Stellar Evolution in Close Binaries: Processes and Outcomes

TL;DR

This paper analyzes how close binary evolution, governed by Roche-lobe overflow and mass-transfer physics, diverges from single-star evolution. It contrasts standard NS-LMXB models with non-standard ingredients like irradiation feedback and evaporation, showing that these effects help explain Spider pulsars and episodic mass transfer. It also discusses BH binaries, where accretion-driven spin-up may not fully account for observed spins, implying natal spin is important, and explores blue straggler formation via mass transfer or collisions. The work underscores the need for self-consistent, parameter-free theories to capture the diverse outcomes of binary evolution in astrophysical systems.

Abstract

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.

Figures (8)

• Figure 1: The donor mass vs. orbital period plane for CBSs containing a pulsar. Minimum mass vs. orbital period for pulsars belonging to binary systems with quasi-circular orbits (with eccentricity $e\leq 10^{-3}$) were selected from the ATNF database ATNF. The steep relationship of $P_{orb}(M_{2})$ shown with a thick blue line is that of Masa_periodo. Also, with black thin lines we included a set of evolutionary tracks corresponding to solar composition objects with masses for the donor star and the NS of $M_{2}= 1.50\;M_{\odot}$, and $M_{NS}= 1.40\;M_{\odot}$ respectively, for initial orbital periods of $P_{orb}= 0.50, 0.75, 1.0, 1.5, 3.0, 6.0,$ and $12.0$ d ordered from bottom to top. The thick magenta line corresponds to a model with $M_{2}= 1.50\;M_{\odot}$, $M_{NS}= 1.40\;M_{\odot}$, and $P_{orb}= 0.50$ d with moderate evaporation (see text (§ \ref{['evaporando']}) for further details). The regions corresponding to BWs (evaporation of the companion) and RBs (accretion and recycling) are highlighted. Also, the region corresponding to pulsed mass transfer due to IFB is indicated. Notice that the latter and RBs region overlap.
• Figure 2: Mass transfer rate as a function of donor mass for a solar composition donor star and NS of $M_{2}= 1.50\;M_{\odot}$, and $M_{NS}= 1.40\;M_{\odot}$ respectively, for an initial orbital period of $P_{orb}= 1.0$ d. Blue lines correspond to an irradiated model with $\alpha_{ifb}= 0.10$, whereas black lines correspond to a non-irradiated model.
• Figure 3: Same models as Figure \ref{['Fig:Mdot_vs_m2']}, but for mass transfer rates as function of time. Notice that, in the case of models with IFB, most of the time the system remains detached.
• Figure 4: Hertzsprung-Russell Diagram for the same models included in Figures \ref{['Fig:Mdot_vs_m2']}-\ref{['Fig:Mdot_vs_t']}. The track corresponding to non-irradiated models is presented in black line. The irradiated model is depicted in blue for detached stages, whereas semi-detached stages with mass transfer are denoted with red lines.
• Figure 5: The difference between the radius of the Roche lobe and that of the star for the same models included in Figures \ref{['Fig:Mdot_vs_m2']}-\ref{['Fig:Mdot_vs_t']}.