The Coolest Radio-Emitting Star Yet: A Brown Dwarf Hotter Than a Pizza Oven Pulses Every 1.9 Hours

Astronomers have found the coldest star yet known to produce radio waves. The object, called WISE J0623, is a brown dwarf: too small to count as an ordinary star and too massive to be a planet. The team behind the finding, led by University of Sydney PhD candidate Kovi Rose and Professor Tara Murphy, published it on July 13 in the Astrophysical Journal Letters.

By size, WISE J0623 is roughly a Jupiter. Its magnetic field, though, is far stronger than the Sun's. That makes it one of only a small handful of known ultra-cool dwarfs that emit repeating radio bursts. Its temperature is about 700 Kelvin, or 420 °C (788 °F), about what a commercial pizza oven reaches. That is hot by human standards and very cold for a star.

The Milky Way holds more than 100 billion stars, yet radio waves have been detected from fewer than 1,000 of them. Part of the reason is physical. Optical light comes from thermal radiation off a star's hot outer layer, while radio emission arises when electrons speed up and interact with magnetized gas around the star. That is also why radio detections are useful: they carry information about a star's atmosphere and magnetic field, which could ultimately say more about whether life could survive on planets orbiting it. The other reason is technological. Radio telescopes historically could only pick up very bright sources, so most stellar detections over past decades were flares from highly active stars or bursts from interacting binary systems. Cool brown dwarfs cannot sustain the atmospheric activity that drives radio emission in hotter stars, which has made them especially hard to find.

The discovery came from the Australian SKA Pathfinder, at Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory in Western Australia. It has 36 antennas, each 12 meters across, and can image large patches of sky in a single observation. It has already surveyed nearly 90 percent of the sky and identified close to three million radio sources, most of them active galactic nuclei, the black holes at the centers of distant galaxies.

Sorting stars from that crowd relied on circular polarization, in which the electric field of a radio wave rotates in a corkscrew as it travels. The only astronomical objects known to emit a significant fraction of circularly polarized light are stars and pulsars, which are rotating neutron stars. The team selected only highly circularly polarized sources from an earlier survey and found WISE J0623. In the paper's comparison image, switching to polarized light leaves a single object visible.

One question remained. The detection came from a 15-minute observation, so it could have been a one-off event. Earlier work on other cool brown dwarfs had linked their radio emission to magnetic fields, with bursts typically repeating at the star's rotation rate. The team followed up with CSIRO's Australian Telescope Compact Array and the MeerKAT telescope, operated by the South African Radio Astronomy Observatory.

Those observations showed a pattern. Every 1.9 hours, WISE J0623 produces two bright, circularly polarized bursts, followed by a half-hour delay before the next pair. The authors describe it as the coolest brown dwarf detected through radio waves and the first case of persistent radio pulsations at this temperature.

The team expects the same search method to turn up even cooler brown dwarfs in future surveys. They say studying these missing-link objects should improve understanding of stellar evolution and of how giant exoplanets develop magnetic fields.