P. L. Dufton

The VLT-FLAMES survey of massive stars: Surface chemical compositions of B-type stars in the Magellanic Clouds

We present an analysis of high-resolution FLAMES spectra of approximately 50 early B-type stars in three young clusters at different metallicities, NGC 6611 in the Galaxy, N 11 in the Large Magellanic Cloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using the tlusty non-LTE model atmospheres code, atmospheric parameters and photospheric abundances (C, N, O, Mg and Si) of each star have been determined. These results represent a significant improvement on the number of Magellanic Cloud B-type stars with detailed and homogeneous estimates of their atmospheric parameters and chemical compositions. The relationships between effective temperature and spectral type are discussed for all three metallicity regimes, with the effective temperature for a given spectral type increasing as one moves to a lower metallicity regime. Additionally the difficulties in estimating the microturbulent velocity and the anomalous values obtained, particularly in the lowest metallicity regime, are discussed. Our chemical composition estimates are compared with previous studies, both stellar and interstellar with, in general, encouraging agreement being found. Abundances in the Magellanic Clouds relative to the Galaxy are discussed and we also present our best estimates of the base-line chemical composition of the LMC and SMC as derived from B-type stars. Additionally we discuss the use of nitrogen as a probe of the evolutionary history of stars, investigating the roles of rotational mixing, mass-loss, blue loops and binarity on the observed nitrogen abundances and making comparisons with stellar evolutionary models where possible.

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

Aims. We have previously analysed the spectra of 135 early B-type stars in the Large Magellanic Cloud (LMC) and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and Small Magellanic Cloud (SMC) metallicities. Methods. We provide chemical compositions for ~50 Galactic and ~100 SMC early B-type stars and compare these to the LMC results. These samples cover a range of projected rotational velocities up to ~300 km s-1 and hence are well suited to testing rotational mixing models. The surface nitrogen abundances are utilised as a probe of the mixing process since nitrogen is synthesized in the core of the stars and mixed to the surface. Results. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, which is comparable to that found previously in the LMC. The identification of non-enriched rapid rotators in the SMC is not possible due to the relatively high upper limits on the nitrogen abundance for the fast rotators. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models.

The VLT-FLAMES survey of massive stars: Rotation and nitrogen enrichment as the key to understanding massive star evolution

Rotation has become an important element in evolutionary models of massive stars, specifically via the prediction of rotational mixing. Here we study a sample of stars, including rapid rotators, to constrain such models and use nitrogen enrichments as a probe of the mixing process. Chemical compositions (C, N, O, Mg, and Si) have been estimated for 135 early B-type stars in the Large Magellanic Cloud with projected rotational velocities up to ~300 km s-1 using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including rotational mixing, have been generated attempting to reproduce these observations by adjusting the overshooting and rotational mixing parameters and produce reasonable agreement with 60% of our core hydrogen burning sample. We find (excluding known binaries) a significant population of highly nitrogen-enriched intrinsic slow rotators (vsin i 50 km s-1) incompatible with our models (~20% of the sample). Furthermore, while we find fast rotators with enrichments in agreement with the models, the observation of evolved (log g < 3.7 dex) fast rotators that are relatively unenriched (a further ~20% of the sample) challenges the concept of rotational mixing. We also find that 70% of our blue supergiant sample cannot have evolved directly from the hydrogen-burning main sequence. We are left with a picture where invoking binarity and perhaps fossil magnetic fields is required to understand the surface properties of a population of massive main-sequence stars.

The VLT-FLAMES Survey of Massive Stars: Observations in the Galactic Clusters NGC 3293, NGC 4755 and NGC 6611 ⋆

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R � 25 000), in fields centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are supplemented by a further 50 targets observed with the Fibre-Fed Extended Range Optical Spectrograph (FEROS, R = 48 000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC 3293 and NGC 4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.

Rotating massive main-sequence stars: II. Simulating a population of LMC early B-type stars as a test of rotational mixing

Context. Rotational mixing in massive stars is a widely applied concept, with far-reaching consequences for stellar evolution, nucleosynthesis, and stellar explosions. Aims. Nitrogen surface abundances for a large and homogeneous sample of massive B-type stars in the Large Magellanic Cloud (LMC) have recently been obtained by the ESO VLT-FLAMES Survey of Massive Stars. This sample is the first to cover a broad range of projected stellar rotational velocities, with a large enough sample of high quality data to allow for a statistically significant analysis. Here we use the sample to provide the first rigorous and quantitative test of the theory of rotational mixing in massive stars. Methods. We calculated a grid of stellar evolution models, using the VLT-FLAMES sample to calibrate some of the uncertain mixing processes. We developed a new population-synthesis code, which uses this grid to simulate a large population of stars with masses, ages, and rotational velocity distributions consistent with those from the VLT-FLAMES sample. The synthesized population is then filtered by the selection effects in the observed sample, to enable a direct comparison between the empirical results and theoretical predictions. Results. Our simulations reproduce the fraction of stars without significant nitrogen enrichment. However, the predicted number of rapid rotators with enhanced nitrogen is about twice as large as found observationally. Furthermore, two groups of stars (one consisting of slowly rotating, nitrogen-enriched objects and another consisting of rapidly rotating un-enriched objects) cannot be reproduced by our single-star population synthesis. Conclusions. Physical processes in addition to rotational mixing appear to be required to understand the population of massive mainsequence stars from the VLT-FLAMES sample. We discuss the possible role of binary stars and magnetic fields in the interpretation of our results. We find that the population of slowly rotating nitrogen-enriched stars is unlikely to be produced via mass transfer and subsequent tidal spin-down in close binary systems. A conclusive assessment of the role of rotational mixing in massive stars requires a quantitative analysis that also accounts for the effects of binarity and magnetic fields.

