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Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes, astronomers discovered five companions in transit near the burning mass limit of hydrogen in close orbits around the stars of the main sequence TOI-148, TOI-587, TOI-681, TOI-746 and TOI-1213.
Artist’s impression of objects resembling brown dwarfs around TOI-148, TOI-587, TOI-681 and TOI-746b. Image credit: Roger Thibaut.
Brown dwarfs are cold, dark objects that have masses between gas giant planets, such as Jupiter or Saturn, and low mass stars.
Sometimes called failing stars, these objects are too small to sustain hydrogen fusion reactions at their core, but they have star-like attributes.
They are often defined with a lower limit of 13 Jupiter masses, the approximate mass at which an object can begin to ignite the deuterium fusion in its nucleus, and with an upper limit of 80 Jupiter masses, the approximate mass at which a object becomes massive enough to fuse hydrogen nuclei into helium nuclei within its nucleus (main characteristic of a main sequence star).
However, these limits are not clear because the exact masses where the fusion of deuterium and hydrogen occurs depends on the chemical composition of the object.
A defining characteristic of brown dwarfs is their relatively low rate of occurrence (less than 1%) in close orbits (less than 5 AU) around main sequence stars compared to giant planets and other stars, or the “desert. brown dwarfs “.
“We still don’t know exactly where the mass limits of brown dwarfs are, which distinguish them from low-mass stars that can burn hydrogen for billions of years, while a brown dwarf will have a short combustion phase then a colder life, ”said Dr Nolan Grieves, astronomer at the Geneva Observatory at the University of Geneva.
“These limits vary depending on the chemical makeup of the brown dwarf, for example, or how it was formed, as well as its initial radius.”
In the new research, Dr Grieves and his colleagues discovered and characterized five brown dwarf-type companions in transit around five stars: TOI-148 (also known as UCAC4 260-199322), TOI-587 (HD 74162), TOI-681 (TYC 8911-00495-1), TOI-746 (TYC 9177-00082-1) and TOI-1213 (TYC 8970-00020-1).
These objects – TOI-148b, TOI-587b, TOI-681b, TOI-746b and TOI-1213b – have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 Jupiter masses, and rays between 0.81 and 1.66 rays of Jupiter.
Their masses are near the uncertain lower limit of hydrogen nucleus fusion (73-96 Jupiter masses), which separates brown dwarfs and low mass stars.
“Each new discovery reveals additional clues about the nature of brown dwarfs and allows us to better understand how they form and why they are so rare,” said Dr Monika Lendl, also from the University’s Geneva Observatory. from Geneva.
“Brown dwarfs are believed to shrink over time because they burn off their deuterium stores and cool down,” said Professor François Bouchy, also from the Geneva Observatory at the University of Geneva.
“We found that the two older objects, TOI-148b and TOI-746b, have a smaller radius, while the two younger companions have larger radii.”
“Even with these additional objects, we still lack the numbers to draw definitive conclusions about the differences between brown dwarfs and low-mass stars,” Dr. Grieves said.
“More studies are needed to find out more. “
The discovery is reported in an article in the journal Astronomy & Astrophysics.
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Nolan is crying et al. 2021. Settlement of the Brown Dwarf and the Stellar Limit: Five stars with companions in transit near the burning hydrogen mass limit. A&A 652, A127; doi: 10.1051 / 0004-6361 / 202141145
