[Paper Review] Rotational mixing in massive binaries: detached short-period systems

S. E. de Mink, Matteo Cantiello+4 more|ArXiv.org|Feb 10, 2009

Stellar, planetary, and galactic studiesPhysics and Astronomy60 references131 citations

TL;DR

This paper proposes using detached, short-period massive binaries—especially eclipsing systems—as precise observational tests for rotational mixing in massive stars. By modeling stellar evolution with calibrated rotational mixing in tidally locked binaries, the study finds that primaries in these systems show strong nitrogen enhancements (up to 0.6 dex) and evolve as compact, blue, Wolf-Rayet-like stars with surface He and N enrichment, offering a new channel for forming tight Wolf-Rayet and massive black hole binaries without mass transfer.

ABSTRACT

Models of rotating single stars can successfully account for a wide variety of observed stellar phenomena, such as the surface enhancements of N and He. However, recent observations have questioned the idea that rotational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In these systems the tides spin up the stars to rapid rotation. We use a state-of-the-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We discuss the surface abundances expected in massive close binaries (M1~20 solar masses) and we propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their parameters can be determined with high accuracy, allowing for a direct comparison with binary evolution models. In more massive close systems (M1~50 solar masses, Porb

Motivation & Objective

• To test the hypothesis that rotational mixing is the dominant mechanism for nitrogen and helium surface enhancements in massive main-sequence stars.
• To identify a clean observational test for rotational mixing by leveraging detached, short-period massive binaries with high-precision parameters.
• To explore whether rotational mixing can explain the formation of tight Wolf-Rayet binaries and massive black hole binaries like M33 X-7 without requiring mass transfer or common-envelope phases.
• To investigate the role of rotational mixing in altering standard binary evolution pathways, particularly in systems where tides synchronize rotation with orbital motion.

Proposed method

• Using a state-of-the-art stellar evolution code that includes rotational mixing, tidal synchronization, and magnetic fields.
• Calibrating rotational mixing efficiency against the VLT-FLAMES survey of massive stars under the assumption that rotational mixing drives observed surface abundance anomalies.
• Modeling detached, short-period massive binaries with orbital periods ≤3 days and primary masses ≈20–50 M☉.
• Simulating evolution under the assumption of no mass transfer or common-envelope phases, ensuring the system remains detached throughout core hydrogen burning.
• Assessing the impact of rotational mixing on surface abundances, radius evolution, and Roche lobe overflow timing.
• Comparing model predictions with observed systems such as M33 X-7 and Cyg X-1 to evaluate the viability of the proposed evolutionary scenario.

Experimental results

Research questions

• RQ1 Can detached, short-period massive binaries with high-precision parameters serve as clean tests for rotational mixing?
• RQ2 To what extent does rotational mixing lead to nitrogen surface enhancements in tidally locked massive stars?
• RQ3 Does rotational mixing alter the standard binary evolution sequence, particularly the order of Roche lobe overflow?
• RQ4 Can rotational mixing alone explain the formation of tight Wolf-Rayet binaries with main-sequence companions?
• RQ5 Is there a viable evolutionary pathway for massive black hole binaries like M33 X-7 that avoids mass transfer and common-envelope phases?

Key findings

• Primaries in massive short-period binaries (M₁ ≈ 20 M☉, P_orb ≤ 3 days) show nitrogen surface enhancements of up to 0.6 dex in the Small Magellanic Cloud due to rotational mixing.
• In systems with M₁ ≈ 50 M☉ and P_orb ≤ 2 days, rotational mixing efficiently mixes centrally produced helium throughout the envelope, keeping the star compact and blue during core hydrogen burning.
• These stars appear as 'Wolf-Rayet stars in disguise'—core hydrogen-burning stars with strong surface He and N enhancements—due to efficient mixing.
• Contrary to standard binary evolution, the less massive secondary star fills its Roche lobe first in these systems, due to the primary's compactness from rotational mixing.
• The primary remains within its Roche lobe throughout core hydrogen burning in a significant fraction of parameter space, avoiding mass transfer and common-envelope phases.
• This new evolutionary path, labeled Case M, provides an alternative formation channel for tight Wolf-Rayet binaries and may explain massive black hole binaries such as M33 X-7.