Brown dwarfs are astronomical objects with masses between those of planets
and stars. The question of where exactly the limits of their mass lie
remains a matter of debate, especially since their constitution is very
similar to that of low-mass stars. So how do we know whether we are dealing
with a brown dwarf or a very low mass star? An international team, led by
scientists from the University of Geneva (UNIGE) and the Swiss National
Centre of Competence in Research (NCCR) PlanetS, in collaboration with the
University of Bern, has identified five objects that have masses near the
border separating stars and brown dwarfs that could help scientists
understand the nature of these mysterious objects. The results can be found
in the journal Astronomy & Astrophysics.
Like Jupiter and other giant gas planets, stars are mainly made of hydrogen
and helium. But unlike gas planets, stars are so massive and their
gravitational force so powerful that hydrogen atoms fuse to produce helium,
releasing huge amounts of energy and light.
‘Failed stars’
Brown dwarfs, on the other hand, are not massive enough to fuse hydrogen and
therefore cannot produce the enormous amount of light and heat of stars.
Instead, they fuse relatively small stores of a heavier atomic version of
hydrogen: deuterium. This process is less efficient and the light from brown
dwarfs is much weaker than that from stars. This is why scientists often
refer to them as ‘failed stars’.
“However, 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”, points out Nolan Grieves, a
researcher in the Department of Astronomy at the UNIGE’s Faculty of Science,
a member of the NCCR PlanetS and the study’s first author. “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”, 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 rather rare. “So far, we have only
accurately characterised about 30 brown dwarfs”, says the Geneva-based
researcher. Compared to the hundreds of planets that astronomers know in
detail, this is very few. All the more so if one considers that their larger
size makes brown dwarfs easier to detect than planets.
New pieces to the puzzle
Today, the international team characterized five companions that were
originally identified with the Transiting Exoplanet Survey Satellite (TESS)
as TESS objects of interest (TOI) - 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. This places them on the borderline between brown dwarfs and
stars.
These five new objects therefore 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”,
says Monika Lendl, a researcher in the Department of Astronomy at the UNIGE
and a member of the NCCR PlanetS.
One of the clues the scientists found to show these objects are brown dwarfs
is the relationship between their size and age, as explained by François
Bouchy, professor at UNIGE and member of the NCCR PlanetS: “Brown dwarfs are
supposed to shrink over time as they burn up their deuterium reserves and
cool down. Here we found that the two oldest objects, TOI 148 and 746, have
a smaller radius, while the two younger companions have larger radii.”
Yet these objects are so close to the limit that they could just as easily
be very low-mass stars, and astronomers are still unsure whether 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. Further studies are needed to find out more”, concludes
Grieves.
Reference:
Nolan Grieves, François Bouchy, Monika Lendl, Theron Carmichael, Ismael
Mireles, Avi Shporer, Kim K. McLeod, Karen A. Collins, Rafael Brahm, Keivan
G. Stassun, Sam Gill, Luke G. Bouma, Tristan Guillot, Marion Cointepas,
Leonardo A. Dos Santos, Sarah L. Casewell, Jon M. Jenkins, Thomas Henning,
Louise D. Nielsen, Angelica Psaridi, Stéphane Udry, Damien Ségransan, Jason
D. Eastman, George Zhou, Lyu Abe, Abelkrim Agabi, Gaspar Bakos, David
Charbonneau, Kevin I. Collins, Knicole D. Colon, Nicolas Crouzet, Georgina
Dransfield, Phil Evans, Robert F. Goeke, Rhodes Hart, Jonathan M. Irwin,
Eric L. N. Jensen, Andrés Jordán, John F. Kielkopf, David W. Latham,
Wenceslas Marie-Sainte, Djamel Mékarnia, Peter Nelson, Samuel N. Quinn, Don
J. Radford, David R. Rodriguez, Pamela Rowden, François–Xavier Schmider,
Richard P. Schwarz, Jeffrey C. Smith, Chris Stockdale, Olga Suarez,
Thiam-Guan Tan, Amaury H. M. J. Triaud, William Waalkes, Geof Wingham.
Populating the brown dwarf and stellar boundary: Five stars with transiting
companions near the hydrogen-burning mass limit. Astronomy &
Astrophysics, 2021; 652: A127
DOI: 10.1051/0004-6361/202141145
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