Empirical calibration of thermohaline and rotational mixing in massive stars

Determine empirically the efficiency of thermohaline mixing in massive stars and characterize its interplay with rotational mixing, in order to reduce uncertainty in predicted surface-abundance signatures of binary mass gainers.

Background

The paper explains that accreted helium-rich material induces thermohaline mixing in the envelope of a massive mass gainer. Initially, rapid thermohaline mixing homogenizes the envelope, whereas subsequent slower thermohaline mixing, assisted by rotational mixing, transports material through the CN-processing region and raises the surface N/C ratio.

The predicted displacement of mass-gainer models above the CNO-equilibrium-plus-dilution line depends on the efficiency of these mixing processes. Because this efficiency has not been empirically calibrated and the interaction between thermohaline and rotational mixing is not fully understood, quantitative predictions of the surface-abundance evolution remain uncertain. Resolving this problem would improve the use of CNO fingerprints to reconstruct binary mass-transfer histories.

References

Notably, the efficiency of thermohaline mixing in massive stars has so far not been calibrated empirically, nor has its interplay with rotational mixing been fully understood, such that the elevation of our gainer models above the ``CNO-eq. + dilution''-line must be considered uncertain (cf., Extended Data Fig.\ref{fig_mock}).

— Chemical fingerprints of binary mass transfer in massive stars  (2608.11940 - Jin et al., 12 Aug 2026) in Results from detailed binary evolution models, paragraph beginning “Notably, the efficiency of thermohaline mixing”

For example, a simple scenario of C underabundance caused by mixing of CN-cycled product (e.g., due to rapid rotation or binary mass transfer) is unlikely because any N enrichment is not observed. This problem remains yet to be further investigated.

— On the photospheric abundances of the B-type fast rotator Regulus  (2609.00860 - Takeda, 1 Sep 2026) in Abstract