The VLT-FLAMES Survey of Massive Stars
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Reports from Observers The VLT-FLAMES Survey of Massive Stars Christopher Evans1, 2 The Large and Small Magellanic Clouds (Maeder and Meynet, 2001). There are Stephen Smartt 3 (LMC and SMC) are our nearest cos- also suggestions that the distribution of Daniel Lennon 2 mic neighbours to the Milky Way. Detailed stellar rotation rates may depend on Philip Dufton 3 studies of stars and gas regions in the the metal abundance (Maeder et al. 1999). Ian Hunter 3 Clouds over the past 50 years have re- Rohied Mokiem4 vealed that conditions in them are very Understanding the physical processes Alex de Koter 4 different to those seen in our galaxy. Com- in massive stars has far-reaching implica- Mike Irwin 5 pared to the Sun, the Clouds are found tions, from the feedback of kinetic en- to be metal-poor (“metals” meaning ele- ergy into the interstellar medium and met- ments heavier than helium). The metal al enrichment of their host galaxies, 1 UK Astronomy Technology Centre, content, or metallicity, in the LMC is 40 % to issues such as the progenitors of su- Edinburgh, United Kingdom of that found in the Sun, and drops to pernova explosions and gamma-ray 2 Isaac Newton Group of Telescopes, 20 % in the SMC. These different abun- bursts. The unique combination of high- La Palma, Spain dances mean that the Clouds are an resolution, multi-object spectroscopy 3 Queen’s University of Belfast, excellent laboratory to test our under- from FLAMES provides us with an excel- Northern Ireland standing of the role of metals in star for- lent opportunity to expand on previous 4 University of Amsterdam, mation and stellar evolution. studies, which were limited to a few tens the Netherlands of stars by the available instrumentation. 5 Institute of Astronomy, Cambridge, Interest in massive stars in the Magellanic The FLAMES Survey of Massive Stars has United Kingdom Clouds has been particularly strong in observed a large sample of O and B-type recent years. The evolution of O and early stars, in a range of environments (i.e. the B-type stars is dominated by the ef- Milky Way, the LMC and the SMC) to fully We have observed an unprecedented fects of mass-loss from their strong stellar investigate the role of metallicity on stellar sample of 800 massive stars in open winds, with the most massive O stars evolution. cluster fields in the Magellanic Clouds already losing a significant amount of their and Milky Way, primarily with the initial mass over their core hydrogen- multi-fibre FLAMES instrument. The sur- burning lifetimes. Stellar winds are accel- Target fields vey addresses the role of environment, erated by momentum transfer from via stellar rotation and mass-loss, on photons in the radiation field to metal ions Our FLAMES fields were centred on the evolution of the most massive stars, (such as carbon, nitrogen, and iron) in seven stellar clusters, selected to sample which are the dominating influence on the outer atmosphere of the star. It follows a range of age and metallicity as summa- the evolution of young, star-forming gal- that the intensity of these winds, and rised in Table 1. Our targets were selected axies. therefore the mass lost by a star over its from images taken with the Wide-Field lifetime, are expected to be dependent on Imager (WFI) on the 2.2-m telescope at the metallicity of the region in which the La Silla, most of which were from the ESO star formed (Kudritzki et al., 1987). Obser- Imaging Survey pre-FLAMES programme. vational evidence of this has been rela- We have observed 750 stars with tively scarce, limited to small samples of FLAMES, using six of the standard wave- O stars (Puls et al. 1996) and luminous length settings of the Giraffe spectro- B-type supergiants (Evans et al. 2004; graph at high resolution (R ~ 20 000). This Crowther et al. 2006). gives continuous wavelength coverage from 385–475 nm in the blue, with addi- Another factor that strongly affects the tional coverage from 638–662 nm. The evolution of a star is its rate of rota- red region includes the Hα Balmer line, an tion. Stellar evolutionary models that in- important diagnostic of mass-loss from clude the effects of rotation predict stellar winds and invaluable for identifica- enhanced amounts of helium and nitrogen tion of Be-type stars. The brightest 50 at the surface of the atmosphere. The stars in the three Milky