Stellar Evolution in Close Binaries: Processes and Outcomes

We discuss some aspects of stellar evolution in binary systems. While single stars can swell following the chemical evolution of their interior, stars belonging to binary systems cannot overflow the size of the Roche lobe and hydrostatic equilibrium is strictly impossible. The system is forced to exchange mass between its members through the inner Lagrangian point. In the first part of the paper, we discuss the standard evolution of binaries that have a non-degenerate donor star and a compact companion. We show that the model fails when to account for the occurrence of binary pulsars when they predict a long-standing mass transfer episode. Models including irradiation feedback and evaporation in close binaries are examined next. Following these sections, we discuss the case of systems with a black hole (BH). We show that if BHs are born non-rotating, binary interaction seems insufficient to speed them up, an indication that BH rotation is a feature present at birth. Finally, we discuss Blue Straggler Stars detected in open and globular clusters. Since they cannot be understood as single-born stars, we evaluate one of the proposed channels is mass transfer in close binaries, and discuss its viability and the limitations of the present models.

💡 Research Summary

The paper provides a comprehensive review of stellar evolution in close binary systems (CBS), focusing on the physical processes that govern mass transfer and their observable consequences. After a brief introduction to single‑star evolution, the authors describe the Roche‑lobe geometry, the Eggleton approximation for the lobe radius, and the classification of binaries into detached, semi‑detached, and contact configurations. They emphasize that mass transfer stability depends critically on the donor’s envelope structure (radiative versus convective), the mass ratio, and the orbital period. Standard 1‑D binary evolution models, which neglect the effects of X‑ray irradiation and donor evaporation, predict long‑lasting, continuous mass‑transfer episodes for low‑mass X‑ray binaries (LMXBs) and their descendants, the so‑called “spider” pulsars (black widows and redbacks). However, observed binary pulsars often lie outside the predicted region in the orbital‑period versus donor‑mass plane, indicating a failure of the standard approach.

To resolve this discrepancy, the authors introduce non‑standard ingredients: irradiation feedback (IFB) and evaporation. IFB reduces the effective radiating area of the donor’s heated hemisphere, temporarily enhancing the mass‑transfer rate, while subsequent cooling causes detachment. This leads to a cyclic, pulsed mass‑transfer behavior that matches the observed distribution of spider pulsars. Evaporation further strips the donor, shortening the mass‑transfer phase.

The paper then examines binaries containing black holes. Calculations show that a non‑rotating black hole cannot be spun up sufficiently by accretion alone, implying that high spins observed in black‑hole binaries must be primordial.

Finally, the authors discuss blue straggler stars (BSS) in open and globular clusters. While mass transfer in close binaries is a plausible formation channel, current 1‑D models cannot accurately reproduce the required mass‑transfer efficiencies, common‑envelope evolution, or the observed BSS population, highlighting the need for multi‑dimensional simulations and improved physics.

In conclusion, the study demonstrates that traditional binary evolution models are insufficient to explain several key observational classes. Incorporating irradiation, evaporation, and realistic black‑hole spin histories, as well as advancing beyond 1‑D approximations, are essential steps toward a more complete understanding of stellar evolution in close binary systems.