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An Astrophysical Laboratory: Understanding and Exploiting the Young Massive Cluster Westerlund 1

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An Astrophysical Laboratory: Understanding and Exploiting the Young Massive Cluster Westerlund 1 Astronomical Science An Astrophysical Laboratory: Understanding and Exploiting the Young Massive Cluster Westerlund 1 Simon Clark1 sient high-energy phenomena in the Uni- on the interplay of these parameters. At Ignacio Negueruela2 verse: supernovae, gamma-ray bursts a given stellar mass, do these factors Ben Ritchie1 and X-ray binaries. It is therefore some- combine to drive sufficient mass loss to Paco Najarro3 what unsettling that the mechanisms of permit a type Ibc rather than a type II SN? Norbert Langer4 their birth, the processes governing their Is the pre-SN core rotating rapidly Paul Crowther5 subsequent evolution on and beyond the enough to permit a gamma-ray burst? Liz Bartlett6 main sequence (MS) and the nature of Indeed, is the core massive and compact Danielle Fenech7 their deaths and ultimate fate — neutron enough to form a black hole rather than Carlos González-Fernández8 star or black hole — are shrouded in a neutron star, and, if the latter, is it pow- Simon Goodwin5 uncertainty. ered by the extraction of rotational or Marcus Lohr1 magnetic energy? Raman Prinja7 Nevertheless, our current understanding is sufficient to enable us to identify the physical agents driving massive stellar Observational tests 1 Department of Physics and Astronomy, evolution. A central theoretical tenet is The Open University, Milton Keynes, that the evolutionary pathway of a star How, then, may one quantify the roles United Kingdom is dependent on its initial mass, but, for played by these various evolutionary 2 Departamento de Física, Ingeniería the most massive stars, powerful stellar agents? Ideally, one would like to study de Sistemas y Teoría de la Señal, winds and, in their latter evolutionary a homogeneous stellar population where Universidad de Alicante, Spain stages, violent instabilities act to continu- the competing physical effects might 3 Departamento de Astrofísica, Centro de ously strip their hydrogen-rich outer be distinguished. An obvious solution is Astrobiología, (CSIC-INTA), Madrid, layers away. The efficiency of such pro- to employ massive stellar clusters or Spain cesses is likely to contribute to the associations as laboratories, since both 4 Argelander Institut für Astronomie, dichotomy between type II (H-rich) and the age and metallicity of the stars may Bonn, Germany type Ibc (H-poor/depleted) supernovae be expected to be well constrained and 5 Department of Physics and Astronomy, (SNe). Since stellar winds are driven by furthermore they furnish a statistically University of Sheffield, United Kingdom radiation pressure, this in turn introduces robust sample size. 6 Astrophysics, Cosmology and Gravity an additional explicit metallicity depend- 5 Centre (ACGC), Astronomy Department, ence into massive stellar evolution. With a mass likely > 10 MA, the University of Cape Town, South Africa 30 Doradus star-forming complex (R136) 7 Department of Physics & Astronomy, Moreover, the internal dynamics of such within the Large Magellanic Cloud is a University College London, United King- stars, and hence the degree of mixing compelling target (Figure 1, bottom), more dom of chemically processed and unpro- so because the low foreground extinction 8 Institute of Astronomy, University of cessed material they experience, will also permits observations of traditional “blue- Cambridge, United Kingdom play a profound role in their evolution. end” (400–500 nm) spectral diagnostics Since it is likely that both the (differential?) for massive stars. These factors drove rotation of such stars and the presence the development and implementation of Westerlund 1 provides a unique oppor- (or absence) of an internal magnetic field the VLT/FLAMES Tarantula survey (Evans tunity to probe the physics of massive will play key roles in determining the effi- et al., 2011), which encompasses ~ 800 O stars, from birth to death and beyond, ciency of such processes, these too must and B stars and, to date, has resulted as well as the formation and evolution be incorporated into theoretical models. in about 20 refereed publications on the of a super star cluster that appears properties and evolution of its members. destined to evolve into a globular clus- Finally, it has recently been recognised Given this undoubted success, what is ter. We highlight the result of current that a large proportion of massive stars the motivation for additional studies? studies of this cluster, its diverse stellar are born in binary systems, and that in constituents and immediate environ- many cases these are close enough to We may identify a number of factors. ment, concluding with a summary permit interactions on or beyond the Most importantly one would wish to of future research avenues enabled by MS (e.g., Sana et al. 2013; de Mink et al., study differing metallicity environments in ESO facilities. 