Astronomical News

Report on the Workshop Ecology of Blue Straggler Stars held at ESO Vitacura, Santiago, Chile, 5–9 November 2012

Henri Boffin1 systems, stellar evolution and stellar (talk by F. Ferraro), in open clusters (talks Giovanni Carraro1 ­populations, as well as the dynamics of by R. Mathieu and J. Ahumada) and in Giacomo Beccari 1 clusters. The chosen format of the event nearby dwarf galaxies (Y. Momany). Field (i.e., a low number of participants and a blue stragglers have also been identified large amount of time devoted to discus- from their anomalous kinematics and 1 ESO sion) resulted in a very lively workshop, chemical abundances (G. Preston). These which was much appreciated by the par- reviews revealed the fantastic advances ticipants. The discussions were triggered made in the field in the past decades. The last workshop dedicated to by the very detailed and useful invited blue straggler stars (BSS) took place talks — no less than 12 were given — Ferraro and his group discussed the chal- 20 years ago and there have been a providing a unique and insightful view on lenges associated with observing these great number of subsequent develop- the field. relatively faint stars in overcrowded stellar ments since in both observations and regions, such as the cores of globular theory. The wide range of BSS topics The introductory talk by R. Cannon clusters, and the key advances obtained covered at the workshop are briefly reminded us of the first discovery of blue through high-resolution ultraviolet (UV) summarised; the published proceed- straggler stars in 1953 by Alan Sandage photometry. The speakers provided de­ ings will form the first book on the sub- in the globular cluster M53, as well as the tailed evidence that BSS are a common ject. fact that the only previous proper work- population found in each globular cluster shop on BSS took place in October 1992, properly observed, with some clusters at the Space Telescope Institute, i.e. (e.g., M80) having more than 100 identi- The existence of blue straggler stars 20 years ago! It was also Cannon who fied BSS. The observations of many (BSS), which appear younger, hotter, and recalled that the germ of the idea that ­clusters allow a comparative analysis of more massive than their siblings, is BSS are due to binaries was planted by BSS in globular clusters with different at odds with a simple picture of stellar none other than Fred Hoyle, only two physical parameters to be made, show- evolution, as such stars should have years after their discovery, as well as ing the clear connection that is emerging exhausted their nuclear fuel and evolved by John Crawford in the same year, and between BSS properties and the parent long ago to become cooling white dwarfs. then further developed by William cluster dynamical evolution. Indeed, With the many advances in observational McCrea in 1964. the shape of the BSS radial distribution and theoretical work on blue straggler provides a record of the degree of dynam- stars, the time was considered ripe for ical evolution experienced to date by a dedicated workshop to summarise Observations of BSS the p­ arent cluster, indicating how efficient the current state of the field, identify dynamical friction has been, while the the many still open questions and define As is typical for astronomical confer- presence of double BSS sequences in in a coordinated way new avenues of ences, the first few days were dedicated research to address these issues. The to a review of the observational evidence, resulting workshop brought together clearly demonstrating that BSS are ubiq- Figure 1. The conference photograph spec ­ialists in binaries and multiple star uitous: they exist in globular clusters in the grounds of ESO Vitacura.

48 The Messenger 151 – March 2013 the colour–magnitude diagram could tes- seem to indicate that mass transfer from As R. Cannon also reminded us during tify to, and even date, the occurrence of a giant star may be much more stable his introductory talk, the few good re- the cluster core collapse event. Ferraro’s than previously thought, leaving a much views about BSS date back to the 1990s group also presented exhaustive chemical wider parameter range to explain the and no dedicated book exists on this and kinematical analysis of BSS in globu- existence of BSS. However, it is not cer- exciting topic. We have thus decided to lar clusters. tain that this mass transfer happens early remedy this situation and make use of enough that plenty of mass is still availa- the unique expertise gathered by the R. Mathieu presented a fine study of ble to be transferred, a pre-requisite for a workshop to produce the first ever book the BSS population in the open cluster BSS. dedicated to BSS. The book will be pub- NGC 188, allowing him and his col- lished by Springer in its Astronomy and leagues to perform “micro-astronomics”, H. Perets, M. Davies and A. Geller Astrophysics Library Series and will con- i.e., determination of the binary frequency ­presented the possible formation chan- tain major chapters from all the invited and study of the distributions of the nels for BSS in various environments, speakers, edited by us. We aim at pro- orbital parameters (period, eccentricity S. McMillan reviewed the dynamical evo- ducing a book that will appeal to a large and secondary mass). G. Preston did lution of globular clusters and how BSS community of researchers and especially the same for the BSS in the field, and the fit into the picture, while C. Knigge and to graduate students. The book should conclusion of both these studies is that A. Sills showed detailed confrontations appear by the year’s end. In the mean- BSS present a high binary fraction, with between the theory and observations. In time, you can still view the presentations orbital periods of 1000 days, low orbital some cases, data exist that are precise and watch some of the recorded talks on eccentricities and typical secondary enough to derive detailed information on the conference web page1. masses of 0.5 MA. Whether this is similar the characteristics of some blue straggler to the population of BSS in globular systems (namely mass, temperature and ­clusters is still unknown, but such prop- luminosity), to enable comparison with Acknowledgements erties are, as explained by H. Boffin, very evolutionary tracks and mass transfer It is a pleasure to thank all the participants for con- reminiscent of the population of barium modelling. There were indeed a few sur- tributing so actively to the workshop, and especially and other peculiar red giant stars, and prises: as several reviewers indicated, it the invited speakers for providing very clear and thus possibly the result of wind mass is still unclear how to produce BSS with exhaustive reviews of their respective areas. Special transfer from an asymptotic giant branch their observed luminosities, which appear thanks go to Bob Mathieu for providing an illuminat- ing and entertaining summary talk, from which we (AGB) star. The fact that most field BSS to be much too low for their mass, or unashamedly borrowed to write the present report. are s-process enriched — the exact sig- how one can even produce BSS with Warm thanks also go to the Local Organising Com- nature of barium stars — is thus perhaps such high masses. Similarly, it was gen- mittee for the smooth organisation of the workshop. no surprise, even though much more erally assumed that two mechanisms work is required to connect the two cate- were responsible for blue straggler stars Links gories of stars. — merger and mass transfer — both of which can operate in the same cluster: 1 Ecology of BSS workshop pages: http://www.eso. the exact preponderance of one mech­ org/sci/meetings/2012/bss2012.html Interpreting BSS anism over the other possibly depending on the cluster’s property, that is, the blue Following the observational talks, the straggler’s ecology. The wealth of obser- next days were devoted to the theory and vations reviewed during the workshop the interpretation of the data. As men- showed that it is still impossible to con- tioned, Boffin reviewed the scenario of clude whether stellar collisions do occur, mass transfer by winds, while N. Ivanova and could create some BSS. More work covered Roche lobe overflow. If the latter is clearly still required in this area. mass transfer scenario is most likely to occur in the very short period systems The last invited talk, by L. Deng, showed (with orbital period of a few days or less), the importance of BSS in stellar popu­ the crucial question is to know under lation synthesis, and how the presence which conditions mass transfer from an of BSS can dramatically alter the observ­ AGB star could be stable, so as to avoid ables of distant systems — their neglect the common-envelope (CE) phase. For- can lead to large errors in the inferred mation of a CE is thought to cause a dra- spectrophotometric properties of galax- matic shrinkage of the orbit and thus ies. The last word was given to R. Mathieu would not allow the long orbital periods who brilliantly summarised the workshop observed in open clusters to be explained. and led the final discussion. The recent results from Ivanova’s group

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