# The eccentric behaviour of windy binary stars

**Authors:** M. I. Saladino, O. R. Pols  
**arXiv:** [1906.02038](https://arxiv.org/abs/1906.02038)  
**Date:** 2019-06-05  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1906.02038v1/

## Key points

- Study by M. I. Saladino and O. R. Pols on orbital evolution of carbon-enhanced metal-poor stars, CH stars, and barium stars in low- and intermediate-mass binaries during the AGB phase
- Observations show significant eccentricities up to 0.9 in AGB binary progeny despite theoretical predictions of circularized orbits due to tidal effects
- Numerical simulations by Saladino and Pols reveal wind mass transfer plays a role in circularizing the orbit over longer timescales compared to tidal effects for eccentricity below 0.4
- Low initial wind velocities and pseudo-synchronization of donor star can lead to wind Roche-lobe overflow near periastron, causing orbit shrinkage and decreased eccentricity
- Wind interaction can pump eccentricity at a similar rate as tidal circularization under specific conditions
- Study highlights the need for broader range of numerical models to determine if hydrodynamical eccentricity pumping counteracts tidal circularization effectively

## Summary

In the study titled "The eccentric behaviour of windy binary stars" by M. I. Saladino and O. R. Pols, the authors investigate the orbital evolution of carbon-enhanced metal-poor stars, CH stars, and barium stars in low- and intermediate-mass binaries during the asymptotic giant branch (AGB) phase of stellar evolution. Theoretical models suggest that if their initial orbital periods are short, these systems should have circularized orbits due to tidal effects. However, observations show that many AGB binary progeny exhibit significant eccentricities up to 0.9. To understand this discrepancy, Saladino and Pols conduct numerical simulations incorporating wind mass transfer in AGB binaries using a hydrodynamical code for modeling the AGB wind and an N-body code for evolving stellar dynamics. The results reveal that wind mass transfer plays a role in circularizing the orbit over longer timescales compared to tidal effects when the eccentricity is below 0.4. Interestingly, for scenarios with low initial wind velocities and pseudo-synchronization of the donor star, a structure resembling wind Roche-lobe overflow near periastron is observed. In such cases, interactions between gas and stars are stronger than those with high initial wind velocities, causing the orbit to shrink while decreasing eccentricity. Furthermore, one model demonstrates that wind interaction can pump eccentricity at a similar rate as tidal circularization under specific conditions. Although based on a limited sample size of models, this study provides valuable insights into how eccentric binaries evolve through winds' interactions. The authors emphasize the need for a broader range of numerical models encompassing various binary parameters to determine if there exists a regime where hydrodynamical eccentricity pumping effectively counteracts tidal circularization. Such investigations could potentially elucidate the puzzlingly high eccentricities observed in descendants of AGB binaries and shed light on the complex dynamics governing these systems.