Evolutionary stages of the progenitors of the Gaia black hole systems. After leaving the main sequence, the massive star (blue) expands and fills its Roche lobe, initiating a non-conservative mass-transfer phase. Eventually, the currently observed system, consisting of a black hole and a low-mass stellar companion, is formed. © MPA/A. Olejak
In addition to the spectacular black hole systems that produce high-energy transients, there should exist a much larger hidden population of black holes that remain silent and can be detected only through their gravitational influence on companion stars. So far, the ESA Gaia mission has identified three such quiescent black hole binaries, with many more potentially to be released in December this year. However, two of these systems pose a major challenge to our understanding of binary evolution because of their unexpectedly wide orbits and extreme mass ratios. A new study by the Max Planck Institute for Astrophysics proposes a solution: if most of the mass transferred from the massive star escapes the system without carrying away a significant fraction of the orbital angular momentum, the binary can survive without its orbit shrinking or the stars merging. These findings suggest that low-angular-momentum mass loss may also play an important role in other types of systems, with significant implications for our understanding of how binary systems evolve.
Black hole discoveries are often associated with some of the most energetic phenomena in the Universe: powerful X-ray outbursts from matter falling onto a black hole, or spectacular mergers detected through gravitational waves. Yet the vast majority of black holes are expected to be remarkably quiet. They may drift through the Galaxy alone or orbit a normal star without producing any detectable radiation, making them almost impossible to find.
In recent years, the European Space Agency’s Gaia mission has opened a new window for discovering these hidden black holes. Gaia detects their presence through the subtle motion they induce on their stellar companions. By precisely measuring the positions and movements of billions of stars in the Milky Way, Gaia can reveal tiny “wobbles” caused by the gravitational pull of an unseen companion.
So far, three such “sleeping” black holes have been publicly confirmed through Gaia observations. These systems consist of a relatively low-mass star orbiting a black hole at a large distance, with orbital periods ranging from over a hundred to a few thousand days. Because the black holes are not actively feeding on their companions, they remain invisible, detectable only through their gravitational influence.
However, two of the discovered systems, Gaia BH1 and Gaia BH2, came as a surprise. According to the standard picture of binary star evolution, systems like these should be extremely difficult, if not impossible, to form.
Black hole discoveries are often associated with some of the most energetic phenomena in the Universe: powerful X-ray outbursts from matter falling onto a black hole, or spectacular mergers detected through gravitational waves. Yet the vast majority of black holes are expected to be remarkably quiet. They may drift through the Galaxy alone or orbit a normal star without producing any detectable radiation, making them almost impossible to find.
In recent years, the European Space Agency’s Gaia mission has opened a new window for discovering these hidden black holes. Gaia detects their presence through the subtle motion they induce on their stellar companions. By precisely measuring the positions and movements of billions of stars in the Milky Way, Gaia can reveal tiny “wobbles” caused by the gravitational pull of an unseen companion.
So far, three such “sleeping” black holes have been publicly confirmed through Gaia observations. These systems consist of a relatively low-mass star orbiting a black hole at a large distance, with orbital periods ranging from over a hundred to a few thousand days. Because the black holes are not actively feeding on their companions, they remain invisible, detectable only through their gravitational influence.
However, two of the discovered systems, Gaia BH1 and Gaia BH2, came as a surprise. According to the standard picture of binary star evolution, systems like these should be extremely difficult, if not impossible, to form.
A Problem for Stellar Evolution
Massive stars rarely evolve in isolation. Most are born in binary systems, where the interactions between the two stars can profoundly influence their lives. Through processes such as mass transfer, one star can lose material to its companion, altering the evolution and even the final fate of the entire system. One important process is called Roche-lobe overflow, a stage in which an expanding star spills material onto its companion. While Gaia BH3 is in a wide enough orbit to avoid this Roche-lobe overflow, Gaia BH1 and Gaia BH2 are expected to undergo this phase during their evolution.
The systems Gaia BH1 and Gaia BH2 each contain a black hole with a mass of about 9 times that of the Sun orbiting a much lighter companion star. Their black hole progenitors must once have been much more massive stars – around 20 times the mass of the Sun – paired with significantly smaller companions. Such massive stars reach the end of their main sequence phase quickly and then expand dramatically. Eventually, they should have been transferring material onto their companion stars.
This mass transfer phase creates a major theoretical challenge for the Gaia systems. Binary systems with such an extreme difference in stellar masses between the two stars were traditionally expected to undergo unstable mass transfer, leading to a so-called common envelope phase. During this phase, the smaller companion becomes engulfed inside the envelope of the massive star, and the system is expected either to merge into a single star or to emerge with a much tighter orbit – unlike the wide black hole binaries observed by Gaia.
