Tidal Interactions Enhance or Suppress Chemical Mixing in Massive Binaries

Scientists are tackling a long-standing puzzle in stellar evolution: how internal mixing within close binary star systems affects their chemical compositio. Luca Sciarini, Sophie Rosu, Sylvia Ekström, et al, from the University of Geneva, present new modelling which investigates whether tidal forces enhance or suppress the mixing of materials inside these interacting stars. Previous work predicted tidal forces would always boost mixing, yet observations haven’t confirmed this, creating a significant discrepancy. This research is important because it systematically examines the complex interplay between tidal interactions, angular momentum transport, and chemical mixing, revealing that the outcome isn’t straightforward , tides can actually reduce mixing depending on the underlying assumptions used in stellar models, potentially explaining the diverse range of observed chemical abundances in binary systems.

The research, published in Astronomy & Astrophysics, reveals a surprising sensitivity to the assumptions made regarding angular momentum transport (AMT) within the stars. This work addresses a long-standing discrepancy between theoretical models and observations, which have failed to show a clear correlation between orbital period and nitrogen enrichment in binary systems. Researchers computed extensive grids of binary star models, employing various AMT treatments at solar metallicity, and systematically compared them to single-star models with identical initial conditions.

The team achieved a crucial breakthrough by independently assessing the role of tidal interactions, revealing that the outcome, enhanced or suppressed mixing, is heavily dependent on the chosen AMT model. In close systems experiencing tidal forces, magnetic models predict mixing efficiency is primarily determined by the orbital configuration, whereas hydrodynamic models also depend on the initial velocity of the stars. Consequently, hydrodynamic models can exhibit non-monotonic period-enrichment trends, or even correlations where enrichment decreases with increasing period, a result that better accommodates peculiar observed systems. This sensitivity to initial conditions significantly expands the range of parameter space covered by hydrodynamic models, allowing them to explain systems exhibiting mild enrichment at short periods or high enrichment at longer periods.
Experiments show that the adopted AMT assumptions are paramount in accurately modeling binaries with tidal interactions, particularly within the context of chemically homogeneous evolution. The study employed the GENeva Evolutionary Code (genec) with both advective-diffusive AMT and magneto-hydrodynamic models incorporating Tayler-Spruit instabilities to explore these effects. Tides were incorporated as modifiers of stellar rotation, influencing the efficiency of rotational mixing, but the research suggests this framework may be overly simplistic. The findings echo the work of Zahn (1994), who demonstrated that tidal synchronization can reduce mixing and lithium depletion in late-type binaries, offering a parallel explanation for observed discrepancies.

This research establishes a key contrast between magnetic and hydrodynamic models, highlighting how different AMT treatments can lead to divergent predictions regarding chemical mixing in tidally interacting binaries. The investigation systematically computed model variations of single stars with identical initial conditions to isolate the impact of tidal interactions, providing a robust basis for comparison. By exploring a wide range of parameters and AMT assumptions, the study unveils the complex interplay between tidal forces, angular momentum transport, and chemical evolution, offering new insights into the behaviour of massive binary systems and their observed chemical compositions. The work opens avenues for refining stellar models and improving our understanding of the internal processes governing the evolution of these fascinating objects.

Binary Star Models and Tidal Mixing Efficiency

Scientists initiated a comprehensive study to disentangle the complex interplay between tidal interactions and rotational mixing in massive binary stars, addressing discrepancies between theoretical predictions and observational data regarding period-nitrogen enrichment trends. The research team computed extensive grids of binary star models using the GENeva Evolutionary Code (genec), systematically varying angular momentum transport (AMT) treatments at solar metallicity to assess their impact on chemical mixing. Crucially, to isolate the effect of tidal forces, the study also generated corresponding single-star models with identical initial conditions, enabling a direct comparison of mixing efficiencies. Experiments employed two distinct AMT schemes: a purely hydrodynamic model featuring an advective-diffusive approach, and a magneto-hydrodynamic model incorporating magnetic fields to influence angular momentum transport.

These models were run across a parameter space designed to capture the diverse range of close binary systems, allowing scientists to explore how tidal forces modulate internal mixing processes. The hydrodynamic models calculated initial velocities, while magnetic models focused on orbital configuration to determine mixing efficiency. This innovative approach enabled the team to identify a critical distinction, in close tidal systems, magnetic models predict mixing efficiency is primarily dictated by orbital configuration, whereas hydrodynamic models also depend on the assumed initial velocity. The study pioneered a method for systematically evaluating the sensitivity of model predictions to AMT assumptions, revealing that tidal interactions can either enhance or suppress mixing relative to single-star models.

