New analysis has revealed 21 Sun-like stars in mutual orbit around dark objects of neutron star–like masses — rare systems that have escaped destruction by supernova.
Most massive stars are born with at least one stellar sibling. But as the massive ones of these groups mature, they wreak havoc on their families. Yet astronomers have found some that have survived this tumult.
Before exploding as a supernova, a massive star expands, sometimes engulfing any stellar companions. Or, even if the companion avoids being swallowed up, it may yet end up on its own: The supernova imparts a kick on the crushed core of the massive star, causing the newborn neutron star to escape the system. Many of the thousands of neutron stars known in the Milky Way are alone.
But in a new analysis of data from the European Space Agency’s Gaia mission, Kareem El-Badry (Caltech) and colleagues have found 21 survivors: “dark” neutron stars in mutual orbit with Sun-like stars. The results are published in the Open Journal of Astrophysics.
We know most neutron stars by their radio emission. However, the 21 new ones are dark, found instead by the tugs on their companion suns, which wobble on the sky. Gaia saw these wobbles as it charted the precise positions of more than a billion Milky Way stars. With so many stars to look at, chances improve for finding even the rarest of systems.
The Gaia catalog released in 2022 includes data for 170,000 binary systems. A recent study found that in 177 of those binaries, a luminous star orbits a dark companion — either a white dwarf, a neutron star, or a black hole. El-Badry’s team followed up on 50 of these that were thought to host a neutron star. The researchers obtained spectra from numerous ground-based telescopes, including from La Silla and Las Campanas observatories in Chile, Fred Lawrence Whipple Observatory in Arizona, and Keck Observatory on Mauna Kea in Hawai‘i. The spectra show how fast the suns are moving back and forth in their binary orbits, which reveals their dark companions’ exact mass.
Of the 50 systems observed, 21 systems show evidence of a luminous, Sun-size star and a dark, neutron star–mass object.
“We still do not have a complete model for how these binaries form,” explains El-Badry. “In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution.
“We think the majority of such binaries are destroyed,” he adds. “We can only detect the few that survive.”
One in a Million
The supernova that births a neutron star is usually asymmetric, imparting a kick up to 250 km/s (550,000 mph). Indeed, young pulsars — spinning neutron stars that remain visible for some 10 million years due to their rotationally powered lighthouse beams — are usually isolated. When they aren’t, the companions are typically massive themselves, which might make them more likely to hold onto the neutron star even after its supernova-given kick.
Some close neutron star binaries have been found orbiting less massive stars. But while those systems are easy to find, since the neutron star produces bright X-rays as it pulls material from its companion’s atmosphere, they’re exceedingly rare — perhaps just one in 10 to 100 million.
The 21 dark neutron stars that the team found, on the other hand, seem to be slightly more common, perhaps just one in 1 million. The elongated orbits of the suns and neutron stars, which mutually orbit their common center of mass, support the survivor scenario.
The team acknowledges, though, that some of these supposed neutron stars might end up being surprisingly massive white dwarfs or even a tighter pair of less massive white dwarfs. If further data finds that to be the case, the rarity of these dark stellar cores will need to be reeavaluated.
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