Maunakea, Hawaiʻi – Most stars in our universe come in pairs. While our own Sun is a loner, many stars like our Sun orbit similar stars, while a host of other exotic pairings between stars and cosmic orbs pepper the universe. Black holes, for example, are often found orbiting each other. One pairing that has proved to be quite rare is that between a Sun-like star and a type of dead star called a neutron star.
Now, astronomers led by Caltech’s Kareem El-Badry have uncovered what appear to be 21 neutron stars orbiting in binary systems with stars like our Sun. Neutron stars are dense burned-out cores of massive stars that exploded. On their own, they are extremely faint and usually cannot be detected directly. They are heavier than Sun-like stars, but the two objects mutually orbit each other around a common center of mass. As the neutron stars orbit, they tug on the Sun-like stars, causing their companions to shift back and forth in the sky. Using the European Space Agency’s Gaia mission, the astronomers were able to catch these telltale wobbles to reveal a new population of dark neutron stars.
“Gaia is continuously scanning the sky and measuring the wobbles of more than a billion stars, so the odds are good for finding even very rare objects,” says El-Badry, an assistant professor of astronomy at Caltech and an adjunct scientist at the Max Planck Institute for Astronomy in Germany.
Data from 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, were used to follow up the Gaia observations and learn more about the masses and orbits of the hidden neutron stars.
The researchers used three Keck Observatory instruments to study the 21 binary systems that Gaia discovered in finer detail: the DEep Imaging and Multi-Object Spectrograph (DEIMOS), Echellette Spectrograph and Imager (ESI), and the High-Resolution Echelle Spectrometer (HIRES).
ANIMATION: Astronomers have discovered 21 stars like our Sun in orbit around neutron stars—heavy, compact remains of massive stars that previously exploded. The hidden neutron stars were discovered through their gravitational effects alone. Though the neutron stars are heavier than Sun-like stars, the two objects mutually orbit one another around a common center of mass. As the neutron stars orbit around, they tug on the Sun-like stars, causing them to wobble. The European Space Agency’s Gaia mission detected this wobble by observing the orbits of the Sun-like stars (yellow dots) over a period of three years. The Sun-like stars are green in this animation, and the neutron stars (and their orbits) are purple. Credit: Caltech/Kareem El-Badry
While neutron stars have previously been detected in orbit around stars like our Sun, those systems have all been more compact. With little distance separating the two bodies, a neutron star (which is heavier than a Sun-like star) can steal mass away from its partner. This mass transfer process makes the neutron star shine brightly at X-ray or radio wavelengths. In contrast, the neutron stars in the new study are much farther from their partners—on the order of one to three times the distance between Earth and the Sun.
That means the newfound stellar corpses are too far from their partners to be stealing material from them. They are instead quiescent and dark. “These are the first neutron stars discovered purely due to their gravitational effects,” El-Badry says.
The discovery comes as somewhat of a surprise because it is not clear how an exploded star winds up next to a star like our Sun.
“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.”
The huge star would have knocked the little star around, likely temporarily engulfing it. Later, the neutron star progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron stars and Sun-like stars careening in opposite directions.
“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 says.
Gaia was able to find the unlikely companions due to their wide orbits and long periods (the Sun-like stars orbit around the neutron stars with periods of six months to three years).
“If the bodies are too close, the wobble will be too small to detect,” El-Badry says. “With Gaia, we are more sensitive to the wider orbits.”
Gaia is also most sensitive to binaries that are relatively nearby. Most of the newly discovered systems are located within 3,000 light-years of Earth—a relatively small distance compared, for example, to the 100,000 light-year-diameter of the Milky Way Galaxy.
The new observations also suggest just how rare the pairings are.
“We estimate that about one in a million solar-type stars is orbiting a neutron star in a wide orbit,” he said.
El-Badry also has an interest in finding unseen dormant black holes in orbit with Sun-like stars. Using Gaia data, he has found two of these quiet black holes hidden in our galaxy. One, called Gaia BH1, is the closest known black hole to Earth at 1,600 light-years away.
“We don’t know for sure how these black hole binaries formed either,” El-Badry says. “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.”
ABOUT DEIMOS
The DEep Imaging and Multi-Object Spectrograph (DEIMOS) boasts the largest field of view (16.7arcmin by 5 arcmin) of any of the Keck Observatory instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.
ABOUT ESI
The Echellette Spectrograph and Imager (ESI) is a medium-resolution visible-light spectrograph that records spectra from 0.39 to 1.1 microns in each exposure. Built at UCO/Lick Observatory by a team led by Prof. Joe Miller, ESI also has a low-resolution mode and can image in a 2 x 8 arc min field of view. An upgrade provided an integral field unit that can provide spectra everywhere across a small, 5.7 x4.0 arc sec field. Astronomers have found a number of uses for ESI, from observing the cosmological effects of weak gravitational lensing to searching for the most metal-poor stars in our galaxy.
ABOUT HIRES
The High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding exoplanets. Astronomers also use HIRES to study important astrophysical phenomena like distant galaxies and quasars, and find cosmological clues about the structure of the early universe, just after the Big Bang.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.