The VLT-FLAMES survey of massive stars: Evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds. ⋆,⋆⋆

We present an analysis of high resolution VLT-FLAMES spectra of 61 B-type stars with relatively narrow-lined spectra lo cated in 4 fields centered on the Milky Way clusters; NGC3293 & NGC4755 and the Large and Small Magellanic cloud clusters; NGC2004 and NGC330. For each object a quantitative analysis was carried out using the non-LTE model atmosphere code TLUSTY; resulting in the determination of their atmospheric parameters and phot ospheric abundances of the dominant metal species (C, N, O, Mg, Si, Fe). The results are discussed in relation to our earlier work on 3 younger clusters in these galaxies; NGC6611, N11 and NGC346 paying particular attention to the nitrogen abundances which are a n important probe of the role of rotation in the evolution of s tars. This work along with that of the younger clusters provides a consistent dataset of abundances and atmospheric parameters for over 100 B-type stars in the three galaxies. We provide effective temperature scales for B-type dwarfs in all three gal axies and for giants and supergiants in the SMC and LMC. In each galaxy a dependence on luminosity is found between the three classes with the unevolved dwarf objects having significantly higher e ffective temperatures. A metallicity dependence is present b etween the SMC and Galactic dwarf objects, and whilst the LMC stars are only slightly cooler than the SMC stars, they are significantly hotter than the ir Galactic counterparts.

The VLT-FLAMES Tarantula Survey - XII. Rotational velocities of the single O-type stars

Context. The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars. Aims. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The sample is large enough to obtain statistically significant information and to search for variations among subpopulations – in terms of spectral type, luminosity class, and spatial location – in the field of view. Methods. We measured projected rotational velocities, ν_esini, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P(ν_e), of the equatorial rotational velocity, ν_e. Results. The distribution of νesini shows a two-component structure: a peak around 80 kms^(-1) and a high-velocity tail extending up to ~600 kms^(-1). This structure is also present in the inferred distribution P(ν_e) with around 80% of the sample having 0 < ν_e ≤ 300 kms^(-1) and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O- and early B-type stars. Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the high-velocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts.

The present-day chemical composition of the SMC from UVES spectra of the sharp-lined, B-type dwarf AV 304

High-resolution spectroscopic VLT/UVES observations are presented for the B-type main-sequence star, AV 304, in the Small Magellanic Cloud (SMC). These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As AV 304 is located within the hydrogen burning main-sequence band, its chemical composition should reflect that of the SMC interstellar medium (ISM). A detailed line-by-line differential analysis has been undertaken relative to a Galactic comparison star. A general metal deficiency for the a-process elements O, Si & S of -0.43 +/- 0.05 dex is found for AV 304, with iron having a similar underabundance. Oxygen may be relatively over-abundant by similar to0.1 dex and carbon and aluminium underabundant by similar to0.2 dex. A large nitrogen underabundance (of -1.2 dex relative to hydrogen and -0.7 dex relative to iron) is found. This is interpreted in terms of the CNO bi-cycle having been suppressed in the SMC. Furthermore, the large nitrogen deficiency is in excellent agreement with that found for SMC H II regions. Indeed, this represents a first for stellar astrophysics - confirming the low base-line nitrogen composition of the SMC ISM (viz. 12+log(N/H) similar to 6.66 +/- 0.10 dex).

The present-day chemical composition of the LMC

High-resolution observations of five OB-type main-sequence stars in the Large Magellanic Cloud (LMC) have been obtained with the UCLspectrograph on the 3.9-m Anglo-Australian Telescope. These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As these stars are located within the hydrogen burning main-sequence band, their surface abundances should reflect those of the present-day interstellar medium. Detailed line-by-line differential analyses have been undertaken relative to Galactic comparison stars. We conclude that there exists a general metal deficiency of −0.31 ± 0.04 dex within the LMC, and find no significant abundance variations between cluster and field stars. There is also tentative evidence to suggest a lower oxygen to iron abundance ratio, and an over-deficiency of magnesium relative to the other α-elements. These are discussed in terms of previous abundance analyses and models of discontinuous (or bursting) star formation within the LMC. Finally, there is some evidence to suggest a greater chemical enrichment of material within the H  region LH104.

Electron-impact-excitation collision strengths for Be-like ions: II. Intermediate-energy region and collision rates

Abstract Intermediate-energy collision strengths calculated using the R-matrix method are presented for four Be-sequence ions, C III (2.6–8.0 Ry), O V (4.4–12.0 Ry), Ne VII (8.4–20.0 Ry), and Si XI (11.0–34.0 Ry). The six ionic states (2s2) 1S, (2s2p)3P0, 1P0 and (2p2)3Pe, 1De, 1Se, corresponding to ten fine-structure levels, are included, leading to 29 independent transitions per ion. High-energy analytical expressions have also been calculated for the collision strengths. These results have been combined with previously published low-energy collision strengths to deduce effective collision strengths (that is, collision rates) for ranges of electron temperature appropriate to the four ions.