Way clusters were importance of this process is thought observed separately with FEROS on the to depend on the initial metal abundance 2.2-m. The survey is introduced at length Table 1: Summary of fields observed with VLT-FLAMES. Metallicity “Young clusters” “Old clusters” (< 5 Myrs) (10–20 Myrs) Milky Way Solar NGC 6611 NGC 3293 and NGC 4755 LMC 0.4 * Solar N11 NGC 2004 SMC 0.2 * Solar NGC 346 NGC 330 36 The Messenger 122 – December 2005 Figure 1: V-band WFI image of FLAMES targets in Figure 2: Two FLAMES spectra of note. The lines N11 in the LMC. O-type stars are marked as open identified in N11-026, from left to right, are N IV λ4058; blue circles, B-type stars by yellow circles. Star #26 is Si IV λλ4089-4116; N V λλ4604-4620; N III λλ4634- an early O-type star in the north of the field, with its 4640-4642. The lines identified in NGC 346-023 are spectrum shown in Figure 2. The solid black lines are Fe II λλ4549-4556-4584-4629. For clarity both spec- simply the gaps between CCDs in the WFI mosaic tra have been smoothed by a 1.5 Å FWHM filter. array. 2.0 Fe II NGC 346-023 Be 1.5 N IV Si IV N III N 11-026 O2.5 III(f*) 1.0 N V 0.5 4000 4100 4200 4300 4400 4500 4600 4700 Wavelength (Å) by Evans et al. (2005), together with a a two-stage starburst. Star #26 and the The FLAMES data are uniquely powerful discussion of the Milky Way data. A simi- other O stars beyond the central region in another regard. With repeat obser- lar paper giving a thorough overview of of the field are likely members of this vations at each of the different wavelength the LMC and SMC data will also be pub- “second generation” of star formation. settings, the survey took over 100 h lished in due course. The distribution of of VLT time to complete. The survey was spectral types in the whole survey is sum- Naturally we have found a number of therefore undertaken in service mode, marised in Table 2. Be-type stars in the survey, with some entailing observations at many different displaying permitted Fe II emission lines. epochs in Periods 71 and 72. This means A V-band WFI image of the N11 region in The morphology of both the Hα and that we are very sensitive to the detec- the LMC is shown in Figure 1. Most of Fe II profiles suggest that we are observing tion of binaries, enabling firm lower limits the FLAMES targets are highlighted, dis- the stars from a range of perspectives. to be put on the binary fraction in our tinguishing between O and B-type stars. Single-peaked emission is seen in some, fields, of interest in the context of star for- There is significant structure in the N11 compared to others which show twin- mation and the initial mass function – region, with many subtle gaseous arcs peaked profiles, usually interpreted as ob- such spectroscopic monitoring has rarely and filaments seen only in the near-IR. The serving the star ‘edge-on’ through a been done before, and certainly not dense nebula to the north of centre is circumstellar disc. One of these Be-type with such high-quality data as that from Lucke-Hodge 10 in which, as one might stars in our NGC 346 field is shown in FLAMES. In some cases the spectro- expect, a number of O-type stars are Figure 2. These high-resolution data will scopic data are sufficiently well-sampled found. provide new insights into the physical enough to yield periods, and to con- properties and nature of Be-type stars. strain the properties of the individual com- The hottest normal stars are those classi- ponents. The FLAMES data provide us fied as types O2 and O3, with three of these discovered in Lucke-Hodge 10 by Parker et al. (1992). There are still only Table 2: Overview of the distribution of spectral types in the FLAMES survey. ~ ten O2-type stars known anywhere, so the statistical sample for attempting Field O Early-B Late-B AFG Total studies of these objects remains small. (B0-3) (B5-9) Star #26 in our N11 field (marked in Fig- NGC 3293 – 48 51 27 126 ure 1, with the FLAMES spectrum shown NGC 4755 – 54 44 10 108 in Figure 2) is particularly interesting NGC 6611 13 28 12 32 85 and is classified as O2.5 III (f*). Aside from NGC 330 6 98 11 10 125 providing a further example of one of NGC 346 19 84 2 11 116 these extreme objects, its location away NGC 2004 4 (+ 1 WR) 101 6 7 119 from the centre is also of note. The gen- N11 43 77 – 4 124 eral consensus is that this region is Total 86 490 126 101 803 The Messenger 122 – December 2005 37 Reports from Observers Evans C.