Unraveling the mystery of brown dwarfs

Brown dwarfs are astronomical bodies whose mass is between that of the heaviest gaseous planets and the lightest stars, and they are not massive enough to sustain continuous nuclear reactions like other stars. I mean, they are not really stars. The debate comes because the boundary between them and low-mass stars has not yet been exactly established. An international team, led by scientists from the University of Geneva (UNIGE) and the Swiss National Center for Research Competence (NCCR) PlanetS, in collaboration with the University of Bern, has identified five objects that have masses near the border that separates the stars and brown dwarfs that could help

Scientists understand the nature of these mysterious objects. The results can be found in the magazine ‘
Astronomy & Astrophysics

Like Jupiter and other gas giant planets, stars are composed primarily of hydrogen and helium. But unlike these worlds, stars are so massive and their gravitational pull so powerful that hydrogen atoms fuse to produce helium, releasing enormous amounts of energy and light. However, brown dwarfs are a sort of ‘failed stars’, as they are not massive enough to fuse hydrogen and therefore cannot produce the enormous amount of light and heat that stars do. Instead, they fuse relatively small deposits of a heavier atomic version of hydrogen: deuterium. This process is less efficient and the light from brown dwarfs is much weaker than that of the rest of their ‘older sisters’.

‘However, we do not yet 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 stage short combustion and then a cooler life, “he says Nolan Grieves, researcher at the Department of Astronomy of the Faculty of Sciences of the UNIGE, member of NCCR PlanetS and first author of the study. “These limits vary depending on the chemical composition of the brown dwarf, for example, or the way it was formed, as well as its initial radius,” he explains.

To get a better idea of ​​what these mysterious objects are, we need to study examples in detail. But it turns out that they are quite rare. “So far, we have only accurately characterized about 30 brown dwarfs,” says Grieves. Compared to the hundreds of planets known to astronomers, this number is very low. Even more so considering that their larger size makes brown dwarfs easier to detect than planets.

New pieces of the puzzle

Specifically, the international team characterized five ‘companions’: TOI-148, TOI-587, TOI-681, TOI-746 and TOI-1213. These are called ‘companions’ because they orbit their respective host stars. They do so with periods of 5 to 27 days, have radii between 0.81 and 1.66 times that of Jupiter, and are between 77 and 98 times more massive. That is, they are on the border between brown dwarfs and stars.

Therefore, these five new objects contain valuable information. “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,” he says. Monika Lendl, researcher at the Department of Astronomy at UNIGE and member of the NCCR Planets.

One of the clues that scientists found to show that these objects are brown dwarfs is the relationship between their size and age, as he explains Francois Bouchy, UNIGE professor and member of NCCR PlanetS: “Brown dwarfs are supposed to shrink over time as they burn their deuterium reserves and cool down. Here we find that the two oldest objects, TOI 148 and 746, have a smaller radius, while the two youngest companions have a larger radius. ‘

However, these objects are so close to the limit that they could easily be very low-mass stars, and astronomers are still not sure if they are brown dwarfs. “Even with these additional objects, we still lack the numbers to draw definitive conclusions about the differences between brown dwarfs and low-mass stars. More studies are needed to find out more, ”concludes Grieves.


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