First direct detection of a brown dwarf with a radio telescope

Astronomers at ASTRON have used the LOFAR radio telescope, a pathfinder of the SKA, to discover a brown dwarf.

The discovery of the object dubbed Elegast, opens up a new path that uses radio telescopes to discover faint objects that are close-cousins of Jupiter-like exoplanets.

Brown dwarfs are objects between the mass of giant planets and small stars. They are sometimes called ‘failed stars’ because they don’t have enough mass to sustain the fusion required to become a star.

Radio waves emitted by brown dwarfs carry information about their magnetic field strength. Until now radio observations could only measure strong magnetic fields. LOFAR’s low frequencies of observation make it sensitive to magnetic fields comparable to that of a fridge magnet, which is within the range postulated to exist on the coldest brown dwarfs and large exoplanets.

“Magnetic fields control the atmospheric properties and radiation environment around exoplanets and radio observations are our best hope of measuring them. With this discovery, we have taken an important step towards realising the promise of radio astronomy to exoplanet science,” said Dr Harish Vedantham, ASTRON staff scientist and lead author of the study, which has been published in the Astrophysical Journal Letters.

With this discovery, we have taken an important step towards realising the promise of radio astronomy to exoplanet science

Dr Harish Vedantham, ASTRON

“We could not have picked out Elegast in our standard radio images from among the crowd of millions of galaxies, but it immediately stood out when we made circularly polarised images,” said Dr Tim Shimwell, ASTRON staff and project scientist of the LOFAR survey that led to the discovery. Polarised radiation twists in the form of a helix as it travels. Astronomers made the images with a polarisation filter similar to the one used in anti-glare sunglasses, but implemented in software. The group then used infrared followup observations from the Gemini telescope and NASA’s Infrared Telescope Facility to confirm that Elegast was indeed a cold brown dwarf.

Elegast is the first object of its kind that has been directly identified in radio images. The group is now busy acquiring follow-up observations to measure its magnetic field and compare the results with theory. They are also sifting through LOFAR data to identify more objects like Elegast.

By Mischa Brendel (ASTRON)

Artist’s impression of Elegast. The blue loops depict the magnetic field lines. Charged particles moving along these lines emit radio waves that LOFAR detected. Some particles eventually reach the poles and generate aurorae similar to the northern lights on Earth.

Credit: ASTRON / Danielle Futselaar