Results demonstrate that hydrodynamic models can produce non-monotonic period-enrichment trends, or even correlations, challenging the previously assumed direct relationship between orbital period and nitrogen abundance. This sensitivity extends the range of parameter space covered by hydrodynamic models, allowing them to accommodate observed systems exhibiting mild enrichment at short periods or high enrichment at longer periods, previously difficult to reconcile with theoretical predictions. Furthermore, the research highlights the importance of accurately modelling AMT in binary systems undergoing tidal interactions, particularly within the context of chemically homogeneous evolution. By meticulously comparing single and binary star models with varying AMT treatments, the team established a robust framework for interpreting observational data and refining stellar evolution models, ultimately improving our understanding of chemical mixing processes in massive stars. The detailed modelling work provides a crucial step towards resolving the observed discrepancies between theory and observation in close binary systems.

Tidal Forces Modulate Mixing in Binary Stars

Scientists have revealed that tidal interactions in close binary star systems can either enhance or suppress chemical mixing, a finding highly sensitive to assumptions regarding angular momentum transport (AMT). The research, utilising the GENeva Evolutionary Code (genec), systematically examined grids of binary models alongside single-star variations at solar metallicity to independently assess the role of tidal forces. Investigations demonstrated that tides can lead to either increased or decreased mixing compared to single stars with identical initial conditions, contingent upon the specific AMT treatment employed. A key distinction emerged between magnetic and hydrodynamic models; in close tidal systems, magnetic models predict mixing efficiency is primarily dictated by orbital configuration, while hydrodynamic models also depend on the assumed initial velocity.

Consequently, hydrodynamic models can exhibit non-monotonic period-enrichment trends, or even correlations where increased period corresponds to decreased enrichment, a surprising result challenging previous predictions. Measurements confirm that the sensitivity of hydrodynamic model predictions to initial conditions expands the range of period-enrichment parameter space, accommodating peculiar observed systems displaying mild enrichment at short periods or high enrichment at longer periods. The team meticulously computed model variations, allowing for a direct comparison of tidal effects against single-star evolution. Results demonstrate that the interplay between tidal torques and rotational mixing is profoundly influenced by the chosen AMT treatment.

Specifically, the study highlights the importance of AMT assumptions when modelling binaries undergoing tidal interactions, particularly within the context of chemically homogeneous evolution. Tests prove that the adopted AMT significantly impacts the predicted mixing efficiency, influencing the distribution of chemical elements within the stellar interior. Furthermore, the work suggests that tidal interactions can, under certain conditions, lead to “tidally-suppressed mixing”, where binary stars exhibit less chemical mixing than their single-star counterparts, a counterintuitive finding. This sensitivity to initial conditions extends the parameter space covered by hydrodynamic models, enabling them to better reconcile with observed systems exhibiting unusual enrichment patterns. The research provides a crucial step towards refining stellar models and improving our understanding of chemical evolution in binary star systems.

Tidal Mixing Hinges on Angular Momentum Transport

Scientists have investigated the complex interplay between tidal interactions and rotational mixing in binary star systems, revealing that tides can either enhance or suppress chemical mixing compared to single stars. Their research demonstrates that the outcome is critically dependent on the assumptions made regarding angular momentum transport (AMT). Specifically, the study highlights a key distinction between magnetic and hydrodynamic models; magnetic models predict mixing efficiency is primarily determined by orbital configuration, while hydrodynamic models also depend on initial stellar velocity. This work establishes that hydrodynamic models can produce non-monotonic or even correlated period-enrichment trends, potentially explaining observed peculiarities in binary systems, such as mild enrichment at short periods or high enrichment at longer periods.

The authors acknowledge that their hydrodynamic models are sensitive to initial conditions, representing a limitation, but this sensitivity simultaneously expands the range of possible period-enrichment scenarios they can accommodate. Future research could focus on refining AMT treatments and exploring the impact of different initial conditions to further constrain the models and improve predictions of chemical evolution in close binary systems. These findings are significant because they underscore the importance of carefully considering AMT assumptions when modelling binaries undergoing tidal interactions, particularly within the context of chemically homogeneous evolution.