J0008+0128: the invisible object Gaia found that breaks star models
Astronomers found a hidden compact object about 20 km across, orbiting a red giant. Thousands of simulations could not explain how it formed.
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giftdetect· the right gift in minutesSomething invisible is orbiting a red giant a few thousand light-years away, and nobody can say how it got there. The object, named J0008+0128, is heavier than the models of stellar evolution allow, at least if it is what astronomers suspect: a neutron star.
You cannot see it, of course. It is roughly 20 km across, so researchers studied it through its partner, the red giant, which is easy to observe. They measured the giant's radial velocity (how fast it moves toward or away from us) using the LAMOST and SDSS/APOGEE surveys plus the Nordic Optical Telescope. Astrometric data from the European Gaia spacecraft filled in the motion of the stars and pinned down the orbit. High-resolution spectra showed no lines that could belong to a second ordinary star.
What makes J0008+0128 so strange
The red giant weighs between 1.2 and 2.7 solar masses. The hidden companion comes in at 1.8 to 2.8 solar masses. The orbit takes about 1,220 days and is fairly close to circular, with an eccentricity of about 0.1. It is also too wide for the companion to have grown by pulling mass off its partner.
That combination is the problem. If the companion is a neutron star, it belongs to a very heavy class that has never been observed and does not even show up in models. The team ran thousands of simulations, including the POSYDON binary evolution models, and none reproduced the system.
How could it have formed?
The researchers see two options. One is that an unusually massive neutron star formed directly in a supernova. The other is that the system was once a triple, and two of its stars later merged: a neutron star with an ordinary star, or two neutron stars.
Why it matters
Pinning down the object's properties would help locate the line between neutron stars and black holes, and show which dying stars end up as which. It would also reveal how dense matter can get, and how much of it can resist collapsing into a black hole. If it really is a neutron star, the authors say, the finding could even point to something new in physics.
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