Astronomers can now study the phenomena behind gravitational waves and better understand the mysteries of the cosmos, according to our partner The Conversation.
The fusion of black holes emits, for example, gravitational waves which can be detected by specific observatories such as the European LIGO-Virgo.
The analysis of this phenomenon was carried out by Sylvain Chaty, university professor and astrophysicist at CEA.
Gravitational astronomy was born during the detection, in 2015, of gravitational waves coming from the merger of two black holes.
Since then, many other events, mergers of black holes but also of neutron stars, have been observed.
Astronomers now have a new messenger to study the most violent phenomena in the Universe at the origin of these gravitational waves, and thus learn more about the laws and origins of our cosmos.
This event also revealed something surprising: the black holes that were detected to merge were much more massive than what seemed likely until then.
Artist's vision of the binary black hole at the origin of the source of gravitational waves GW170104 © LIGO / Caltech / MIT / Sonoma State (Aurore Simonnet)
This has raised many questions about the formation and evolution of black holes, and in particular pairs of black holes - those that can merge leaving behind a storm of gravitational waves that we sometimes detect billions of years older. late (gravitational waves travel to us at the speed of light).
How are a couple of black holes born?
The most common story of these progenitors is as follows: two stars, often massive, are born in the same interstellar cloud.
They exchange matter during their lifetime, before eventually collapsing one after the other during two supernova events, thus forming a duo of black holes.
This couple then continues inexorably to get closer, for a time which can reach a few billion years, before finally merging.
The fusion of black holes then emits gravitational waves, which can be detected by gravitational wave detectors like LIGO-Virgo or future detectors, for example the
European
Einstein Telescope
or the
American
Cosmic Explorer
.
Simulation of the merger of two supermassive black holes (NASA Goddard)
Stellar couple: a life full of pitfalls
While this scenario is broadly known, the precise evolutionary conditions leading a pair of stars to transform into binary black holes then intended to merge with each other remain undetermined.
The history of this stellar pair is indeed strewn with pitfalls, and many parameters will or will not lead to the merger of two black holes: let us quote among others the initial mass of each star, their composition, their orbital separation, or even their speed of rotation.
The fate of the couple also depends on the properties of the collapse of each star into a black hole during its supernova, the moment at which it passes through the phase of "common envelope" with its partner, and the efficiency of the mass transfer to. his partner during evolution.
This “common envelope” phase is a crucial yet unrecognized stage in the life of the stellar couple - it's that relatively brief moment in which an envelope of gas begins to fully submerge the binary, just after the first supernova.
During this phase, an important transfer of mass takes place between the two stars, and the orbit which separates them decreases considerably.
Identifying the stellar progenitors of black hole binary fusions is therefore a subject at the heart of astrophysicists' concerns, making it possible to better predict the number of these fusions.
In order to unveil this mystery, several studies have already been conducted with the aim of determining the parameters of the progenitors of stellar black hole fusions of great mass, greater than 20 times the mass of the Sun - as we have seen, the existence of such massive stellar black holes seemed unlikely before their detection by LIGO-Virgo.
Numerical simulations showing 10 black hole fusions detected by Ligo and Virgo © Teresita Ramirez / Geoffrey Lovelace / SXS Collaboration / Ligo Virgo Collaboration
Conversely, few studies have yet looked at the progenitors of less massive stellar black holes (less than 10 times the mass of the Sun) that could merge.
If these "light" black holes are by nature easier to form, their fusion is not for all that guaranteed and the couple can very well break if the required conditions are not met - the stars then leave each one on their own. .
A code to trace the history of a stellar life
This is precisely the goal that we have set ourselves, within the framework of a collaboration between astrophysicists from the Astrophysics, Instrumentation, Modeling (CNRS / CEA / University of Paris) and Astroparticle & Cosmology (CNRS / University of Paris) laboratories: to characterize the properties of progenitor stars at the origin of “light” stellar black hole fusions.
To do this, we have reproduced the evolution of these couples of massive stars (they are indeed stars called "massive" - several times the mass of the Sun - which will ultimately collapse into "light" black holes) by adjusting crucial parameters, before comparing the results obtained with LIGO-Virgo detections.
