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Astronomers have discovered 21 “one in a million” binary systems in which sun-like stars orbit a hidden “dead star”, a very dense and dense object called a neutron star.
One of the neutron stars found in this binary is one of the largest dead stars ever seen.
Although we are used to seeing stars as solitary like our sun, at least half of the stars under the mass of the sun are in binary systems. This proportion increases to 75% for more massive stars. As a result, binaries also contain stellar remnants such as neutron stars and black holes (born from the death of massive stars), with other dead stars and with massive stars.
One binary configuration that has turned out to be rare, however, is a neutron star orbiting a sun-like star. Using the European Space Agency’s (ESA) Gaia spacecraft, astronomers have discovered a new population of these elusive binary systems.
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Neutron stars are often found orbiting normal stars like the sun, but in neighborhoods where these dead stars feed on their companions like cosmic vampires. What makes this cluster of ordinary neutron star binaries so special is that they are much more widely separated, making them something that could challenge our understanding of how systems like these are born .
“Binary evolution models predict that neutron stars and ordinary star binaries should be born mainly in tight orbits, with the neutron star and the companion nearly touching,” discovery team leader Kareem El-Badry, a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA ), said Space.com. “These binaries are much wider than this, with separations about 300 to 1,000 times larger than stars.
“This means that alternative models of neutron stars may be needed.”
These neutron stars resist feeding on their stellar companion
Neutron stars are born when stars with at least eight times the mass of the sun exhaust their fuel for nuclear fusion, ending the outward push that supports them against the inward push of their own gravity.
As the cores of such stars collapse, their outer layers are blown away in massive supernova explosions. This leaves a stellar core with one to two times the mass of the sun, and about 12 miles (20 kilometers) across.
One of the newly discovered neutron stars is right at the edge of that theoretical mass range, at 1.9 times the mass of the Sun, making it among the most massive neutron stars ever observed.
What is really remarkable about these binaries is that they raise the question of how they survived the transformation of one star into a neutron star.
During this process, the giant star should violently attack its small interstellar companion, probably even swallowing it, albeit temporarily. When the soon-to-be neutron star exploded in a supernova explosion, current models suggest that the smaller star should have “kicked off”, ending the binary’s life.
“The discovery of these new systems shows that at least some binaries survive these cataclysmic processes, although models cannot yet fully explain how,” said El-Badry.
When neutron stars were discovered orbiting “living” main sequence stars, they tended to scavenge matter from those stars like cosmic vampires. Such binaries occur because the neutron star and the main sequence star are close enough to each other to facilitate material transfer.
This material has angular momentum, so it cannot fall directly into the neutron star, and instead forms a swirling cloud of diffuse material called an “accretion disk.” Although the material is gradually fed from the accretion disk to the neutron star, the result when it hits the surface is only smooth. This cannibalistic feeding process can release as much energy in a second as it takes the sun to radiate in a million years.
This energy emission, much of which is poured into powerful high-speed jets, makes these neutron donor star binaries very visible to astronomers, especially in X-ray and radio wavelengths. Without such emissions across the electromagnetic spectrum, the more widely separated binaries of quiet and dark neutron stars are more obscure.
“Most neutron star binaries are discovered through X-rays or radio data that originate from the current or past,” said El-Badry. “These neutron stars don’t believe anything and they don’t produce X-rays or radio at a detectable level, so they’re harder to find.”
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Gaia is able to make discoveries like this because of its ability to accurately measure the position and movement of billions of stars against the background sky. Precise tracking of the star’s motion reveals the gravitational influences exerted on these stars, which are the visible components of these binaries, from their dark neutron star counterparts, even though the two objects in the duo are widely separated.
This impact is felt as a tiny dip in the star’s motion caused by the tug of the companion neutron star. This is the first time that neutron stars have been detected by their gravitational influence alone.
“Gaia can detect the tiny shifts of sun-type stars in the plane of the sky, and measure their position with a precision comparable to the width of a human hair seen from 3,000 miles. [5,000 kilometers] away,” El-Badry said. “It’s the only astronomical facility that can do this right now.”
Gaia is actually more sensitive to the wider orbits and longer orbital periods of these systems. El-Badry explained that if these neutron stars were closer to their companions, the mass they would produce would be too small for the ESA spacecraft to see.
Gaia was also helped by the fact that these binaries are close to Earth, but a distance of 3,000 light years or so. This may be a huge distance, but compared to the 100,000 light-year width of the Milky Way, it is relatively small.
While the detection of 21 of these binary systems is a big deal, El-Badry said it’s unlikely to change the idea of the rarity of ordinary wide neutron star binaries in the cosmos.
“We estimate that there are about one in every million solar-type stars orbiting a neutron star in a wide orbit,” he explained. “We weren’t that surprised to find them, though! We had hoped that a population like this would be detectable with Gaia, and characterizing these objects has been one of the main projects of my research group for the past few years.”
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El-Badry explained that the next step for this research is for the team to make models to examine the formation and evolution of neutron star-normal star binaries in the future.
The CfA researcher also plans to use Gaia to search for a wide binary with a normal star and a non-feeding black hole.
“We don’t know for sure how these black hole binaries formed,” El-Badry said. “It is clear that there are gaps in our models of the evolution of binary stars. Finding more of these dark companions and comparing their population statistics with the predictions of different models will help us understand how they form. put together.”
Gaia has already demonstrated its ability to detect silent and dark black holes.
In April 2023, Gaia detected two supermassive black holes, named Gaia BH1 and Gaia BH2, located 1,560 light-years and 3,800 light-years away from Earth, respectively. Just this year, the Gaia space telescope discovered BH3, just 2,000 light years from Earth. Here are the three closest black holes to our planet that have ever been detected.
The team’s research is published on 15 July in the Open Journal of Astrophysics.