2014). Since this can lead to the removal order to determine the effect of chemical of the outer layers of a mass donor, the composition on stellar evolution. Sec- rejuvenation/growth of the mass recipient ondly, star formation across the ~ 200 pc Massive stars play a central role in the and, in extreme cases, their merger, it extent of 30 Dor appears to have pro- evolution of galaxies, from the early is apparent that both the binary fraction ceeded in multiple bursts over ~10 Myr Universe through to the current epoch, as well as the physical properties (e.g., (Walborn & Blades, 1997). Therefore, via their intense radiation fields and the mass ratio and orbital separation) also identifying the co-eval populations ideally deposition of chemically enriched gas profoundly impact stellar evolution. required to most effectively investigate and solid-state material into the inter stellar stellar evolution is non-trivial, especially medium. Moreover they are ultimately One can thus readily envisage how the since the most likely locations to host responsible for the most energetic, tran- endpoints of the stellar lifecycle depend them — dense individual clusters such as 30 The Messenger 159 – March 2015 Figure 1. Top: Optical image of West- R136 — are difficult to probe due to erlund 1 (~ 7.3 pc on a side) taken with crowding. Despite this extended star for- the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope, with mation history, 30 Dor appears to lack BVR filters, showing the position of the cool hypergiants which help drive the both the magnetar and putative binary extensive mass loss required to facilitate companion Wd1-5. the formation of H-depleted Wolf–Rayet Middle: Wide-field optical image (~ 175 pc on a side) of the field centred stars (WRs). Finally one might also wish on Wd1 (the fuzzy orange blob in the to explore whether the environment in centre). which the stars form affects their life Bottom: HST composite-colour image cycle, e.g., via the dynamical formation, of the star- forming complex 30 Dora- dus in the Large Magellanic Cloud modification and disruption of binaries. (size 185 pc by ~ 146 pc), from a com- bination of Hubble Space Telescope Historically, it was thought that the mas- Advanced Camera for Surveys/Wide sive stellar aggregates required to enable Field Channel (ACS/WFC) and Wide Field Camera 3 (WFC3/UVIS) images such studies were absent from the Gal- (i-band) and WFI ([O III] and Hα narrow- axy. However the advent of near- and band filters). mid-infrared surveys revealed an ever- expanding population of heavily obscured young massive clusters, alleviating this apparent deficiency. Nevertheless, given the effort expended in uncovering this population, it is frustrating that possibly DSS the most massive cluster in the Galaxy appears to have been hidden in plain sight: in 1961 Bengt Westerlund simply characterised his discovery as a very young “heavily reddened” cluster, with Westerlund 1 (Wd1; Figure 1, top and middle) languishing in relative obscurity for the next forty years. In retrospect the detection of an unprece- dented cohort of cool, short-lived yellow hypergiants (YHGs) and red supergiants (RSGs) in the earliest observations of Wd1 obviously required a substantial population of massive progenitors; a clear indication that the cluster was worthy of follow-up observations. Motivated instead by the unusual radio properties of Wd1, the discovery that it hosted unprece- dented numbers of both blue super- and hypergiants (BSG/BHGs) and WRs came as a complete surprise (Clark et al., 2005). Analysis of these populations implied a distance of ~ 5 kpc, a radius of < 2 pc (in comparison to the 200 pc extent of 30 Dor; Figure 1, bottom) and an apparent 5 integrated mass of ~10 MA, revealing that Wd1 was the first direct Galactic NASA, ESA and Lennon D. (ESA/STScI) analogue of the super star clusters that characterise the young stellar population of starburst galaxies. Intriguingly, the simultaneous presence of both WRs and RSGs was unexpected, suggesting that we either view Wd1 at a privileged point in its evolution, or that it comprises two or more stellar The Messenger 159 – March 2015 31 Astronomical Science Clark S. et al., Understanding and Exploiting the Young Massive Cluster Wd1 Figure 2. Montage of representative red-end optical "l spectra of evolved OB stars in Wd1 showing the smooth progression in both spectral type and lumi- 6.!AHM nosity class expected for a coeval stellar population (from Clark et al., in prep.). 6.((( of photometrically selected targets, S RD but no such population was identified. EE Entirely unexpectedly, Wd1 appears to N 6. K@A have formed in splendid isolation. Given TW "((( k both its coevality and the absence of 6!K@A additional, contaminating populations it thus serves as a “gold-standard” labora- tory for the analysis of massive stellar -NQL@KHRDC 6A! K@ evolution. 'D( This, however was not the only surprise. 6! K@ Ostensibly designed to search for binary -( candidates, a multi-epoch radial velocity #(! (RV) survey of the OB star population of /@RBGDMRDQHDR Wd1 was undertaken with VLT/FLAMES (Ritchie et al., 2009).
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