Schematic overview of a wide binary orbiting inside the Milky Way. While moving through the Galaxy its ellipticity gets modulated by the gravity of the Galaxy and fly-bys from ambient stars, leading to close encounters (inset). Credit: Jakob Stegmann et al 2024 ApJL 972 L19
Illustration of two equal-mass objects moving around each other on a circular orbit (left panel) and more and more elliptical orbits (towards the right). While all objects remain widely separated for most of the time, those moving on a very elliptical trajectory encounter each other very closely once per orbit. Credit: A. Price-Whelan/Creative Commons CC-BY-SA licence
The groundbreaking detections of gravitational waves from merging pairs of black holes have left us with an intriguing question: how do black holes get close enough to merge? Scientists at MPA show that some of them may have started out as massive stars orbiting one another at extremely large separations — 1,000 to 10,000 times the distance between Earth and Sun. Once these stars end their lives and form black holes, the gravity of the entire galaxy in which they reside could slowly deform the shape of their orbit leading to a close encounter and merger of the black holes.
A large fraction of stars are not alone. Observations show that, unlike our Sun, many of them are orbited by a stellar companion and form a so-called binary. The separation at which these binary stars orbit one another closely determines their evolution. On the one hand, stars on very tight orbits are prone to exchange mass leading to a complex interactive stellar evolution. For massive stars, these interactions may leave behind a close binary black hole which could eventually merge due to the energy-loss from gravitational-wave emission. On the other hand, binary stars at wider separations were previously thought to evolve rather unspectacularly, effectively as single stars, leaving behind binary black holes which are too far apart to merge.
In a recent study, published in the The Astrophysical Journal Letters, a group of researchers led by MPA research fellow Jakob Stegmann question this standard lore of binary physics and show that it is only true as long as the binaries are considered to be in isolation. In reality, they are embedded in a galactic environment in which wide binaries separated by more than 1,000 Earth-Sun distances are vulnerable to perturbations from the gravity of the host galaxy and from fly-bys of ambient stars. Taking into account this galactic influence, the study shows that the dynamics of wide binaries can give rise to extreme interactions between stars and compact remnants.
These interactions are a consequence of the extremely low binding energy that holds very wide binary black holes together. Thus, the gravitational pull of the entire host galaxy can slowly deform the shape of the orbit on which the two black holes move around each other and make it more and more elongated. On these highly elliptical orbits the two black holes remain widely separated for most of the time, but pass close to each other once per orbit (see animation). This leads to a counterintuitive result: In order to bring two black holes closer than a few kilometres so that they can merge, we could nevertheless start with a wide separation of more than 1,000 times the distance between Earth and Sun. The clue lies in the ellipticity of their orbit which slowly grows due to the disturbing effect of the galaxy’s gravity.
This mechanism of driving two black holes closer together could also be relevant for the evolution of wide low-mass binary stars. Recently, researchers at MPIA in Heidelberg have searched for wide binaries in the data from the ESA-led mission Gaia. Surprisingly, they found that about ten percent of all low-mass stars possess a distant stellar companion. While systems like those are not massive enough to develop black holes, in this case the MPA study shows that the gravity of the galaxy could drive the stars to a head-on collision. These collisions would not lead to detectable emission of gravitational waves, but could be visible as energetic flares, so-called Luminous Red Novae.
The results of this study represent progress in investigating the plethora of evolutionary pathways of binary stars and their compact remnants. While previous work on wide binaries has mostly focused on ruling out the existence of a distant companion to our Sun (referred to as the “Nemesis hypothesis”), on the one hand, and understanding the upper limit of their separation to remain bound, on the other hand, little attention has been paid to studying the interactions between wide binary stars. With future data releases of Gaia expanding the catalogue of wide binary stars at an unprecedented rate, the MPA study makes an important step towards understanding their co-evolution with the Milky Way. Investigating their dynamics in detail allows us to understand how systems previously thought uneventful could in fact lead to some of the most energetic transients in the Universe.
A large fraction of stars are not alone. Observations show that, unlike our Sun, many of them are orbited by a stellar companion and form a so-called binary. The separation at which these binary stars orbit one another closely determines their evolution. On the one hand, stars on very tight orbits are prone to exchange mass leading to a complex interactive stellar evolution. For massive stars, these interactions may leave behind a close binary black hole which could eventually merge due to the energy-loss from gravitational-wave emission. On the other hand, binary stars at wider separations were previously thought to evolve rather unspectacularly, effectively as single stars, leaving behind binary black holes which are too far apart to merge.
In a recent study, published in the The Astrophysical Journal Letters, a group of researchers led by MPA research fellow Jakob Stegmann question this standard lore of binary physics and show that it is only true as long as the binaries are considered to be in isolation. In reality, they are embedded in a galactic environment in which wide binaries separated by more than 1,000 Earth-Sun distances are vulnerable to perturbations from the gravity of the host galaxy and from fly-bys of ambient stars. Taking into account this galactic influence, the study shows that the dynamics of wide binaries can give rise to extreme interactions between stars and compact remnants.
These interactions are a consequence of the extremely low binding energy that holds very wide binary black holes together. Thus, the gravitational pull of the entire host galaxy can slowly deform the shape of the orbit on which the two black holes move around each other and make it more and more elongated. On these highly elliptical orbits the two black holes remain widely separated for most of the time, but pass close to each other once per orbit (see animation). This leads to a counterintuitive result: In order to bring two black holes closer than a few kilometres so that they can merge, we could nevertheless start with a wide separation of more than 1,000 times the distance between Earth and Sun. The clue lies in the ellipticity of their orbit which slowly grows due to the disturbing effect of the galaxy’s gravity.
This mechanism of driving two black holes closer together could also be relevant for the evolution of wide low-mass binary stars. Recently, researchers at MPIA in Heidelberg have searched for wide binaries in the data from the ESA-led mission Gaia. Surprisingly, they found that about ten percent of all low-mass stars possess a distant stellar companion. While systems like those are not massive enough to develop black holes, in this case the MPA study shows that the gravity of the galaxy could drive the stars to a head-on collision. These collisions would not lead to detectable emission of gravitational waves, but could be visible as energetic flares, so-called Luminous Red Novae.
The results of this study represent progress in investigating the plethora of evolutionary pathways of binary stars and their compact remnants. While previous work on wide binaries has mostly focused on ruling out the existence of a distant companion to our Sun (referred to as the “Nemesis hypothesis”), on the one hand, and understanding the upper limit of their separation to remain bound, on the other hand, little attention has been paid to studying the interactions between wide binary stars. With future data releases of Gaia expanding the catalogue of wide binary stars at an unprecedented rate, the MPA study makes an important step towards understanding their co-evolution with the Milky Way. Investigating their dynamics in detail allows us to understand how systems previously thought uneventful could in fact lead to some of the most energetic transients in the Universe.
Author:
Jakob Stegmann
tel:2237
stegmaja@mpa-garching.mpg.de
Original Publication
Jakob Stegmann, Alejandro Vigna-Gómez, Antti Rantala, Tom Wagg, Lorenz Zwick, Mathieu Renzo, Lieke A. C. van Son, Selma E. de Mink, and Simon D. M. White
Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-wave Sources
Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-wave Sources
https://iopscience.iop.org/article/10.3847/2041-8213/ad70bb
Source | DOI
