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Ecology of Blue Straggler Stars Astrophysics and Space Science Library 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 Printed on acid-free paper 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.
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