In the universe, most stars are organized in pairs. Although our Sun is solitary, many stars similar to the Sun orbit other stars. Particularly interesting are combinations like black holes orbiting each other. However, a rare combination is that between a Sun-like star and a neutron star.

A team of astronomers, led by Kareem El-Badry from Caltech, has identified 21 binary systems where neutron stars orbit Sun-like stars. Neutron stars are dense cores of massive stars that have undergone supernova explosions. These stars are dark and very faint, making direct observation difficult. Although significantly more massive than Sun-like stars, both stars orbit a common center of mass. As a result, neutron stars cause slight changes in the positions of their stellar partners.

Thanks to the Gaia mission of the European Space Agency, astronomers have been able to discover this new population of dark neutron stars. The Gaia mission continuously scans the sky, measuring tiny shifts in over a billion stars, increasing the chances of finding rare objects.

New Results
The study, published in The Open Journal for Astrophysics, uses data from several Earth-based telescopes, including the W. M. Keck Observatory on Maunakea in Hawaii, the La Silla Observatory in Chile, and the Whipple Observatory in Arizona. These telescopes helped confirm Gaia mission observations and enabled more detailed study of the masses and orbits of hidden neutron stars.

Although neutron stars have previously been detected in orbits around Sun-like stars, these systems are usually much more compact. In such systems, the neutron star may acquire mass from its stellar companion, causing the emission of bright X-rays or radio waves. In contrast, the neutron stars in this study are much farther from their partners, about one to three times the distance between Earth and the Sun, meaning no mass transfer occurs and they remain dark.

Gravitational Effects
These neutron stars were detected solely because of their gravitational effects. "These are the first neutron stars discovered solely because of their gravitational effects," says El-Badry. The discovery is surprising because it is unclear how an exploded star can end up near a Sun-like star.

"We still don't have a complete model of how these binary systems form," explains El-Badry. In principle, the progenitor of the neutron star should become enormous and interact with the Sun-like star during its late evolution. The massive star would likely temporarily engulf the smaller star. Later, the progenitor of the neutron star would explode in a supernova, which models suggest should disrupt the binary systems, sending the neutron stars and Sun-like stars in opposite directions.

Unexpected Discoveries
"The discovery of these new systems shows that at least some binary systems survive these cataclysmic processes, although models still cannot fully explain how," says El-Badry.

Gaia managed to find these unusual partners due to their wide orbits and long periods. Sun-like stars orbit neutron stars with periods ranging from six months to three years. "If the bodies are too close, their movement will be too small for detection," says El-Badry. "With the Gaia mission, we are more sensitive to wider orbits." Gaia is also most sensitive to binary systems that are relatively close to Earth. Most newly discovered systems are within 3,000 light-years of Earth - which is a relatively small distance compared to the Milky Way's diameter of 100,000 light-years.

These new observations also suggest how rare such pairs are. "We estimate that about one in a million Sun-like stars orbits a neutron star in a wide orbit," said El-Badry.

Dark Black Holes
El-Badry is also investigating invisible dormant black holes in orbit with Sun-like stars. Using Gaia mission data, he has found two such quiet black holes hidden in our galaxy. One of them, named Gaia BH1, is the closest known black hole to Earth, only 1,600 light-years away.

"We still don't know exactly how these binary black holes formed," says El-Badry. "There are obvious gaps in our models of binary star evolution. Finding more of these dark companions and comparing their population statistics with predictions from various models will help us piece together how they form."

The paper titled "A population of neutron star candidates in wide orbits from Gaia astrometry" was funded by the National Science Foundation, the European Research Council, and the Gordon and Betty Moore Foundation. Other Caltech authors include graduate student Natsuko Yamaguchi and astronomy professor Andrew Howard. Additional authors are Hans-Walter Rix and René Andrae from the Max Planck Institute for Astronomy, David Latham and Allyson Bieryla from the Center for Astrophysics/Harvard & Smithsonian, Sahar Shahaf from the Weizmann Institute of Science, Tsevi Mazeh from Tel Aviv University; Lars Buchhave from the Technical University of Denmark, Howard Isaacson from UC Berkeley and the University of Southern Queensland; Alessandro Savino from UC Berkeley and Ilya Ilyin from the Leibniz Institute for Astrophysics Potsdam.

Source: California Institute of Technology