Until now, brown dwarfs have largely been found from infrared sky surveys. Using observations from a radio telescope and a pair of observatories on Maunakea, astronomers- for the first time- have discovered a cold brown dwarf, also known as a ‘super planet’ BDR J1750+3809.
BDR J1750+3809 was first identified using data from the Low-Frequency Array (LOFAR) telescope in Europe and then confirmed using telescopes on the summit of Maunakea, namely the International Gemini Observatory and the NASA InfraRed Telescope Facility (which is operated by the University of Hawai’i).
Directly finding these objects with sensitive radio telescopes, such as LOFAR, is a significant breakthrough. It exhibits that astronomers can detect too cold and faint objects in infrared surveys even identify free-floating gas-giant exoplanets.
Astronomer Michael Liu and graduate student Zhoujian Zhang at the UH Institute for Astronomy (IfA) said, “This work opens a whole new method to finding the coldest objects floating in the Sun’s vicinity, which would otherwise be too faint to discover with the methods used for the past 25 years.”
The way that brown dwarfs are radio emitters allowed the international collaboration of astronomers behind this result to develop a novel observing strategy. Radio emissions have recently been identified from only a handful of cold brown dwarfs found and listed by infrared surveys before being seen with radio telescopes. The team decided to flip this strategy, using a sensitive radio telescope to discover cold, faint radio sources and afterward perform subsequent infrared observations with Maunakea telescopes to categorize them.
Harish Vedantham, a lead author of the study and astronomer at ASTRON in the Netherlands, said, “We asked ourselves, ‘Why point our radio telescope at cataloged brown dwarfs?'”
“Let’s just make a large image of the sky and discover these objects directly on the radio.”
“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.”
“Cold brown dwarfs are the closest things to exoplanets that astronomers can currently detect with radio telescopes, and this discovery could be used to test theories predicting the magnetic field strength of exoplanets. Magnetic fields are an important factor in determining the atmospheric properties and long-term evolution of exoplanets.”
Having discovered various tell-tale radio signatures in their observations, the team needed to recognize possibly fascinating sources from background galaxies. To do as such, they looked for a special type of radio waves that were circularly polarized—a feature of light from stars, planets, and brown dwarfs, however not from background galaxies.
Having found a circularly polarized radio source, the team then turned to archive imagery, the Gemini-North Telescope, and the NASA IRTF to provide the measurements required to identify their discovery.
NASA IRTF is equipped with a sensitive spectrometer, SpeX, a workhorse for studying brown dwarfs for the past 20 years, including an upgrade five years ago funded by the National Science Foundation. The team used SpeX to obtain a spectrum of BDR J1750+3809, which revealed the characteristic signature of methane in the atmosphere. Methane is the hallmark of the coldest brown dwarfs and also abundant in the atmospheres of our solar system’s gas-giant planets.
John Rayner, IRTF Director and astronomer at the UH IfA, said, “These observations highlight the increased efficiency of SpeX following its NSF-funded upgrade with state-of-the-art infrared arrays and electronics in 2015.”
- H. K. Vedantham et al. Direct Radio Discovery of a Cold Brown Dwarf, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abc256