In order to reproduce the evolution of these stellar couples, we used a public code, MESA, capable of precisely simulating the evolution of stars, based on the modeling of their internal structure, from the resolution, at each time step. , hydrodynamic equations of physics, as well as the interactions between stars within the couple.
We adapted MESA to include steps related to black hole formation and mass transfer occurring during the common envelope phase.
Thus, starting from a relatively classic evolution scenario (the two stars are born at the same time in the same interstellar cloud), we have carried out more than 66,000 hydrodynamic simulations of stars on the computer of the APC laboratory. If this number is modest compared to the millions of simulations carried out within the framework of the population synthesis models usually used, it is because these stellar evolution simulations require much more computation time, being based on the resolution, at each no time, hydrodynamic equations. However, they also prove to be much more precise, thanks to a realistic simulation of the interior of stars, and of the changes brought about by the transfer of matter and angular momentum.
Infographic of the evolution of stellar progenitors from a black hole fusion, as determined by this study © adapted from Garcia et al. 2021 by Sylvain Chaty and Elsa Couderc
66,000 hydrodynamic simulations of stars
These 66,000 simulations therefore constitute as many combinations of parameters that could be tested and compared to the rates detected by LIGO-Virgo of black hole fusions in this mass range.
We have thus predicted fusion rates of between 0.2 and 5.0 per year in our "local universe", i.e. a volume of universes within a radius of 1 giga parsec (which corresponds to 3.26 billion years -light, about 1/14 of the distance to the horizon of our observable universe), distance at which we can observe galaxies with enough detail but where the effects of cosmic evolution are weak.
This corresponds to 1.2 and 3.3 detections per year of this type of fusion of "light" black holes of less than 10 solar masses (ie rates comparable to the events detected by LIGO-Virgo during the first observation campaigns) .
This detection rate is indeed that achieved by LIGO-Virgo, and this makes it possible to draw up a more precise profile of the stellar progenitors of light black holes which can merge.
Which stars can give birth to black holes that merge?
Black hole mergers known in November 2017: only five of these mergers are true because the signal LVT151012 (second from the left on the diagram), detected by Ligo, does not allow us to conclude anything. The masses of these black holes, before and after fusion, are estimated in solar masses and are indicative given the inevitable uncertainties of the measurements. © California Institute of Technology (adaptation 20 Minutes)
Our study shows that to obtain a fusion of "light" black holes, it is necessary to start from pairs of stars of specific masses (from 25 to 65 solar masses) with a particular initial separation (between 30 and 200 solar rays).
The two stars must follow an evolution during which they exchange matter.
The main result of this study is that the fate of stellar progenitors strongly depends on the initial masses of stars, the loss of mass due to stellar winds and the initial orbital separation.
The two stars follow a similar evolution, with a first episode of stable mass transfer before the formation of the first black hole, then a second episode of unstable mass transfer leading to a common envelope phase, which will then be ejected.
This common envelope phase plays a fundamental role, because only the progenitors surviving this phase are then able to merge in a time shorter than the lifetime of the universe (Hubble time).
Our folder "BLACK HOLES"
Identify the progenitors
Our study also proposes a new method of identifying the progenitors of compact objects, such as black holes or neutron stars, from precise hydrodynamic simulations of stellar evolution, thus getting closer every day to a better understanding of the origins. some of the most violent gravitational phenomena in our universe.
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This analysis was written by Sylvain Chaty, University professor and astrophysicist at CEA.
The original article was published on The Conversation website.
Declaration of interests
This study was carried out by researchers from the French laboratories Astrophysics, Instrumentation, Modeling (AIM, CNRS / CEA / University of Paris) and Astroparticle & Cosmology (APC, CNRS / University of Paris). This work benefited from the financial support of the UnivEarthS Laboratory of Excellence (ANR-10-LABX-0023 and ANR-18-IDEX-0001, From the evolution of binaries to the fusion of compact objects). The simulations were all carried out on the APC laboratory cluster.
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