When a massive star dies in a supernova explosion, it’s not great news for any planets or stars that happen to be nearby. Generally, the catastrophic event crisps nearby worlds and sends companion stars careening through space. So, astronomers were pretty surprised to find 21 neutron stars—the crushed stellar cores left over after supernova explosions—orbiting in binary systems with Sun-like stars.
A team led by Caltech’s Kareem El-Badry detected these cosmic oddities using observations made by the European Space Agency’s Gaia mission. Its astrometrical data revealed “wobbles” in the orbits of the Sun-like companions. The team then followed up with spectral observations of the objects. Essentially, this work helped uncover a new population of what the team terms “dark” neutron stars still in orbital dances with their sunlike partners. Now the trick is to explain why these unusual pairs exist, according to El-Badry.
“We still do not have a complete model for how these binaries form,” he said. “In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution.”
Surviving a Supernova?
It seems counterintuitive to think of the nearby star surviving the nearby catastrophe. The process itself begins as the massive progenitor star ages and expands. That pushes the smaller star around. Just before the supernova occurred, the dying star probably engulfed the companion for a time. Some theories suggest that the engulfment itself could destroy the smaller star. Others say that it affects the star but doesn’t completely obliterate it.
At some point, the larger star’s core collapses when it runs out of fuel. All the other layers come crashing down on the core. The temperatures and pressures in the event compress what’s left of the core into a ball of neutrons. Then, the outer layers rebound off the core and blast out into space. That’s the part we see as the supernova explosion. The outburst should eject it from the system if there’s still a companion star. However, for these strange binaries, that didn’t happen. The neutron star and a companion remain.
Now it’s El-Badry’s team task to figure out why. “The discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how,” he said. In a paper about the finding, the team also suggests that they cannot rule out that the neutron stars may be ultramassive white dwarfs or white dwarf binaries.
The Search for Neutron Stars and their Companions
The Gaia mission aims to scan the sky and look for “wobbles” in the motions of more than a billion stars. The orbits of planets around the stars cause wobbles. However, the gravitational tug of nearby black holes, neutron stars, or more massive stars also induces them.
Neutron stars are massive balls of neutrons about 20 km across but denser than the Sun. They’re created as the collapsing stellar layers crush the core of the supernova progenitor star. As the neutron star and its companion orbit around a common center of mass, the neutron star tugs on its companion and that makes it shift back and forth—creating the telltale “wobble”. Gaia detected those wobbles, and then scientists used data from follow-up observations at several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawai‘i; La Silla Observatory in Chile; and the Whipple Observatory in Arizona. That gave them more information about the masses and orbits of the hidden neutron stars.
Now, there have been neutron stars in orbit with other Sun-like stars, those orbits have been pretty tight and close-in. In those cases, the mass transfer between the two companions makes the neutron star brighter in X-ray or radio wavelengths. That’s not true for the 21 systems El-Badry’s team studied. They are much farther apart in wider orbits. This limits how much material the neutron star can steal from its companion. As a result, those objects are dark and quiet. “These are the first neutron stars discovered purely due to their gravitational effects,” El-Badry said.
Tracing the Tale from Supernova to Binary
So, now astronomers have a population of neutron star/Sun-like star binaries to explain. Now, the team will work to figure out the real story of why these rare pairings still exist. “We estimate that about one in a million solar-type stars is orbiting a neutron star in a wide orbit,” El-Badry said.
He’s also interested in similar matchups between dormant (and largely invisible) black holes and Sun-like stars. There are two that he knows about, including one called Gaia BH1, which is only 1,600 light-years away from us. The fact that these odd couples also exist opens up a lot of questions. “We don’t know for sure how these black hole binaries formed either,” El-Badry said. “There are clearly gaps in our models for the evolution of binary stars. Finding more of these dark companions and comparing their population statistics to predictions of different models will help us piece together how they form.”
For More Information
Sun-Like Stars Found Orbiting Hidden Companions
A Population of Neutron Star Candidates in Wide Orbits from Gaia Astrometry