These two stars orbit so close together that the entire system would fit inside the Sun
A record-breaking binary star system has been discovered in which two objects are so close together that they fit completely within the Sun.
Called ZTF J2020+5033, the binary system is 457 light years away and consists of a high-mass brown dwarf and a low-mass red dwarf. This is the closest orbit ever discovered for a brown dwarf, and the distance between the two objects is less than half the radius of the Sun.
Few brown dwarfs have been discovered in close proximity to other small fixed stars in a binary. ZTF J2020+5033 may provide clues to why, according to a team led by Harvard-Smithsonian Center for Astrophysics astrophysicist Kareem El-Badry.
The findings have been submitted to The Open Journal of Astrophysics and are available on the preprint server arXiv.
Brown dwarfs do not strictly fit the definition of stars, occupying a zone between small stars and massive planets that is neither. Brown dwarfs, which are 13 to 80 times more massive than Jupiter, have enough mass to cause deuterium fusion in their cores, but do not have hydrogen, the power source of a perfect star.
They are difficult to find because they are quite small and dim. Approximately 5,000 brown dwarfs are known to exist in the Milky Way Galaxy, most of which are isolated. Only about 1 percent of sun-like stars and low-mass stars form binaries with brown dwarfs within a few astronomical units.
Nonetheless, such binaries are being searched for by astronomers. Brown dwarfs interacting with a companion star can help us measure their properties and better understand their formation and evolution.
El-Badry and colleagues used the Zwicky Transient Facility to search for low-mass binary stars that may contain brown dwarfs and discovered ZTF J2020+5033. Follow-up surveys using various data sets, including Gaia data, allowed precise measurements of this system and confirmed its characteristics.
The red dwarfs in this system are also relatively small, with a radius of 17.6 percent of the Sun’s and a mass of 13.4 percent.
The brown dwarf, on the other hand, is just under the upper mass limit of this enigmatic object. Its radius is about the same as Jupiter’s, but its mass is 80.1 times greater.
Other properties suggest that both objects are quite old, which raises the question of how they arrived at their current positions. El-Badry and his colleagues believe that both objects must have once been significantly larger than they are today, at least five times farther apart.
When matter escapes from a star, it is slowed by the star’s magnetic field for a considerable distance before finally escaping. Like a spinning ice skater stretching its arms to slow down, this mass distribution slows the star’s rotation and, in the case of binary stars, shrinks its orbit. Based on the tight orbits in this binary system, such “magnetic braking” appears to be an efficient process even for low-mass stars and brown dwarfs.
This implies that the orbit of ZTF J2020+5033 should continue to shrink in the future. Brown dwarfs are smaller and less massive than red dwarfs, but their surface gravity is slightly greater. This means that as the brown dwarf approaches the red dwarf, it will begin to take material away from the red dwarf.
If magnetic braking is involved in orbital decay, this material transfer should begin within tens of millions of years. Although we cannot see this system, the discovery of this system in our immediate vicinity suggests that such close-in low-mass binary stars are relatively common. It may just be that they are too faint to be found very often. However, with the improvement of our telescope technology, such a phenomenon may be revealed in the very near future.
Source: These two stars orbit so close together that the entire system would fit inside the Sun
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