More and more black holes are found orbiting a luminous massive stellar companion. The future of these systems holds a fundamental puzzle: once the companion star expands and begins to lose mass onto the black hole, will the interaction remain stable or will the black hole plunge into the star and destroy it from within? Using state-of-the-art computational models, a team led at MPA has identified a surprisingly simple rule: the interaction is stable as long as the distance between the black hole and the star remains larger than about ten times the radius of the Sun. The newly found separation threshold will play a key role in determining which systems survive to form gravitational-wave sources and will help interpret the growing population of LIGO/Virgo/Kagra detections. Binaries that fail to remain stable, however, are no less remarkable. Such black hole-star mergers could be the explanation for luminous fast blue optical transients, linking these rare and powerful explosions to the violent end states of binary evolution.
Black holes are invisible by nature, but some of them reveal their presence by orbiting a luminous companion star. Over the last few years, astronomers have discovered several black holes in binaries with a massive stellar companion – at least ten times heavier than the Sun – by carefully tracking the motion of the visible star. These systems are likely just the tip of the iceberg: population studies suggest that hundreds more may be hidden in our Milky Way.
Massive stars do not stay compact forever. Within a few million years, the stars we see today will expand by factors of tens to a hundred, until the black hole’s gravity pulled their outer layers away. This process, known as mass transfer, lights up the system as an X-ray binary, with hot gas spiralling into the black hole via an accretion disk. Crucially, this mass exchange does not only transform the star itself, but also reshapes the entire binary: depending on how mass and angular momentum are redistributed, the orbit can widen or tighten dramatically, in some cases by orders of magnitude.
A long-standing mystery is whether this interaction remains stable or ends catastrophically. In some cases, the black hole may accrete matter peacefully for millions of years, gradually stripping away the hydrogen envelope of its companion and revealing the helium core beneath. In others, the binary becomes dynamically unstable and the black hole plunges deep into the star, destroying it from the inside. In a recent study, a research team led by an MPA fellow used detailed computer simulations with the state-of-the-art stellar evolution code MESA to show that, despite the complex gas dynamics in systems with black hole accretors, the outcome is governed by a surprisingly simple rule: how close the binary orbit becomes.
The team found that stable mass transfer has a hard limit. If the orbit tightens below about ten solar radii – roughly one-twentieth of the Earth-Sun distance – the massive star reacts by rapidly expanding. The black hole then plunges into its stellar companion, spirals through it, and ultimately merges with the helium core, destroying the star and thus the binary. This separation limit is not set by the uncertain details of how mass is exchanged, but by how massive stars respond to mass loss when forced into very tight orbits. Different stars have different “comfort zones”: some trigger instability at slightly wider separations than others do. In every case, however, the threshold can be traced back to the star’s internal structure, in particular to deep layers near the core that are normally hidden from our view.
This orbital size limit has important consequences for gravitational-wave astronomy. Compact orbits are required to form pairs of black holes or neutron stars that later spiral together and merge, producing detectable gravitational waves. The newly identified separation threshold therefore shapes which binaries can become gravitational-wave sources and which cannot, helping to clarify the origins of the growing population of mergers observed across the Universe.
But systems that cross the stability threshold may give rise to something even more dramatic. In a follow-up study, researchers from MPA and Columbia University propose that these “failed” gravitational-wave sources power one of the most mysterious explosions in the Universe: luminous fast blue optical transients, or LFBOTs.
LFBOTs are among the most extreme stellar explosions known. They can shine as brightly as the most luminous supernovae (up to a hundred times brighter than typical stellar explosions) while rising and fading on timescales of just a few days. They launch powerful outflows at tens of percent of the speed of light and emit X-rays that can persist for years after the initial flash. Radio observations add another puzzling clue: these explosions occur inside an enormous cloud of dense gas, extending to distances nearly a hundred times larger than Pluto’s orbit. Such extreme environments have posed a major challenge for models attempting to explain LFBOTs. These events are also exceedingly rare, occurring roughly a thousand times less frequently than ordinary supernovae. Illustration of a black hole absorbing a stellar core, causing radiation in radio, IR, optical/UV, and X-rays.
The new model naturally brings all these pieces together. When a black hole plunges into the star following a dynamical instability, it spirals into the compact helium core, tidally rips it apart, and accretes a fraction of a solar mass in just a few hours. This rapid accretion releases an enormous amount of energy and drives powerful, asymmetric outflows that propagate through what remains of the star, producing the observed brightness, colors, and rapid evolution of LFBOTs..
Crucially, such a merger does not happen overnight. The study shows that before the orbit tightens below the critical separation and a delayed dynamical instability is triggered, the black hole will strip mass from its companion for thousands of years in a long-lived, stable phase. Only a small fraction of this material is accreted; most of it is expelled into space, naturally building the vast and dense circumstellar medium inferred from radio observations. When the final explosion occurs, it does so inside this cocoon – explaining one of the most puzzling features of LFBOTs..
Taken ; together, the new studies led at MPA draw a direct line from the quiet lives of black hole binaries to both gravitational-wave sources and some of the most powerful stellar explosions known. Get too close to a black hole, it seems, and the result is fireworks.
