A Thorne–Żytkow object is a star within a star — a star with a neutron star at its core. These objects are theorized to form in close binary systems, but new research reveals complications in this proposed formation pathway.
A Star Within a Star
The term “star” encompasses a wide variety of objects, from our familiar Sun to roiling supergiants dozens of times as massive and hundreds of times as wide. Certain types of stars are only theorized, like those containing huge amounts of dark matter or with cores composed of strange quarks. One such theorized star — and the subject of today’s article — is a Thorne–Żytkow object, also known as a hybrid star.
After being engulfed by its companion, the neutron star is thought to sink to the star’s core. There, it is hypothesized to energize the surrounding star through accretion and nuclear fusion, creating a curious mix of elements that distinguishes a Thorne–Żytkow object from an ordinary star.
After being engulfed by its companion, the neutron star is thought to sink to the star’s core. There, it is hypothesized to energize the surrounding star through accretion and nuclear fusion, creating a curious mix of elements that distinguishes a Thorne–Żytkow object from an ordinary star.
Diverging Paths
At least, that’s the theory. But as a recent research article led by Tenley Hutchinson-Smith (University of California, Santa Cruz; University of Copenhagen) shows, more work is needed to understand whether X-ray binary systems could truly evolve into Thorne–Żytkow objects. At the core of this question is how the inspiraling neutron star affects the companion that has engulfed it. Does the engulfing star hold on to its extended gaseous envelope, or does it lose its atmosphere and diverge from the Thorne–Żytkow object path? And how long would the Thorne–Żytkow phase last — could the neutron star remain at the center of its companion indefinitely, or does the neutron star eventually gain mass and collapse into a black hole?
The team used as the basis of their exploration the X-ray binary system LMC X-4, which contains a 1.57-solar-mass neutron star and an 18-solar-mass primary star. The stars are locked in a tight gravitational embrace, separated by only 14 solar radii and orbiting one another every 1.4 days
The team used as the basis of their exploration the X-ray binary system LMC X-4, which contains a 1.57-solar-mass neutron star and an 18-solar-mass primary star. The stars are locked in a tight gravitational embrace, separated by only 14 solar radii and orbiting one another every 1.4 days
Total Collapse of the Heart
Using a three-dimensional fluid dynamics simulation, Hutchinson-Smith and collaborators followed the evolution of LMC X-4 as the primary star engulfed the neutron star. As the neutron star spiraled inward, the energy released ejected only a small amount of gas, and the neutron star accreted only a small amount of matter from the companion star. At that point, the formation of a Thorne–Żytkow object seemed inevitable, but the merger of the neutron star with the companion star’s core set the system on a different course.
As the neutron star melded with the companion’s core, it imparted angular momentum to the core. This created an accretion disk that fed the neutron star until it collapsed into a black hole. The collapse launched a relativistic jet and powered gamma-ray emission that was about as bright and as long-lasting as an ultra-long gamma-ray burst. Feedback from accretion onto the black hole ejected nearly all of the gaseous envelope, definitively halting the short-lived Thorne–Żytkow phase.
Thus, Hutchinson-Smith’s team has demonstrated that a Thorne–Żytkow object is unlikely to result from the evolution of an X-ray binary system like LMC X-4 — though this evolution may provide a path to powering ultra-long gamma-ray bursts. This suggests that accretion and feedback leading to the collapse of the neutron star and the ejection of the stellar envelope must be taken into consideration when exploring the formation of Thorne–Żytkow objects.
As the neutron star melded with the companion’s core, it imparted angular momentum to the core. This created an accretion disk that fed the neutron star until it collapsed into a black hole. The collapse launched a relativistic jet and powered gamma-ray emission that was about as bright and as long-lasting as an ultra-long gamma-ray burst. Feedback from accretion onto the black hole ejected nearly all of the gaseous envelope, definitively halting the short-lived Thorne–Żytkow phase.
Thus, Hutchinson-Smith’s team has demonstrated that a Thorne–Żytkow object is unlikely to result from the evolution of an X-ray binary system like LMC X-4 — though this evolution may provide a path to powering ultra-long gamma-ray bursts. This suggests that accretion and feedback leading to the collapse of the neutron star and the ejection of the stellar envelope must be taken into consideration when exploring the formation of Thorne–Żytkow objects.
By Kerry Hensley
Citation
“Rethinking Thorne–Żytkow Object Formation: The Fate of X-Ray Binary LMC X-4 and Implications for Ultra-Long Gamma-Ray Bursts,” Tenley Hutchinson-Smith et al 2024 ApJ 977 196. doi:10.3847/1538-4357/ad88f3
