Astronomers using the Low-Frequency Array (LOFAR) radio telescope have discovered a new brown dwarf, a substellar object not quite massive enough to fuse hydrogen in its core. It is the first substellar object to be discovered through radio observations.
An artist’s impression of BDR J1750+3809. Image credit: Danielle Futselaar / ASTRON.
The newly-detected object, named BDR J1750+3809, is a cold brown dwarf of spectral type T6.5.
Nicknamed Elegast, the object is located 212 light-years away in the constellation of Hercules.
BDR J1750+3809 is the first object of its kind to be directly identified in radio images.
“Radio waves emitted by brown dwarfs carry information about their magnetic field strength,” said lead author Dr. Harish Vedantham, an astronomer in ASTRON and the Kapteyn Astronomical Institute at the University of Groningen, and colleagues.
“Until now radio observations could only measure strong magnetic fields — about a hundred times the strength of a common fridge magnet.”
“LOFAR’s low frequency of observation makes 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.”
“With LOFAR, we want to go down the mass-ladder all the way to Jupiter-like planets that are too faint to have been found in existing infrared surveys, so we decided to search for these objects directly in our radio images,” said co-author Dr. Joe Callingham, a postdoctoral researcher at Leiden Observatory.
Objects such as BDR J1750+3809 stand out in special polarized radio images because the electric field of the radio waves they emit rotates in a characteristic circular pattern as it propagates — a phenomenon called circular polarization.
“We could not have picked out BDR J1750+3809 in our standard radio images from among the crowd of millions of galaxies, but the object immediately stood out when we made circularly polarized images,” said co-author Dr. Tim Shimwell, an astronomer in ASTRON and Leiden Observatory.
Radio detection of BDR J1750+3809 with LOFAR. Image credit: Vedantham et al., doi: 10.3847/2041-8213/abc256.
The astronomers then used infrared follow-up observations from the Gemini telescope and NASA’s Infrared Telescope Facility to confirm that BDR J1750+3809 was indeed a cold brown dwarf.
“These observations really highlight the versatility of Gemini, and in particular the little-used ‘keyhole’ imaging capability of Gemini’s GNIRS spectrograph,” said co-author Dr. Trent Dupuy, an astronomer at Gemini Observatory and the University of Edinburgh.
“This observation showcases both the flexibility and the power of the Gemini Observatories,” said Dr. Martin Still, a researcher at the National Science Foundation who was not involved in the study.
“This was an opportunity where Gemini’s design and operations enabled an innovative idea to develop into a significant discovery.”
As well as being an exciting result in its own right, the discovery of BDR J1750+3809 could provide a tantalizing glimpse into a future when astronomers can measure the properties of exoplanets’ magnetic fields.
“Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life,’ Dr. Vedantham said.
“Because magnetic phenomena of cold brown dwarfs like BDR J1750+3809 are so similar to what is seen on solar system planets, we expect our work to provide a vital datapoint to test theoretical models that predict the magnetic fields of extrasolar bodies.”
The team’s paper was published in the Astrophysical Journal Letters.
_____
H.K. Vedantham et al. 2020. Direct Radio Discovery of a Cold Brown Dwarf. ApJL 903, L33; doi: 10.3847/2041-8213/abc256
