[Paper Review] Rotating Massive Main-Sequence Stars II: Simulating a Population of LMC early B-type Stars as a Test of Rotational Mixing

I. Brott, C. J. Evans+10 more|UvA-DARE (University of Amsterdam)|Feb 3, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy62 references119 citations

TL;DR

This study uses a novel population synthesis model to test rotational mixing in massive early B-type stars in the Large Magellanic Cloud (LMC), using high-precision nitrogen abundance and rotational velocity data. Despite successfully reproducing the fraction of non-enriched stars, the model predicts twice as many rapidly rotating nitrogen-enriched stars as observed, and fails to reproduce two key subpopulations: slowly rotating nitrogen-enriched stars and rapidly rotating unenriched stars, indicating additional physics—such as binarity or magnetic fields—are needed beyond standard rotational mixing models.

ABSTRACT

Rotational mixing in massive stars is a widely applied concept, with far reaching consequences for stellar evolution. Nitrogen surface abundances for a large and homogeneous sample of massive B-type stars in the LMC were obtained by the VLT-FLAMES Survey of Massive Stars. This sample is the first covering a broad range of projected stellar rotational velocities, with a large enough sample of high quality data to allow for a statistically significant analysis. We use the sample to provide the first rigorous test of the theory of rotational mixing in massive stars. We calculated a grid of stellar evolution models, using the 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 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. Our simulations reproduce the fraction of stars without significant nitrogen enrichment. The predicted number of rapid rotators with enhanced nitrogen is about twice as large as found observationally. Furthermore, a group of stars consisting of slowly rotating, nitrogen-enriched objects and another consisting of rapidly rotating un-enriched objects can not be reproduced by our single-star population synthesis. Additional physical processes appear to be required to understand the population of massive main-sequence stars from the 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 produced via mass transfer and subsequent tidal spin-down in close binary systems

Motivation & Objective

• To rigorously test the theory of rotational mixing in massive stars using a large, homogeneous sample of early B-type stars in the Large Magellanic Cloud (LMC).
• To address the long-standing ambiguity in interpreting nitrogen enrichment in massive stars, particularly the presence of slowly rotating, nitrogen-rich stars that challenge rotational mixing models.
• To develop and apply a new population synthesis framework that accounts for the full distribution of stellar masses, ages, and projected rotational velocities observed in the VLT-FLAMES survey.
• To quantify the discrepancy between theoretical predictions and observations, especially for rapidly rotating, unenriched stars and slowly rotating, nitrogen-enriched stars.
• To assess the role of alternative physical mechanisms—such as binarity and magnetic fields—in explaining the observed stellar populations.

Proposed method

• Constructed a grid of stellar evolution models with calibrated overshooting and rotational mixing parameters to match observed main-sequence widening and nitrogen enrichment trends.
• Developed a new population-synthesis code that simulates a synthetic population of massive stars with distributions of mass, age, and projected rotational velocity matching the VLT-FLAMES sample.
• Applied observational selection effects (e.g., signal-to-noise, line broadening) to the synthetic population to enable direct comparison with the observed data.
• Used the observed nitrogen abundance as a function of projected rotational velocity to constrain the efficiency of rotational mixing in the models.
• Compared the predicted number of stars in each observed subpopulation (e.g., rapidly rotating nitrogen-enriched, slowly rotating nitrogen-enriched) with the actual counts from the VLT-FLAMES survey.
• Evaluated competing scenarios, including binary evolution and magnetic braking, to explain the unexplained subpopulations.

Experimental results

Research questions

• RQ1 To what extent can single-star evolutionary models with rotational mixing reproduce the observed distribution of nitrogen surface abundances and projected rotational velocities in early B-type stars in the LMC?
• RQ2 Why are there significantly more rapidly rotating, nitrogen-enriched stars predicted by the model than observed?
• RQ3 What physical mechanism could explain the existence of slowly rotating, nitrogen-enriched stars, given that rotational mixing is unlikely to operate efficiently in such stars?
• RQ4 Can the observed population of rapidly rotating, unenriched stars be explained by single-star models, or does it require additional physics such as binarity or magnetic fields?
• RQ5 Is the observed discrepancy between model predictions and observations robust, or could it be attributed to uncertainties in the modeling or selection effects?

Key findings

• The model successfully reproduces the observed fraction of stars without significant nitrogen enrichment, validating the calibration of overshooting and rotational mixing parameters.
• The model predicts approximately twice as many rapidly rotating, nitrogen-enriched stars as are observed, indicating a significant overprediction of this subpopulation.
• The model fails to reproduce the observed population of slowly rotating, nitrogen-enriched stars (Box 2), which are unlikely to be explained by mass transfer and tidal spin-down in binary systems due to overpopulation of the rapidly rotating, nitrogen-enriched group.
• The model also fails to reproduce the observed population of rapidly rotating, unenriched stars (Box 1), which are underpredicted relative to theoretical expectations.
• The existence of two unexplained subpopulations—slowly rotating nitrogen-rich stars and rapidly rotating unenriched stars—suggests that rotational mixing alone cannot explain the full range of observed properties.
• Magnetic fields or binary interactions are likely required to explain the observed stellar populations, with magnetic braking a plausible explanation for the nitrogen-rich, slow rotators, though no physical prescription for magnetic effects is currently available in models.