Australia’s SKA Pathfinder telescope has detected regularly repeating radio emissions from a brown dwarf star barely hotter than a pizza oven. Kofi Rose, an astronomer at the University of Sydney and one of the study’s authors, wrote on the Sapce Australia website.
At just 425°C, the little ball of gas is cooler than a typical campfire. Meanwhile, the Sun burns nuclear fuel, bringing its temperature to 5,600 degrees Celsius.
The coolest radio-emitting star ever discovered has been identified: a brown dwarf that’s too young for a star and too massive to be a planet. The discovery, published July 14 in Astrophysical Journal Letters, details the detection of a pulsating radio emission from the star WISE J062309.94−045624.6 (WISE J0623).
Despite being the size of Jupiter, this brown dwarf star has a much stronger magnetic field than our sun. There are only a few known supercooled dwarfs that generate frequent radio bursts.
There are more than 100 trillion stars in the Milky Way, so it may come as a surprise that astronomers have detected radio waves emanating from less than 1,000 of them. One reason for this is that radio waves and light waves are generated by different physical processes. Unlike thermal radiation, which comes from a star’s hot outer layer, radio emission is a result of electrons accelerating and interacting with the magnetic gas around the star.
Therefore, we can learn about the atmosphere and magnetic field of stars from radio emissions, which can ultimately tell us more about the possibility of life surviving on the star’s exoplanets.
Another factor is the sensitivity of radio telescopes, which have historically only been able to detect very bright sources. In the past few decades, most stellar observations made by radio telescopes have been flares from very active stars or energetic explosions from binary star systems. But with new radio telescopes with improved sensitivity and coverage, we can also spot fewer bright stars, such as cool brown dwarfs.
WISE J0623 has a temperature of about 700 degrees Kelvin, or 420 degrees Celsius, about the temperature of a commercial pizza oven, which is very hot by human standards, but very cold for a star.
These cool brown dwarfs cannot sustain the levels of atmospheric activity that generates radio emissions in hotter stars, making it difficult for astronomers to find such stars as WISE J0623.
How did they find the coldest brown dwarf?
The brown dwarf was discovered by the ASKAP (Australian SKA Pathfinder) telescope, which is owned by Australia’s national science agency, CSIRO, and operated by CSIRO (Commonwealth Scientific and Industrial Research Organization). The ASKAP telescope is located at CSIRO’s Murchinson Radio Astronomy Observatory in Western Australia. ASKAP consists of 36 rows of parabolic antennas, each 12 meters in diameter.
The telescope can see large regions of the sky with a single observation and has already examined nearly 90% of it. Almost 3 million radio sources have already been identified from this survey, most of them AGNs – black holes at the center of distant galaxies.
How do they know which of the millions of sources is a radio star? One way to do this is to look for circularly polarized radio emissions. Radio waves, like other electromagnetic radiation, oscillate as they travel through space. Circular polarization occurs when the electric field propagates alternating waves in a spiral motion.
In the research, astronomers used the fact that there are two known astronomical objects that emit a significant portion of circularly polarized light: stars and pulsars. They selected only highly circularly polarized radio sources from the telescope, and thus discovered the brown dwarf WISE J0623.
But what is the cause of radio emission from this star?
What does this discovery mean?
Was this a rare, one-time event during the 15-minute observation? Or can it be revealed again?
Previous research has shown that radio emissions detected from other cool brown dwarfs are related to their magnetic fields and are usually repeated by the star’s rotation rate. To investigate this, the researchers conducted follow-up observations with the CSIRO Australian Telescope Compact Array and the MeerKAT telescope operated by the South African Radio Astronomy Observatory in South Africa.
These new observations showed that every 1.9 hours there were two bright, circularly polarized bursts from the star WISE J0623, followed by a half-hour delay before the next pair of bursts.
WISE J0623 is the coolest radio brown dwarf and the first continuous radio pulse event. Using the same research method, the researchers anticipate future studies to discover cooler brown dwarfs.
Studying this missing dwarf ring of stars will help us better understand the evolution of stars and how giant exoplanets developed their magnetic fields.
(Source: Space Australia: https://spaceaustralia.com/)
