Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes, astronomers have discovered five transiting companions near the hydrogen-burning mass limit in close orbits around main-sequence stars TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213.
An artist’s impression of brown dwarf-like objects around TOI-148, TOI-587, TOI-681, and TOI-746b. Image credit: Roger Thibaut.
Brown dwarfs are cool, dim objects that have masses in between gas giant planets, such as Jupiter or Saturn, and low-mass stars.
Sometimes called failed stars, these objects are too small to sustain hydrogen fusion reactions at their cores, yet 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 deuterium fusion in its core, and with an upper limit of 80 Jupiter masses, the approximate mass at which an object becomes sufficiently massive to fuse hydrogen nuclei into helium nuclei within its core (principal characteristic of a main-sequence star).
However, these boundaries are not clear-cut as the exact masses where deuterium and hydrogen fusion occur depend on the chemical composition of the object.
A defining characteristic of brown dwarfs is their relative low occurrence rate (less than 1%) in close orbits (less than 5 AU) around main-sequence stars compared to giant planets and other stars, or the ‘brown dwarf desert.’
“We still do not know exactly where the mass limits of brown dwarfs lie, limits that allow them to be distinguished from low-mass stars that can burn hydrogen for many billions of years, whereas a brown dwarf will have a short burning stage and then a colder life,” said Dr. Nolan Grieves, an astronomer with the Observatoire de Genève at the Université de Genève.
“These limits vary depending on the chemical composition of the brown dwarf, for example, or the way it formed, as well as its initial radius.”
In the new research, Dr. Grieves and colleagues discovered and characterized five transiting brown dwarf-like companions 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 radii between 0.81 and 1.66 Jupiter radii.
Their masses are near the uncertain lower limit of hydrogen core 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 gives us a better understanding of how they form and why they are so rare,” said Dr. Monika Lendl, also from the Observatoire de Genève at the Université de Genève.
“Brown dwarfs are supposed to shrink over time as they burn up their deuterium reserves and cool down,” said Professor François Bouchy, also from the Observatoire de Genève at the Université de Genève.
“We found that the two oldest 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.
“Further studies are needed to find out more.”
The discovery is reported in a paper in the journal Astronomy & Astrophysics.
Nolan Grieves et al. 2021. Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit. A&A 652, A127; doi: 10.1051/0004-6361/202141145