The systems Gaia BH1 and Gaia BH2 each contain a black hole with a mass of about 9 times that of the Sun orbiting a much lighter companion star. Their black hole progenitors must once have been much more massive stars – around 20 times the mass of the Sun – paired with significantly smaller companions. Such massive stars reach the end of their main sequence phase quickly and then expand dramatically. Eventually, they should have been transferring material onto their companion stars.
This mass transfer phase creates a major theoretical challenge for the Gaia systems. Binary systems with such an extreme difference in stellar masses between the two stars were traditionally expected to undergo unstable mass transfer, leading to a so-called common envelope phase. During this phase, the smaller companion becomes engulfed inside the envelope of the massive star, and the system is expected either to merge into a single star or to emerge with a much tighter orbit – unlike the wide black hole binaries observed by Gaia.
An Alternative Mass-Loss Channel
The new study by Max Planck Institute for Astrophysics (MPA) stellar department team provides a possible solution to this puzzle, using detailed stellar evolution calculations to explore a different pathway for the formation of Gaia’s quiet black hole binaries.
Usually, in binary evolution models most of the transferred material is captured and lost from the vicinity of the companion star. Instead, the team investigated a scenario in which the majority of the material escapes directly from the vicinity of the massive star (the donor star).
The crucial difference is how much orbital angular momentum is removed from the system. If matter leaves close to the donor star, it carries away relatively little angular momentum compared with scenarios where mass is lost from the outer regions of the binary. As a result, the orbit does not shrink dramatically, allowing the two stars to avoid a catastrophic merger.
With this alternative mass-loss channel, the binary can survive and naturally evolve into a system with properties similar to Gaia BH1 and Gaia BH2.
Usually, in binary evolution models most of the transferred material is captured and lost from the vicinity of the companion star. Instead, the team investigated a scenario in which the majority of the material escapes directly from the vicinity of the massive star (the donor star).
The crucial difference is how much orbital angular momentum is removed from the system. If matter leaves close to the donor star, it carries away relatively little angular momentum compared with scenarios where mass is lost from the outer regions of the binary. As a result, the orbit does not shrink dramatically, allowing the two stars to avoid a catastrophic merger.
With this alternative mass-loss channel, the binary can survive and naturally evolve into a system with properties similar to Gaia BH1 and Gaia BH2.
Why would stars behave this way?
The obvious next question is whether and when nature can actually produce such a mass loss mode. The study identifies several possible physical mechanisms. Massive stars close to the end of their lives develop high opacity outer layers where radiation pressure can become extremely strong. These layers may drive powerful eruptions similar to those observed in luminous blue variable stars, ejecting material directly away from the donor star.
At the same time, the enormous difference in the sizes of the two stars' gravitational domains may prevent much of the transferred gas from ever reaching the companion. Current one-dimensional stellar evolution models cannot fully capture these complex hydrodynamic processes. Nevertheless, the agreement between our evolutionary models and the observed Gaia systems suggests that these effects deserve much closer attention.
At the same time, the enormous difference in the sizes of the two stars' gravitational domains may prevent much of the transferred gas from ever reaching the companion. Current one-dimensional stellar evolution models cannot fully capture these complex hydrodynamic processes. Nevertheless, the agreement between our evolutionary models and the observed Gaia systems suggests that these effects deserve much closer attention.
Beyond the Gaia black holes
The implications extend far beyond the two unusual black hole binaries. Several other classes of post-interacting binary stars (including systems containing neutron stars, white dwarfs, and stripped Wolf-Rayet stars) seem to face similar difficulties within the standard picture of mass transfer. If low-angular-momentum mass loss proves to be common in certain binaries, it could reshape our understanding of how many compact-object binaries form with important implications for other populations, such as gravitational wave sources.
Future data releases from Gaia are expected to uncover many more dormant black hole binaries, providing a much larger sample against which theoretical models can be tested. Combined with discoveries from other observational techniques and increasingly sophisticated three-dimensional simulations of mass transfer, these observations will help determine whether this alternative evolutionary pathway is indeed a common outcome of binary star evolution.
Future data releases from Gaia are expected to uncover many more dormant black hole binaries, providing a much larger sample against which theoretical models can be tested. Combined with discoveries from other observational techniques and increasingly sophisticated three-dimensional simulations of mass transfer, these observations will help determine whether this alternative evolutionary pathway is indeed a common outcome of binary star evolution.
Author:
Olejak, Aleksandra Olejak
Postdoc
Tel: 2231
Email: aolejak@mpa-garching.mpg.de
Original publication
A. Olejak et al.
Nonconservative Mass Transfer as a Formation Channel for Gaia Black Hole Systems
ApJ1006 13
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