Coldest brown dwarf star ever detected producing radio waves

Astronomers from the University of Sydney have made an intriguing discovery: a small, faint star that happens to be the coldest ever recorded to emit radio waves. This “ultracool brown dwarf” is essentially a gas ball with a surface temperature of approximately 425 degrees Celsius, making it cooler than a typical campfire. In comparison, the sun, a blazing nuclear furnace, has a surface temperature of around 5600 degrees Celsius. While not the coldest star ever observed, this particular star stands out as the coolest one studied using radio astronomy. The research findings were recently published in The Astrophysical Journal Letters.

Kovi Rose, the lead author and a Ph.D. student in the School of Physics at the university, expressed the rarity of finding ultracool brown dwarf stars emitting radio waves. Typically, these stars lack the necessary dynamics to generate the magnetic fields responsible for detectable radio emissions from Earth. Thus, discovering this brown dwarf producing radio waves at such a low temperature is considered a noteworthy achievement.

Gaining a deeper understanding of ultracool brown dwarfs like this one contributes to our knowledge of stellar evolution and the generation of magnetic fields. The internal dynamics of brown dwarfs that occasionally produce radio waves remain somewhat enigmatic. While astronomers have a good understanding of how larger “main sequence” stars, such as the sun, generate magnetic fields and radio emissions, the reason why fewer than 10 percent of brown dwarfs exhibit such emission remains unclear.

It is believed that the rapid rotation of ultracool dwarfs plays a role in generating their strong magnetic fields. When the magnetic field rotates at a different speed from the dwarf’s ionized atmosphere, it can induce electrical current flows. In this case, it is hypothesized that the radio waves are produced by the inflow of electrons into the star’s magnetic polar region. Coupled with the rotation of the brown dwarf star, this phenomenon generates regularly repeating radio bursts.

Brown dwarf stars, named for their low energy and light output, lack the mass required for nuclear fusion like that observed in stars such as our sun. They serve as a “missing link” between the smallest hydrogen-burning stars and the largest gas giant planets like Jupiter.

The star in question, known by its catchy name T8 Dwarf WISE J062309.94−045624.6, is situated approximately 37 light years away from Earth. It was initially discovered in 2011 by astronomers at Caltech in the United States. The star’s radius is estimated to be between 0.65 and 0.95 times that of Jupiter, and its mass is not yet fully understood. However, it is known to be at least four times more massive than Jupiter but no more than 44 times more massive. For comparison, the sun is 1,000 times more massive than Jupiter.

To analyze the star, Kovi Rose utilized new data from the CSIRO ASKAP telescope in Western Australia and supplemented it with observations from the Australia Telescope Compact Array near Narrabri in New South Wales and the MeerKAT telescope in South Africa.

Professor Tara Murphy, co-author of the study and Head of the School of Physics at the University of Sydney, highlighted the significant findings made possible by the ASKAP telescope’s full operations. The ongoing exploration of the radio sky will enhance our understanding of the stars surrounding us and the potential habitability of exoplanetary systems they host.

Source: University of Sydney

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