Astronomers pinpoint rare binary brown dwarf

University of Birmingham
Monday, March 9, 2020

Astronomers working on “first light” results from a newly commissioned telescope in Chile made a chance discovery that led to the identification of a rare binary brown dwarf system.

The observation, published today in Nature Astronomy, was led by researchers at the University of Birmingham, working on the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project. SPECULOOS involves the University of Birmingham in a collaboration with the University of Liège, the University of Cambridge, the University of Bern, the Institute of Astrophysics of the Canaries, and the Massachusetts Institute of Technology.

SPECULOOS’ mission is to investigate planets surrounding ultra-cool dwarfs, a category that includes the smallest stars that exist as well as brown dwarfs. Brown dwarfs are “sub-stellar” objects, meaning they have less mass than a star but more than a planet. This makes brown dwarfs unable to sustain the fusion of hydrogen into helium, the process that characterises normal stars.

Astronomers predict that these ultra-cool dwarfs should host large populations of close-by, potentially habitable rocky planets, offering a wealth of opportunity to explore a diversity of atmospheres and climates, and the chance to search for signatures of biological activity. An example is the 7-planet system TRAPPIST-1, which was discovered by the same team.

The observations were obtained soon after construction of SPECULOOS’ first telescopes in Chile, while they were still being tested. They picked up the unusual signals that led the research team to speculate might show two brown dwarfs in orbit around each other.

Michaël Gillon, Principal Investigator of the SPECULOOS project, said: “Among the first test observations we performed, we turned one of our telescopes to a known brown dwarf. But suddenly the object appeared to get dimmer for about 90 minutes, which indicated an eclipse just took place.”

Artem Burdanov, a postdoctoral researcher at MIT who was involved in commissioning of the SPECULOOS telescopes adds: “We rapidly realised that we were probably looking at two eclipsing brown dwarfs, one passing in front of the other, a configuration which is much rarer than planetary systems.”

The researchers were able to confirm their hypothesis using two more powerful telescopes, the 10m Keck in Hawaii, and the 8m Very Large Telescope. The VLT is based in Chile, next to some of the SPECULOOS telescopes.

The observation of eclipsing brown dwarfs is extremely rare – only one other similar system has been identified, more than 10 years ago. This configuration allows astronomers to measure their radius and mass without making any assumptions.

“We’re been able to measure the individual velocities of the brown dwarfs as they orbit each other, which gave us their masses. In addition to their radii, obtained from the eclipse and luminosity, we could also estimate their age – that’s really rare because usually in measurements of these objects there is at least one other element missing,” says Amaury Triaud, from the School of Physics & Astronomy of the University of Birmingham, who led the analysis. “Models for how brown dwarfs cool down were proposed around 30 years ago – by drawing all these elements together, we’re able to verify these models for the first time. We find the models match remarkably well with the observations, a testament to human ingenuity.”

Burdanov adds: "Also, this discovery has an important implication for the exoplanetology as brown dwarfs cooling models are used to infer the radii and masses of directly imaged exoplanets."

The data that made this discovery possible were obtained by the SPECULOOS-South observatory, one of the facilities participating to the SPECULOOS project. SPECULOOS-South is hosted by the European Southern Observatory at its Paranal Observatory in Chile. SPECULOOS-South is funded by the European Research Council (ERC), as well as the Simons Foundation, the MERAC foundation, and the Science, Technology and Facilities Council.

The SPECULOOS project also involves telescopes in Tenerife (Spain), and in San Pedro Martir (Mexico).

Story Image: It is an artist 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 and the VLT. (Credit: University of Birmingham / Amanda J. Smith)