In a recent study, astronomers have unveiled the existence of 21 neutron stars in binary systems with stars similar to our Sun. This discovery, led by Caltech’s Kareem El-Badry, is a significant milestone in our understanding of stellar evolution and binary star systems. Neutron stars, the dense remnants of massive stars that have exploded in supernovae, are typically faint and challenging to detect. However, the team harnessed the powerful capabilities of the European Space Agency’s Gaia mission to identify these elusive celestial objects.
The Gaia mission continuously scans the sky, measuring the subtle wobbles of over a billion stars. These wobbles, induced by the gravitational pull of neutron stars on their Sun-like companions, provided the key evidence for this discovery. By detecting these slight shifts, El-Badry and his team revealed a new population of dark neutron stars, expanding our understanding of stellar remnants.
Historically, neutron stars have been found in close proximity to their companion stars, often leading to mass transfer and bright emissions at X-ray or radio wavelengths. In stark contrast, the neutron stars in this new study are much farther from their partners—approximately one to three times the distance between Earth and the Sun. This significant separation prevents the neutron stars from accreting material from their companions, rendering them dark and quiescent. As a result, these neutron stars were discovered purely through their gravitational effects, marking a novel method of detection.
The study, published in The Open Journal for Astrophysics, involved a global team of researchers and utilized data from several ground-based telescopes, including the W. M. Keck Observatory in Hawai‘i, La Silla Observatory in Chile, and the Whipple Observatory in Arizona. These follow-up observations were crucial for determining the masses and orbits of the hidden neutron stars, providing further insights into their nature.
The discovery raises intriguing questions about the formation of these binary systems. According to current models, the progenitor of a neutron star should have become enormous and interacted with the Sun-like star during its late-stage evolution. This interaction, followed by a supernova explosion, would typically unbind the binary system, sending the stars in opposite directions. However, the existence of these new systems suggests that some binaries survive these cataclysmic events, challenging existing theoretical models.
Gaia’s sensitivity to wide orbits and long periods was instrumental in detecting these systems. Most of the newly discovered binaries are within 3,000 light-years of Earth, a relatively small distance in galactic terms. El-Badry estimates that about one in a million Sun-like stars has a neutron star companion in a wide orbit, highlighting the rarity of these pairings.
In addition to neutron stars, El-Badry is also focused on uncovering dormant black holes in binary systems with Sun-like stars. Using Gaia data, he has already identified two such black holes, including Gaia BH1, the closest known black hole to Earth at 1,600 light-years away.
The findings of this study underscore the gaps in our understanding of binary star evolution. Astronomers can refine their theories on how these intriguing systems form and evolve by identifying more of these dark companions and comparing their population statistics to model predictions.
The research, 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.
Images are courtesy of Caltech/R. Hurt (IPAC).
The original research article (open access) can be accessed here.
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