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Astronomers have discovered 21 “one-in-a-million” binary star systems in which sun-like stars orbit a hidden “dead star,” an extremely dense and compact object called a neutron star.
One of the neutron stars discovered in this group of binaries is one of the most massive dead stars of its type ever observed.
Although we are used to stars being singletons like our sun, at least half of all stars with masses close to that of the sun exist in binary systems. This percentage increases to 75% for more massive stars. As a result, stellar remnants such as neutron stars and black holes (born from the deaths of giant stars) also exist in binary systems, both with other dead stars and with massive stars.
One binary configuration that has proven to be extremely rare, however, is a neutron star orbited by 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 in orbit around normal stars like the Sun, but at close distances, causing these dead stars to feed on their companions like cosmic vampires. What makes this group of neutron star-normal star binaries so special is that they are much further apart, something that could challenge our understanding of how systems like this are born.
“Binary evolution models predict that neutron stars and normal stellar binaries should be born primarily in close orbits, with the neutron star and the companion star nearly touching,” discovery team leader Kareem El-Badry, a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA), told Space.com. “These binaries are much wider than this, with separations about 300 to 1,000 times larger than the stars.
“This means that alternative models for the formation of normal neutron stars may be needed.”
These neutron stars resist feeding on their stellar companion
Neutron stars form when stars with at least eight times the mass of the sun run out of fuel for nuclear fusion, causing the outward pressure that sustains them to collapse against the inward pressure of their own gravity.
When the cores of such stars collapse, their outer layers are blown away in huge supernova explosions, leaving behind a stellar core with a mass between one and two times that of the Sun, about 12 miles (20 kilometers) wide.
One of the recently discovered neutron stars is right on the edge of that theoretical mass range. It has 1.9 times the mass of the sun, making it one of the most massive neutron stars ever observed.
What makes these binary stars so special is the question of how they survived the transformation of one star into a neutron star.
During this process, the massive star would have violently attacked its smaller stellar companion, probably even swallowing it up, albeit temporarily. When the future neutron star exploded in a supernova explosion, current models suggest that the smaller star would have been “kicked away,” ending the binary’s existence.
“The discovery of these new systems shows that at least some binary stars survive these catastrophic processes, even though models cannot yet fully explain how,” El-Badry said.
When neutron stars are found orbiting “live” main-sequence stars, they tend to grab material from those stars like cosmic vampires. Such binaries are notable because the neutron star and the main-sequence star are close enough to facilitate the transfer of matter.
This matter has angular momentum, so it can’t fall directly into the neutron star, but instead forms a swirling, flattened cloud of 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 anything but soft. This cannibalistic feeding process can release as much energy in a second as it takes the Sun to radiate in a million years.
This emission of energy, much of it in powerful high-speed jets, makes these neutron star-donor binaries very conspicuous to astronomers, especially at X-ray and radio wavelengths. In the absence of such emissions in the electromagnetic spectrum, more distant binaries with quiet and dark neutron stars have proven more elusive.
“Most neutron star binaries are discovered via X-ray or radio data that are the result of current or past accretion,” El-Badry said. “These neutron stars have not accreted anything and do not produce X-rays or radio at a detectable level, so they are harder to find.”
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Gaia is able to make these kinds of discoveries because of its ability to precisely measure the position and motion of billions of stars against the background sky. Precise tracking of stellar motion reveals the gravitational influences exerted on these stars, which are the visible components of these binaries, from their dark neutron star companions, even though the two objects in the pair are far apart.
This influence is detected as a small wobble in the motion of the star, caused by the tug of the companion neutron star. This is the first time that neutron stars have been detected solely through their gravitational influence.
“Gaia can detect the extremely small shifts of the sun’s stars on the plane of the sky and measure their position with a precision comparable to the width of a human hair, viewed from 3,000 miles away [5,000 kilometers] away,” El-Badry said. “It is currently the only astronomical facility that can do this.”
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 companion stars, the wobble they cause would be too small for the ESA spacecraft to see.
Gaia was also helped by the fact that these binary stars are close to Earth, at distances of only 3,000 light-years or so. This may seem like a huge distance, but compared to the Milky Way’s 100,000 light-year width, it is relatively small.
While the discovery of 21 of these binary star systems is a major event, El-Badry said it is unlikely to change opinion about the rarity of wide binaries consisting of neutron stars and normal stars in the cosmos.
“We estimate that about one in a million solar stars orbits a neutron star in a wide orbit,” he explained. “However, we weren’t that surprised to find them! We hoped that a population like this would be detectable with Gaia, and characterizing these objects has been one of the major projects of my research group in recent years.”
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El-Badry explained that the next step in this research is for the team to build models to investigate the formation and future evolution of neutron star-normal star-wide binaries.
The CfA researcher also plans to use Gaia to search for wide binaries with a normal star and an unfeeding black hole.
“We also don’t know for sure how these black hole binaries formed,” El-Badry concluded. “There are clearly holes in our models for binary evolution. By finding more of these dark companions and comparing their population statistics with predictions from different models, we can figure out how they form.”
Gaia has already demonstrated that it can detect quiet and dark black holes.
In April 2023, Gaia discovered two dark black holes, Gaia BH1 and Gaia BH2, 1,560 light-years and 3,800 light-years away from Earth respectively. Only this year, the space telescope discovered Gaia BH3, just 2,000 light-years from Earth. These are the three closest black holes to our planet ever discovered.
The team’s research will be published July 15 in the Open Journal of Astrophysics.