Is TOI-2155 b a dwarf star or a brown dwarf?
The object TOI-2155 b poses a puzzle for astronomers: Is it a red dwarf star or a brown dwarf? The distinction is not so simple: The object lies exactly on the mass boundary between a true star and a celestial body without fusion.
In the constellation Gemini, about 1350 light-years away from us, there is a sun-like star of spectral type F, which has a companion the size of Jupiter. This in itself is nothing special, but the companion of the star TOI-2155 poses a mystery. It cannot be easily classified: With a mass of 80.6 ± 1 Jupiter masses, it is a borderline object between a very massive brown dwarf and a very low-mass representative of a true M-type star. The difference lies in the core zone: If TOI-2155 b is a brown dwarf, then no fusion of hydrogen nuclei into helium nuclei, the energy source of most stars, takes place in its center. If there is fusion, then TOI-2155 b is a true star.
It is not so easy to distinguish between the two possibilities from the outside, as the lead author Md Redyan Ahmed from the Australian University of Sydney notes in the journal "The Astronomical Journal". Using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and other telescopes, it was possible to determine the mass of TOI-2155 b (the abbreviation TOI stands for TESS Object of Interest) very precisely. It is generally assumed that the mass limit for the ignition of hydrogen fusion in the core zone of a celestial body is between 75 and 80 Jupiter masses. But theoretical models suggest that this alone is not enough; other factors such as the further chemical composition and the structure of the object's atmosphere also play a role.
Thus, there is a possibility that TOI-2155 b is the most massive known brown dwarf to date - or one of the least massive red dwarfs. Only further investigations in as many spectral ranges as possible could provide clarification here. Md Redyan emphasizes that it is necessary to find more objects that are on the mass limit to learn more about this boundary area.
The main star TOI-2155 has 1.33 solar masses and 1.7 times the diameter of the sun. With a temperature of about 5800 degrees Celsius, it is slightly hotter than our sun. Its companion orbits it in just 3.72 days and has an average density of 109 grams per cubic centimeter (for comparison: Jupiter, which is about the same size, has 1.33 grams per cubic centimeter).
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