Black holes are invisible by nature, but some of them reveal their presence by orbiting a luminous companion star. Over the last few years, astronomers have discovered several black holes in binaries with a massive stellar companion – at least ten times heavier than the Sun – by carefully tracking the motion of the visible star. These systems are likely just the tip of the iceberg: population studies suggest that hundreds more may be hidden in our Milky Way.
Massive stars do not stay compact forever. Within a few million years, the stars we see today will expand by factors of tens to a hundred, until the black hole’s gravity pulled their outer layers away. This process, known as mass transfer, lights up the system as an X-ray binary, with hot gas spiralling into the black hole via an accretion disk. Crucially, this mass exchange does not only transform the star itself, but also reshapes the entire binary: depending on how mass and angular momentum are redistributed, the orbit can widen or tighten dramatically, in some cases by orders of magnitude.
A long-standing mystery is whether this interaction remains stable or ends catastrophically. In some cases, the black hole may accrete matter peacefully for millions of years, gradually stripping away the hydrogen envelope of its companion and revealing the helium core beneath. In others, the binary becomes dynamically unstable and the black hole plunges deep into the star, destroying it from the inside. In a recent study, a research team led by an MPA fellow used detailed computer simulations with the state-of-the-art stellar evolution code MESA to show that, despite the complex gas dynamics in systems with black hole accretors, the outcome is governed by a surprisingly simple rule: how close the binary orbit becomes.
The team found that stable mass transfer has a hard limit. If the orbit tightens below about ten solar radii – roughly one-twentieth of the Earth-Sun distance – the massive star reacts by rapidly expanding. The black hole then plunges into its stellar companion, spirals through it, and ultimately merges with the helium core, destroying the star and thus the binary. This separation limit is not set by the uncertain details of how mass is exchanged, but by how massive stars respond to mass loss when forced into very tight orbits. Different stars have different “comfort zones”: some trigger instability at slightly wider separations than others do. In every case, however, the threshold can be traced back to the star’s internal structure, in particular to deep layers near the core that are normally hidden from our view.
This orbital size limit has important consequences for gravitational-wave astronomy. Compact orbits are required to form pairs of black holes or neutron stars that later spiral together and merge, producing detectable gravitational waves. The newly identified separation threshold therefore shapes which binaries can become gravitational-wave sources and which cannot, helping to clarify the origins of the growing population of mergers observed across the Universe.
But systems that cross the stability threshold may give rise to something even more dramatic. In a follow-up study, researchers from MPA and Columbia University propose that these “failed” gravitational-wave sources power one of the most mysterious explosions in the Universe: luminous fast blue optical transients, or LFBOTs.
LFBOTs are among the most extreme stellar explosions known. They can shine as brightly as the most luminous supernovae (up to a hundred times brighter than typical stellar explosions) while rising and fading on timescales of just a few days. They launch powerful outflows at tens of percent of the speed of light and emit X-rays that can persist for years after the initial flash. Radio observations add another puzzling clue: these explosions occur inside an enormous cloud of dense gas, extending to distances nearly a hundred times larger than Pluto’s orbit. Such extreme environments have posed a major challenge for models attempting to explain LFBOTs. These events are also exceedingly rare, occurring roughly a thousand times less frequently than ordinary supernovae. Illustration of a black hole absorbing a stellar core, causing radiation in radio, IR, optical/UV, and X-rays.
The new model naturally brings all these pieces together. When a black hole plunges into the star following a dynamical instability, it spirals into the compact helium core, tidally rips it apart, and accretes a fraction of a solar mass in just a few hours. This rapid accretion releases an enormous amount of energy and drives powerful, asymmetric outflows that propagate through what remains of the star, producing the observed brightness, colors, and rapid evolution of LFBOTs..
Crucially, such a merger does not happen overnight. The study shows that before the orbit tightens below the critical separation and a delayed dynamical instability is triggered, the black hole will strip mass from its companion for thousands of years in a long-lived, stable phase. Only a small fraction of this material is accreted; most of it is expelled into space, naturally building the vast and dense circumstellar medium inferred from radio observations. When the final explosion occurs, it does so inside this cocoon – explaining one of the most puzzling features of LFBOTs..
Taken ; together, the new studies led at MPA draw a direct line from the quiet lives of black hole binaries to both gravitational-wave sources and some of the most powerful stellar explosions known. Get too close to a black hole, it seems, and the result is fireworks.
Author:
Dr. Jakub Klencki
Postdoc
2282
jklencki@mpa-garching.mpg.de
Original publication
1. Klencki, Jakub; Podsiadlowski, Philipp; Langer, Norbert; Olejak, Aleksandra; Justham, Stephen; Vigna-Gómez, Alejandro; de Mink, Selma E.
A fundamental limit to how close binary systems can get via stable mass transfer shapes the properties of binary black hole mergers Accepted by A&A
A fundamental limit to how close binary systems can get via stable mass transfer shapes the properties of binary black hole mergers Accepted by A&A
2. Klencki, Jakub; Metzger, Brian D.
Luminous Fast Blue Optical Transients as "Failed" Gravitational Wave Sources: Helium Core− Black Hole Mergers Following Delayed Dynamical Instability
Luminous Fast Blue Optical Transients as "Failed" Gravitational Wave Sources: Helium Core− Black Hole Mergers Following Delayed Dynamical Instability
