I. Brott

Sub-surface convection zones in hot massive stars and their observable consequences

Context. We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. Aims. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. Methods. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. Results. We find the iron convection zone (FeCZ) to be more prominent fo r lower surface gravity, higher luminosity and higher initi al metallicity. It is absent for luminosities below about 10 3.2 L⊙, 10 3.9 L⊙, and 10 4.2 L⊙ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variabil ity. Conclusions. The confirmation of all three trends for the FeCZ as function o f stellar parameters by empirical microturbulent velociti es argues for a physical connection between sub-photospheric convective motions and small scale stochastic velocities i n the photosphere of O- and B-type stars. We further suggest that clumping in the inner parts of the winds of OB stars could be caused by the same mechanism, and that magnetic fields produced in the FeCZ coul d appear at the surface of OB stars as diagnosed by discrete absorption components in ultraviolet absorption lines.

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 ~300u2009kmu2009s-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.

Rotational mixing in massive binaries Detached short-period systems

Models of rotating single stars can successfully account fo r a wide variety of observed stellar phenomena, such as the surface enhancements of N and He observed in massive main-sequence stars. However, recent observations have questioned the idea that ro tational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test for rotational mixing. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In the se systems the tides are thought to spin up the stars to rapid rotation, sync hronous with their orbital revolution. We use a state-of-th e-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We adop t a rotational mixing effi ciency that has been calibrated against observations of rotating stars und er the assumption that rotational mixing is the main process responsible for the observed surface abundances. We find that the primaries of massive close binaries ( M1≈ 20 M⊙, Porb. 3 days) are expected to show significant enhancements in nitrogen (up to 0.6 dex in the Small Magellanic Cloud) for a significant fraction of their core hydrogen-burning lifetime . We propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their p arameters can be determined with high accuracy. For the primary stars of more massive and very close systems ( M1≈ 50 M⊙, Porb. 2 days) we find that centrally produced helium is effi ciently mixed throughout the envelope. The star remains blue and compact during the main sequence evolution and stays within its Roche lobe. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills it s Roche lobe first, contrary to what standard binary evolution theory pre dicts. The primaries will appear as “Wolf-Rayet stars in dis guise”: core hydrogen-burning stars with strongly enhanced He and N at the surface. We propose that this evolution path provides an al ternative channel for the formation of tight Wolf-Rayet binaries with a main-sequence companion and might explain massive black hole binaries such as the intriguing system M33 X-7.

The nature of B supergiants: clues from a steep drop in rotation rates at 22 000 K. The possibility of bi-stability braking

The location of B supergiants in the Hertzsprung-Russell diagram (HRD) represents a long-standing problem in massive star evolution. Here we propose their nature may be revealed utilising their rotational properties, and we highlight a steep drop in massive star rotation rates at an effective temperature of 22 000 K. We discuss two potential explanations for it. On the one hand, the feature might be due to the end of the main sequence, which could potentially constrain the core overshooting parameter. On the other hand, the feature might be the result of enhanced mass loss at the predicted location of the bi-stability jump. We term this effect “bi-stability braking” and discuss its potential consequences for the evolution of massive stars.

The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He ii λ1640 in young star clusters

We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec x 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than m(F555W) = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C IV lambda lambda 1548-51, including 16 O2-3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung-Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5(-0.7)(+0.3) Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He II lambda 1640 emission line. This emission is totally dominated by very massive stars with initial masses above similar to 100M(circle dot). The presence of strong He II lambda 1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100M(circle dot). We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations.

The VLT-FLAMES Tarantula Survey: XXV. Surface nitrogen abundances of O-type giants and supergiants

Context. Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics. Aims. Using a sample of 72 presumably single O-type giants to supergiants observed in the context of the VLT-FLAMES Tarantula Survey (VFTS), we aim to investigate rotational mixing in evolved core-hydrogen burning stars initially more massive than 15M(circle dot) by analysing their surface nitrogen abundances. Methods. Using stellar and wind properties derived in a previous VFTS study we computed synthetic spectra for a set of up to 21 N II-V lines in the optical spectral range, using the non-LTE atmosphere code FASTWIND. We constrained the nitrogen abundance by fitting the equivalent widths of relatively strong lines that are sensitive to changes in the abundance of this element. Given the quality of the data, we constrained the nitrogen abundance in 38 cases;for 34 stars only upper limits could be derived, which includes almost all stars rotating at nu(e) sin i > 200 km s(-1). Results. We analysed the nitrogen abundance as a function of projected rotation rate nu(e) sin i and confronted it with predictions of rotational mixing. We found a group of N-enhanced slowly-spinning stars that is not in accordance with predictions of rotational mixing in single stars. Among O-type stars with (rotation-corrected) gravities less than log g(c) = 3.75 this group constitutes 30 40 percent of the population. We found a correlation between nitrogen and helium abundance which is consistent with expectations, suggesting that, whatever the mechanism that brings N to the surface, it displays CNO-processed material. For the rapidly-spinning O-type stars we can only provide upper limits on the nitrogen abundance, which are not in violation with theoretical expectations. Hence, the data cannot be used to test the physics of rotation induced mixing in the regime of high spin rates. Conclusions. While the surface abundances of 60 70 percent of presumed single O-type giants to supergiants behave in conformity with expectations, at least 30 40 percent of our sample can not be understood in the current framework of rotational mixing for single stars. Even though we have excluded stars showing radial velocity variations, of our sample may have remained contaminated by post-interaction binary products. Hence, it is plausible that effects of binary interaction need to be considered to understand their surface properties. Alternatively, or in conjunction, the effects of magnetic fields or alternative mass-loss recipes may need to be invoked.

How Efficient is Rotational Mixing in Massive Stars

The VLT‐Flames Survey for Massive Stars [1, 2] provides precise measurements of rotational velocities and nitrogen surface abundances of massive stars in the Magellanic Clouds. Specifically, for the first time, such abundances have been estimated for stars with significant rotational velocities. This extraordinary data set gives us the unique possibility to calibrate rotationally and magnetically induced mixing processes. Therefore, we have computed a grid of stellar evolution models varying in mass, initial rotational velocity and chemical composition. In our models we find that although magnetic fields generated by the Spruit‐Taylor dynamo are essential to understand the internal angular momentum transport (and hence the rotational behavior), the corresponding chemical mixing must be neglected to reproduce the observations. Further we show that for low metallicities detailed initial abundances are of prime importance, as solar‐scaled abundances may result in significant calibration errors.

Rotation and Massive Close Binary Evolution

We review the role of rotation in massive close binary systems. Rotation has been advocated as an essential ingredient in massive single star models. However, rotation clearly is most important in massive binaries where one star accretes matter from a close companion, as the resulting spin-up drives the accretor towards critical rotation. Here, we explore our understanding of this process, and its observable consequences. When accounting for these consequences, the question remains whether rotational effects in massive single stars are still needed to explain the observations.

The VLT-FLAMES Tarantula Survey

The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. Here we introduce the scientific motivations of the survey and give an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution.