Coldest star known to emit radio waves is smaller than Jupiter

A brown dwarf star analysed by astronomers at the University of Sydney is the coldest known to emit radio waves.

The “ultracool brown dwarf” is only 425°C on its surface, according to analysis of the wavelengths and brightness of light coming from the star. By comparison, our Sun – a bright, yellow, average-sized star in the prime of its life – has a surface temperature of more than 5,000°C.

Brown dwarfs are also known as failed stars because they are not massive enough to sustain nuclear fusion of hydrogen in their cores. Any smaller and these failed stars are nothing more than gas giant planets like our own Jupiter.

In fact, some brown dwarfs can be more diminutive than some planets.

The brown dwarf, designated T8 Dwarf WISE J062309.94−045624.6, is 37 lightyears from Earth. It was first discovered in 2011 and has a radius between 0.65 and 0.95 that of Jupiter. Its mass is harder to pinpoint, but estimates place it at 4–44 times heavier than Jupiter.

Analysis of the star’s radio-wavelength emissions is published in the Astrophysical Journal.

Why less than 10 percent of brown dwarfs emit radio waves is a mystery. It is believed that ultracool dwarfs’ rapid rotation may play a part in generating strong magnetic fields which can ionise the star’s atmosphere and create electrical currents.

In the case of T8 Dwarf WISE J062309.94−045624.6, it’s thought that electrons flowing into the magnetic polar regions of the star, which is rapidly rotating, are producing regular, repeating radio bursts.

“It’s very rare to find ultracool brown dwarf stars like this producing radio emission,” says lead author Kovi Rose, a University of Sydney PhD student. “That’s because their dynamics do not usually produce the magnetic fields that generate radio emissions detectable from Earth.”

“Deepening our knowledge of ultracool brown dwarfs like this one will help us understand the evolution of stars, including how they generate magnetic fields.”

The analysis was carried out using new data from the CSIRO ASKAP telescope in Western Australia. This was followed with observations from the Australia Telescope Compact Array near Narrabri in New South Wales and South Africa’s MeerKAT telescope.

“We’ve just started full operations with ASKAP and we’re already finding a lot of interesting and unusual astronomical objects, like this,” says co-author Professor Tara Murphy. “As we open this window on the radio sky, we will improve our understanding of the stars around us, and the potential habitability of exoplanet systems they host.”