An international team of scientists led by Carnegie Institution’s Serge Dieterich has demonstrated that brown dwarfs, objects which are unable to sustain stable nuclear fusion of hydrogen, can be more massive than thought previously.
An artist’s conception of Epsilon Indi, a star system approximately 12 light-years from Earth in the constellation Indus: brown dwarfs Epsilon Indi B and C orbit their common center of mass, which in turn orbits the much more distant primary component — the Sun-like star Epsilon Indi A. Image credit: Roberto Molar Candanosa & Sergio Dieterich, Carnegie Institution for Science.
To shine bright, stars need the energy derived from the fusion of hydrogen atoms deep in their interiors. If too small, hydrogen fusion can’t occur, so the object cools, darkens, and turns into something called a brown dwarf.
Many astronomers are trying to determine the mass, temperature, and brightness of objects on both sides of this divide.
“Understanding the boundary that separates stars from brown dwarfs will improve our understanding of how both form and evolve, as well as whether or not they could possibly host habitable planets,” Dr. Dieterich said.
The latest theoretical models predict that the boundary separating stars from brown dwarfs occurs in objects that are between 70 to 73 times the mass of Jupiter, but the new results question this prediction.
Dr. Dieterich and co-authors observed two brown dwarfs — Epsilon Indi B and Epsilon Indi C — that are part of a system that also includes a star of medium luminosity, Epsilon Indi A.
The two brown dwarfs are much too faint to be stars, but their masses are respectively 75 and 70.1 times that of Jupiter.
To the astronomers’ surprise, their findings put Epsilon Indi B and C in what was previously considered the stellar realm, even though we know from other observations that they are not stars.
“Taken together, our results mean that the existing models need to be revised,” Dr. Dieterich said.
“We showed that the heaviest brown dwarfs and the lightest stars may only have slight differences in mass. But despite this, they are destined for different lives — one racing to dim and cool, the other shining for billions of years.”
“An improved definition of the dividing line between stars and brown dwarfs could also help astronomers determine how many of each exist in our Milky Way Galaxy,” said co-author Dr. Alycia Weinberger, an astronomer at Carnegie Institution.
“We are interested in whether stars and brown dwarfs always exist in the same proportion to each other in star-forming regions, which could help us understand the overall habitability of our Galaxy.”
The team’s work was published in the Astrophysical Journal.
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Sergio B. Dieterich et al. 2018. Dynamical Masses of ε Indi B and C: Two Massive Brown Dwarfs at the Edge of the Stellar-substellar Boundary. ApJ 865, 28; doi: 10.3847/1538-4357/aadadc
