VLT-FLAMES observations of young stellar clusters in the Magellanic Clouds

We introduce our VLT-FLAMES survey of massive stars in the Galaxy and the Magellanic Clouds, giving details of the observations in our younger fields in the LMC and SMC. In particular we highlight a new O2.5type star discovered in N11, and Be-type stars in NGC 346 with permitted Fe II emission lines in their spectra. We give an overview of the distribution of spectral types in these fields and summarize the observed binary fraction. 1. Introducing the VLT-FLAMES survey of massive stars We have used the Fibre Large Array Multi-Element Spectrograph (FLAMES) at the Very Large Telescope (VLT) to observe massive stars in 7 fields in the Galaxy and Magellanic Clouds. Our target fields were centered on stellar clusters and were selected to sample a range of metallicity (Z) and age. In practice this means observing fields in the three most readily accessible, yet differing environments, i.e. the Milky Way and the Large and Small Magellanic Clouds (LMC and SMC respectively), as summarized in Table 1. Table 1. Summary of fields observed with VLT-FLAMES ‘Young clusters’ ‘Old clusters’ (<5 Myrs) (10-20 Myr) Milky Way NGC 6611 NGC 3293 & 4755 LMC N11 (inc. LH9/10) NGC 2004 SMC NGC 346 NGC 330 FLAMES has 132 science fibres which are fed through to the Giraffe spectrograph. Notionally blue targets were selected from pre-imaging and the FLAMESGiraffe combination was used to observe a total of 750 stars at high-resolution (R ∼20,000), at six wavelength settings. These observations form an unprecendented high-quality spectroscopic survey of OB-type stars, with coverage in the 1 2 Evans, Lennon & Smartt blue optical region from 3850-4750 Å, and from 6400-6650 Å, to include the Hα Balmer line. The primary motivations and observational details of the survey have been presented by Evans et al. (2005), together with an overview of the Galactic data. A number of parallel studies are now underway using both the Galactic and Magellanic Cloud data to address issues such as: • To determine the vsini distributions of the observed stars, is there evidence for a dependence with Z? (cf. Maeder et al. 1999; Keller 2004). • To obtain physical parameters (including CNO abundances) for a large, homogenous sample of OB stars. These will then be coupled with the vsini results to enable comparisons with evolutionary models that attempt to include the effects of rotation e.g. Maeder & Meynet (2000, 2001). • To explore the Z-dependence of stellar wind mass-loss rates in O-type stars (cf. Vink et al. 2001; see de Koter et al., these proceedings). 2. Census of N11 and NGC346 fields Here we focus on the two younger fields in the Magellanic Clouds, N11 and NGC346. These are the richest in the survey in terms of the numbers of O-type stars, and contain a number of interesting objects. The distribution of spectral types in these two fields is given in Table 2. Table 2. Summary of numbers of stars observed in the N11 and NGC346 fields Field O B0-3 B5-9 Be AFG Total N11 43 67 − 10 4 124 NGC 346 19 57 2 27 11 124 2.1. A new O2.5-type star in the N11 field The N11 field includes the OB associations LH9 and LH10 (Lucke & Hodge, 1970), of particular interest in the context of sequential star-formation. Prior to our survey the best source of spectroscopic information in this region was the study by Parker et al. (1992), which found an apparently younger population in LH10, suggesting that star-formation in that association was triggered by the evolution of the most massive stars in LH9. Owing to observational constraints (such as crowding) we have not observed all of the stars in the Parker et al. study, but we have also explored the spectral content in other nearby regions. Three O3-type stars were reported in LH10 by Parker et al., all of which were considered by Walborn et al. (2002) in their extension of the MK classification scheme to include a new O2 subtype (with one star from Parker et al. reclassified as an O2 giant). In principle the hottest ‘normal’ stars, there are still only ∼10 FLAMES Survey of Massive Stars 3 O2-type stars known. Here we report the discovery of a new star classified as O2.5 III(f). The spectrum (shown in Figure 1) lies between the standards published by Walborn et al. (2002, 2004) and so an intermediate O2.5 subtype is employed. Interestingly the star is not in LH10, nor in any of the other denser gas regions in the field, and is ∼4.5 to the north of LH10. The pre-FLAMES imaging reveals an apparent ionization front in the nearby gas; moving beyond the central region of LH9 and LH10, there is still clearly a lot to learn about the star-formation process in this region. Figure 1. A newly discovered O2.5-type star in the LMC; for clarity the spectrum has been smoothed by a 1.5 Å fwhm filter. The lines identified from left to right are N IV λ4058; Si IV λλ4089-4116; NV λλ4604-4620; NIII λλ4634-4640-4642. 2.2. Analysis of OB-type stars Of the 62 O-type stars observed in these two fields, half were unknown prior to this survey; these data form the core of the analysis by Mokiem et al. (in preparation), which employs an automated approach to the problem of analysing such a large sample with contemporary model atmosphere codes (see Mokiem et al., 2005). The FLAMES survey has also yielded a large number of early B-type spectra, spanning a range of luminosity classes. Our final aim is the consistent analysis of the whole sample; as a first step toward this, 15 of the narrow-lined (i.e. low vsini) stars in NGC346, and 21 in N11, have been analysed by Hunter et al. (in preparation) using the TLUSTY model atmosphere code (Hubeny & Lanz, 1995). 4 Evans, Lennon & Smartt 2.3. Be-type spectra The observations in NGC346 have revealed a number of Be-type stars with (permitted) emission lines from Fe II in their spectra. The morphology of the Hα and Fe II profiles suggests that we are sampling a number of different projection angles; some display single-peaked emission, whereas others show twin-peaked emission, commonly interpreted as viewing a circumstellar disk edge-on. Figure 2 shows the blue-region spectrum of NGC346-023, with some of the stronger Fe II emission lines marked. This star is only 2.8 from the centre of the cluster and appears to be a genuine member; in a young field such as NGC346 one could speculate that these stars might be somewhat different to classical Be stars, more often associated with later products of stellar evolution. Figure 2. A Be-type star in the NGC346 field that displays single-peaked Fe II emission lines; for clarity the spectrum has been smoothed by a 1.5 Å fwhm filter. With regard to the relative numbers of Be-type stars in Table 2, one should not necessarily conclude that these provide further evidence for a Z-dependence of the Be-type phenomenon (cf. Maeder et al., 1999); the selection effects for the Be-stars are not well defined at the current time and we are looking further into this aspect of the survey.