This artist's illustration shows the five brown dwarfs found with satellite TESS. These objects orbit in close orbits for 5-27 days, at least three times closer to the sun than Mercury. Credit: CC-BY-NC-SA 4.0 – Thibaut Roger - UNIGE
Brown dwarfs are astronomical objects that have masses similar to stars and planets. It is still a question to determine exactly where their mass limits lie, particularly since their structure is very similar that of low-mass star. How can we tell if we're dealing with a brown dwarf, or a very small-mass star? In collaboration with the University of Bern and scientists from the University of Geneva, a team of international scientists has identified five objects with masses close to the border separating stars or brown dwarfs. This could aid scientists in understanding the mysterious nature of these objects. The journal Astronomy and Astrophysics has published the results.
Stars are mostly made up of hydrogen and Helium, just like Jupiter and other gas giants. Stars are much more powerful than gas planets because they are so large and have a strong gravitational pull. This allows hydrogen atoms to fuse to create helium and releases a lot of light and energy.
"Failed stars"
Brown dwarfs on the other side aren't large enough to fuse hydrogen, so they can't produce the immense amount of light or heat that stars can. They instead fuse a smaller amount of Deuterium, a heavier version of hydrogen. This is less efficient than stars and the light that brown dwarfs emit is weaker. Scientists often refer to them "failed stars" because they are less efficient.
"But, we don't know where the mass limits of brown dimers lie. These limits allow us to distinguish them from low-mass stars, which can burn hydrogen for billions of years. A brown dwarf, however, will have a shorter burning stage and then a longer life," says Nolan Grieves. He is a researcher at the UNIGE's Faculty of Science and a member of NCCR PlanetS. He explains that these limits can vary depending on the chemical makeup of the brown dwarf or its origin.
We need to examine examples closely in order to get a better understanding of these mysterious objects. It turns out they are quite rare. According to the Geneva-based researcher, "So far we have only accurately identified about 30 brown dwarfs." This is a small number compared to the many planets that astronomers have detailed information on. Brown dwarfs are easier to spot than planets due to their larger sizes.
The puzzle is complete with new pieces
Today's international team identified five companions originally associated with the Transiting Exoplanet Survey Satellite. They were designated TESS objects (TOI), TOI-148 to TOI-587 to TOI-681, TESS-746 and TESS-1213. These companions are known as "companions" since they orbit their host stars. They orbit their host stars for 5 to 27 days. Their radii are between 0.81 and 1.66 times those of Jupiter and they are between 77-98 times larger. They are borderline stars and brown dwarfs.
These five objects contain valuable information. Monika Lendl is a researcher at the UNIGE in the Department of Astronomy and a member of NCCR PlanetS. "Each discovery reveals additional clues regarding the nature of brown dwarfs, gives us a better understanding of why they form and how they form," she says.
Scientists have found a clue that these objects are brown dwarfs. This was explained by Franois Bouchy from UNIGE, a member of NCCR PlanetS. "Brown dwarfs are thought to shrink as they heat up and burn off their deuterium resources. We found that TOI 148, the oldest object, and 746, had a smaller radius than their younger counterparts.
These objects are so close that they could be very low-mass starlets. Astronomers are not sure if they are brown dwarfs. Even with the addition of these objects, there are still insufficient numbers to make definitive conclusions about the differences between low-mass stars and brown dwarfs. Grieves concludes that further research is needed to learn more.
Continue reading: Observations reveal a brown dwarf orbiting star TOI1278
More information: Nolan Grieves et. al., Populating brown dwarf and stellar border: Five stars with transiting partners near the hydrogen-burning limit, Astronomy & Astrophysics (2021). Information from Astronomy and Astrophysics Nolan Grieves, et al., Populating a brown dwarf, and stellar boundary: Five stars that have transiting companions close to the hydrogen-burning limit. (2021). DOI: 10.1051/0004-6361/202141145
