Astronomers identified a red giant star paired with an unseen binary companion after examining data from the Large Sky Area MultiObject Fiber Spectroscopic Telescope and the Apache Point Observatory Galactic Evolution Experiment. phys.org reported on November 13 that the inferred mass of the object makes it an anomaly that pushes the boundaries of stellar physics.
Neutron stars typically have masses around 1.4 times the mass of the sun packed into a sphere roughly 12 miles across. Black holes generally start at five solar masses or higher, leaving a gap that researchers use to classify dense stellar remnants.
Researchers estimate the red giant companion in system J0008+0128 has a mass between 1.8 and 2.8 solar masses, with a best estimate of 2.2 solar masses. That upper range places the object squarely in the mass gap between neutron stars and black holes.
The pair completes an orbit in about 3.3 years along a near-circular path despite the wide separation between the two objects. No light was detected outside the red giant, pointing to a neutron star as the most likely identity for the companion according to the team’s [published study in Science Advances].
Simulations Fail to Explain Wide Orbit Mass Gap
Heavy neutron stars usually occupy tight binaries where they pull material from a companion star to gain mass. This potential neutron star sits too far away from the red giant to steal material, and no signs of mass transfer are present in the data.
The research team conducted around 300,000 POSYDON simulations of isolated binaries to test how the system might have formed. None of those computer models reproduced the specific combination of high mass, wide separation, and near-circular orbit.
“The scarcity of J0008+0128–like systems is unlikely to be driven solely by limited POSYDON sampling, and instead points to a strong evolutionary bias within isolated binary evolution,” the study authors wrote in [DOI: 10.1126/sciadv.aeg3307].
The authors added that forming such a system may require unusual parameters like efficient common envelope ejection, a high birth mass, and minimal supernova kick. Standard binary evolution channels fail to account for the wide, low-eccentricity orbit.
Alternative Theories for J0008+0128 Companion
The unseen companion closely resembles Gaia NS1, a neutron star candidate with a mass of about 1.9 suns in a two-year orbit. J0008+0128 displays more extreme traits while resisting standard classification models.
Other explanations remain possible for the mysterious binary system. The setup could involve a triple system containing two very close, smaller neutron stars and the red giant, or a combination of a neutron star, white dwarf, and red giant.
Researchers cannot completely rule out that the object might be an exceptionally small black hole, though scientists have never identified a black hole that small. Future data collection will determine whether the system requires new theoretical models or points to undiscovered formation channels.
Frequently Asked Questions About J0008+0128
How did astronomers find the hidden companion?
Astronomers detected gravitational effects on the visible red giant star using spectral and radial velocity data from LAMOST and APOGEE, combined with astrometric data from the Gaia telescope.
Why is this system considered an anomaly?
The unseen companion possesses an estimated mass of up to 2.8 solar masses while maintaining a wide, near-circular 3.3-year orbit without drawing material from the red giant.
What makes the mass of this object significant?
An object weighing 2.2 solar masses sits directly in the traditional mass gap between known neutron stars and black holes.