Rotating massive main-sequence stars

Abstract

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.

A comparison of evolutionary tracks for single Galactic massive stars

“…results were confirmed by the calculations of Brott et al (2011b). Nonetheless, Maeder et al (2009) cautioned that many parameters could affect the surface abundances and that they needed to be considered.…”

Population synthesis at the crossroads

“…Many stars with surface enrichment are also fast rotators. While the relation between nitrogen overabundance and rotation velocity is complex and far from unique, there is consensus that rotation is an important, if not the prime driver of mixing in massive stars (Brott et al 2011a). Stellar evolution with rotation has been modeled over the past decade, but it is only now that quantitative evolutionary tracks with rotation have become available for implementation of population synthesis (Brott et al 2011a;Ekström et al 2011).…”

B fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439

“…Brott et al 2011;Potter et al 2012) with the aim of understanding the origin and role of magnetic fields in massive stars. Recent observations indicate that magnetic fields may be responsible for a wide range of phenomena observed in massive stars, such as chemical peculiarity, periodic UV wind-line variability, cyclic variability in Hα and He II λ4686, excess emission in UV-wind lines, and unusual X-ray emission (e.g.…”