Astronomers Record First Direct Discovery of a Cold Brown Dwarf

Researchers of a collaboration between the LOw Frequency ARray (LOFAR) radio telescope in Europe, the Gemini North telescope, and the NASA InfraRed Telescope Facility (IRTF), both on Maunakea in Hawai‘i, have reported the first-ever direct observation of a cold brown dwarf using its radio wavelength emission.

Artist’s impression of the cold brown dwarf BDR J1750+3809. 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. Image Credit: ASTRON/Danielle Futselaar.

Apart from opening the door toward future discoveries of brown dwarfs, the findings of the study are a crucial step in applying radio astronomy to the exhilarating area of exoplanets.

Astronomers have used, for the first time, observations from the LOFAR radio telescope, the NASA IRTF, run by the University of Hawaii, and the international Gemini Observatory, a Program of NSF’s NOIRLab, to find and characterize a cold brown dwarf.

Named BDR J1750+3809, the object is the first substellar object to be found using radio observations—to date, brown dwarfs have been unraveled in large infrared and optical surveys.

The direct observation of such objects using sensitive radio telescopes like LOFAR is a major progress since it shows that astronomers can detect even those objects that are too faint and cold to be discovered through existing infrared surveys—perhaps even large free-floating exoplanets.

In this discovery, Gemini was particularly important because it identified the object as a brown dwarf and also gave us an indication of the temperature of the object. The Gemini observations told us that the object was cold enough for methane to form in its atmosphere—showing us that the object is a close cousin of Solar System planets like Jupiter.

Harish Vedantham, Study Lead Author, Netherlands Institute for Radio Astronomy (ASTRON)

Brown dwarfs constitute substellar objects that straddle the boundary between the smallest stars and the largest planets. Brown dwarfs are sometimes referred to as failed stars and lack the mass to induce hydrogen fusion in their cores; rather, they glow at infrared wavelengths with remnant heat from their formation.

Although brown dwarfs lack the fusion reactions that enable the Sun to shine always, they can emit light at radio wavelengths. The fundamental process that powers this radio emission is well known, since it takes place in the largest planet in the Solar System. The powerful magnetic field of Jupiter speeds up charged particles like electrons, in turn generating radiation—radio waves and aurorae in this case.

Since brown dwarfs are powerful radio emitters, the international team behind this study was able to devise an innovative observation method. Earlier, radio emissions were detected using just a handful of cold brown dwarfs—and they are familiar and cataloged by infrared surveys before are observed through radio telescopes.

The astronomers decided to switch this approach by using a sensitive radio telescope to find cold, faint sources and then carry out follow-up infrared observations using a large telescope, such as the 8-m Gemini North telescope, to classify them.

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 in the radio.

Harish Vedantham, Study Lead Author, Netherlands Institute for Radio Astronomy (ASTRON)

The team discovered a range of tell-tale radio signatures in their observations and had to differentiate potentially fascinating sources from background galaxies. They achieved this by looking for a unique form of circularly polarized light—a feature of light from planets, stars, and brown dwarfs, but not from background galaxies.

Then they found a circularly polarized radio source and used telescopes, such as Gemini North and the NASA IRTF, to offer the measurements needed to identify their discovery.

Gemini North is provided with a range of infrared instruments, one of which is often maintained ready to observe in the event of an exciting astronomical opportunity. For BDR J1750+3809, the Near InfraRed Imager and spectrograph (NIRI), which is Gemini’s key infrared imager, was not available.

Therefore, Gemini astronomers resorted to the unconventional step of using the acquisition camera for the Gemini Near-Infrared Spectrograph (GNIRS) instead. The meticulous work and prudence of Gemini staff enabled this camera to offer sharp, deep, and precise imaging at various infrared wavelengths.

These observations really highlight the versatility of Gemini, and in particular the little-used ‘keyhole’ imaging capability of Gemini’s GNIRS spectrograph,” noted Trent Dupuy, astronomer at Gemini Observatory and the University of Edinburgh. Dupuy is also a co-author of the research paper.

The observations by Gemini North were acquired through Director’s Discretionary Time, which is reserved for programs that require small amounts of observation time with potentially high-impact results.

This observation showcases both the flexibility and the power of the Gemini Observatories. This was an opportunity where Gemini’s design and operations enabled an innovative idea to develop into a significant discovery.

Martin Still, National Science Foundation

The discovery of BDR J1750+3809 is not just an exciting result in its own right but could also offer an appealing glimpse into a future where astronomers can quantify the properties of the magnetic fields of the exoplanets.

Cold brown dwarfs are the nearest objects to exoplanets that can now be detected by astronomers using radio telescopes, and this finding could be employed to test theories that propose the magnetic field strength of exoplanets. Magnetic fields are a crucial factor in identifying the long-term evolution and atmospheric properties of exoplanets.

Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life,” concluded Vedantham. “Because magnetic phenomena of cold brown dwarfs are so similar to what is seen in Solar System planets, we expect our work to provide vital data to test theoretical models that predict the magnetic fields of exoplanets.”

Video Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Lomberg J, J. Chu/J. Pollard, E. Mastroianni, LOFAR / ASTRON, S. Brunier/Digitized Sky Survey 2.

Journal Reference:

Vedantham, H. K., et al. (2020) Direct radio discovery of a cold brown dwarf. The Astrophysical Journal Letters.


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