglerStars:Observationsvs.Theory ...... 266 11.5.1 GlobularClusters...... 266 11.5.2 OpenClusters ...... 270 11.5.3 FieldBSSs ...... 272 11.6 Summary...... 273 References...... 274 12 Models of Individual Blue Stragglers ...... 277 Alison Sills 12.1 Introduction...... 277 12.2 Collisional Models ...... 278 12.3 BinaryMassTransferModels ...... 286 12.4 ParametrisedModels...... 290 12.5 FutureDirections...... 291 References...... 293 13 Blue Stragglers in Globular Clusters: Observations, Statistics and Physics ...... 295 Christian Knigge 13.1 Straw-ManModelsforBlueStragglerFormation...... 295 13.2 All Theory Is Grey: Binary Coalescence and Dynamical Encounters in Practice...... 298 13.3 The Search for the Smoking Gun Correlation I: TheNearConstancyofBlueStragglerNumbers...... 300 13.4 Do Clusters Deplete Their Reservoir of Binary Blue StragglerProgenitors?...... 302 13.5 The Search for the Smoking Gun Correlation II: TheCoreMassCorrelation...... 305 13.6 AlternativeConstraintsonFormationChannels...... 307 13.6.1 RadialDistributions ...... 308 13.6.2 Double Blue Straggler Sequences ...... 309 13.7 The Search for the Smoking Gun Correlation III: Once More,withBinaryFractions...... 310 13.8 SummaryandOutlook...... 315 References...... 316 Contents xix

14 Blue Stragglers in Clusters and Integrated Spectral Properties of Stellar Populations...... 317 Yu Xin and Licai Deng 14.1 Introduction...... 317 14.2 M67: Setting Up the Scheme ...... 320 14.3 ISEDsofGalacticOpenClusters...... 321 14.4 TheMassiveStarClustersintheLMCandSMC ...... 322 14.5 BuildingUpanEmpiricalSSPLibrary ...... 325 14.5.1 A General BSS Distribution Function in Stellar Populations ...... 326 14.5.2 BuildingtheEmpiricalSSPLibrary...... 333 14.6 DiscussionsandProspectives...... 339 References...... 341

Index ...... 343