Astronomers catch rare eclipse of double brown dwarf system

  • An artist’s view of one of the SPECULOOS telescopes, with the eclipsing binary brown dwarf in the sky. The third red dot is a third nearby brown dwarf, which is also part of the same system. The book on the side shows the data that led to the discovery. On the left page is the eclipse captured by SPECULOOS while the right page shows the data from Keck Observatory and the VLT. (University of Birmingham,Amanda J. Smith/Special to West Hawaii Today)

Astronomers working on “first light” data from a newly commissioned telescope in Chile made a chance discovery that led to the identification of a rare eclipse of two brown dwarfs.

The result, which includes data taken from W. M. Keck Observatory on Maunakea in Hawaii to help confirm the discovery, was published Monday in the journal Nature Astronomy.

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Sometimes called “failed stars,” brown dwarfs occupy a gray zone between stars and giant planets. They are unable to sustain the fusion of hydrogen into helium, a process that powers the light from normal stars like the sun; yet they appear to form like stars, only with less mass. They provide a critical link in scientists’ understanding of star and planet formation.

The chance discovery was led by an international team of researchers, including scientists at University of California at San Diego, working on a project called SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars), which aims to find planets orbiting the smallest stars, including brown dwarfs.

The project finds planets by detecting periodic dips in a star’s brightness as a planet passes in front of it, an event called a planetary transit. Astronomers predict that the smallest stars and brown dwarfs could host large populations of close-in, potentially habitable rocky planets, like the seven-planet system TRAPPIST-1 discovered in 2017.

“This is a great example of scientific serendipity,” said Adam Burgasser, professor of physics at UC San Diego and co-leading author on the study. “While searching for planets, we found an eclipsing brown dwarf binary, a system that is uniquely suited for studying the fundamental physics of these faint celestial objects.”

Helping to confirm the finding was the 10-meter Keck II telescope on Maunakea, and the 8-meter Very Large Telescope (VLT) on Cerro Paranal in Chile. Both observatories are equipped with sensitive spectrometers that can measure the velocities of celestial objects.

In the current case, astronomers detected the velocities of both brown dwarfs as they orbited one another in the constellation Libra.

“From the very first spectrum we obtained, we could tell we had an exciting binary discovery,” said Burgasser, who led the spectroscopic analysis with current and former graduate students at UC San Diego’s Cool Star Lab. “It was thrilling to see the absorption lines move back and forth in perfect synchronicity, which allowed us to measure the mass of the binary.”

The detection of an eclipsing brown dwarf binary is extremely rare because the system needs to be precisely aligned with our line-of-sight to move in front of each other. Only one other such system has been identified to date.

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The eclipsing binary is also part of a brown dwarf triple system — another rarity — with a third component orbiting at a much wider separation.

“Collecting a combination of mass, radius, and age is really rare for a star, let alone for a brown dwarf,” said Amaury Triaud, Birmingham Fellow at the University of Birmingham in the UK who was the primary author of the study. “Usually one or more of these measurements is missing. By drawing all these elements together, we were able to verify theoretical models for how brown dwarfs cool, models which are over 30 years old. We found the models match remarkably well with the observations, a testament to human ingenuity.”

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