Breaking New Ground in Astronomy: The Least Warm Star that Radiates Radio Waves

A group of researchers have thoroughly examined an exceptionally cool brown dwarf star, which is known to be the chilliest star to emit radio waves. While it isn’t the absolute coldest star, this celestial body discovered by astronomers from Caltech in 2011, emits radio waves at a lower temperature than any star previously studied. It stands as a link between the smallest hydrogen-burning stars and enormous gas giants such as Jupiter.

This brown dwarf star, which is cooler than a campfire and smaller than Jupiter, represents a unique discovery.

A team of astronomers from the University of Sydney have conducted a study on the most frigid star known to emit radio waves to date, an ultracool brown dwarf. This research contributes to the understanding of star evolution and magnetic field generation.

Uncovering the Coldest Star Emitting Radio Waves

Sydney University astronomers have established that a dim, small star currently holds the record for being the coldest star to emit radio waves.

Their study subject is an ‘ultracool brown dwarf,’ essentially a gas orb with a temperature around 425 degrees Celsius (800 °F), cooler than an average campfire, and it doesn’t ignite nuclear fuel.

In contrast, the Sun’s surface temperature, a nuclear blaze, is about 5,600 degrees Celsius (10,000 °F). Although it’s not the coldest star ever found, it is the most tepid star to have been studied with radio astronomy thus far.

The results were disclosed on July 13 in The Astrophysical Journal Letters.

A visual representation shows the relative size of a common brown dwarf star. In the case of this study’s star, the brown dwarf is smaller than Jupiter (ranging between 0.65 and 0.95 of its radius) but possesses greater mass, between four and 44 times Jupiter’s mass. Credit: NASA/JPL

Understanding Magnetic Field Generation in Ultracool Brown Dwarfs

The primary author and PhD student at the School of Physics, Kovi Rose, stated, “Ultracool brown dwarfs emitting radio waves are seldom encountered because their dynamics don’t typically generate the magnetic fields that lead to detectable radio emissions on Earth.

“Stumbling upon this brown dwarf emitting radio waves at such a low temperature is an intriguing discovery.”

Kovi Rose, the primary author and PhD student from the School of Physics and Sydney Institute for Astronomy at the University of Sydney. Credit: The University of Sydney

Rose suggests that enhancing our knowledge about ultracool brown dwarfs like this one will contribute to a greater understanding of star evolution, including how they generate magnetic fields.

Challenges in Understanding Magnetic Field Generation

The process of how the internal dynamics of brown dwarfs occasionally result in radio waves is still somewhat ambiguous. Despite astronomers having a good understanding of how larger ‘main sequence’ stars like the Sun create magnetic fields and radio emissions, the reasons for less than 10 percent of brown dwarfs emitting such emissions is not entirely clear.

It’s postulated that the swift rotation of ultracool dwarfs plays a part in producing their powerful magnetic fields. When the magnetic field rotates at a different speed than the dwarf’s ionized atmosphere, it can create electric currents.

Radio Wave Emission in the Studied Dwarf

In this specific case, it’s theorized that the radio waves are created by the influx of electrons to the star’s magnetic polar region. This process, combined with the brown dwarf’s rotation, results in the creation of regular repeating radio bursts.

Brown dwarf stars are frequently referred to as such due to their limited energy or light emission, as they are not large enough to initiate the nuclear fusion that occurs in other stars, like our Sun.

The Significance of Brown Dwarfs

Mr. Rose underscored, “These stars serve as a kind of intermediary between the smallest hydrogen-burning stars and the largest gas giant planets, like Jupiter.”

The star, informally named T8 Dwarf WISE J062309.94−045624.6, is roughly 37 light-years away from Earth and was discovered in 2011 by astronomers at Caltech in the United States.

Characteristics of T8 Dwarf WISE J062309.94−045624.6

The star’s radius is between 0.65 and 0.95 times that of Jupiter. While its mass is not precisely defined, it’s at least four times more massive than Jupiter but not more than 44 times. For comparison, the Sun is 1000 times more massive than Jupiter.

Mr. Rose carried out the star’s analysis using fresh data from the CSIRO ASKAP telescope in Western Australia and supplemented it with observations from the Australia Telescope Compact Array near Narrabri in NSW and the MeerKAT telescope in South Africa.

Co-author Professor Tara Murphy, Head of School, School of Physics at the University of Sydney. Credit: The University of Sydney

Prospects for Future Research

Professor Tara Murphy, co-author and the Head of the School of Physics at the University of Sydney, commented: “We’ve just begun full operations with ASKAP, and we’re already uncovering numerous intriguing and unusual astronomical objects, like this one.

“As we open this window on the radio sky, we will enhance our understanding of the stars around us, and the potential habitability of exoplanet systems they host.”

Reference: “Periodic Radio Emission from the T8 Dwarf WISE J062309.94–045624.6” by Kovi Rose, Joshua Pritchard, Tara Murphy, Manisha Caleb, Dougal Dobie, Laura Driessen, Stefan W. Duchesne, David L. Kaplan, Emil Lenc, and Ziteng Wang, 13 July 2023, The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ace188

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More about Coldest Star Emitting Radio Waves

• The Astrophysical Journal Letters
• CSIRO ASKAP Telescope
• Australia Telescope Compact Array
• MeerKAT Telescope
• Brown Dwarfs
• Magnetic Fields of Stars
• Radio Waves
• Star Evolution