The brown dwarf, which is too huge to be a planet but too small to be an ordinary star, is the coolest star yet discovered to emit radio waves.
Our research, which was just released today [July 13] in the Astrophysical Journal Letters, describes how WISE J0623’s pulsed radio emission was discovered.
This dwarf star is about as big as Jupiter, but it has a magnetic field that is far stronger than the Sun’s. It will now be counted among the select group of recurrent radio burst-producing ultra-cool dwarfs that are currently recognized.
Getting radio stars to make waves
You might be surprised to learn that, despite the Milky Way galaxy having over 100 billion stars, less than 1,000 of them have been found to emit radio waves. One explanation is that distinct physical processes produce optical light and radio waves, respectively.
Radio emission, as opposed to thermal (heat) radiation, is caused by the acceleration of electrons and their interactions with the magnetized atmosphere surrounding a star.
Because of this, scientists may use radio emission to learn more about the magnetic fields and atmospheres of stars, which in turn may help us understand whether life is possible on any planets that orbit these stars.
The sensitivity of radio telescopes, which formerly could only identify very bright objects, is another reason.
Over the past few decades, radio telescopes have mostly found stars by the detection of flares from extremely active stars or explosive bursts from the interaction of binary (two) star systems. New radio telescopes, however, have increased sensitivity and coverage, allowing us to discover dim stars like cold brown dwarfs.
Around 700 Kelvin is the temperature of WISE J0623. That’s the same as a commercial pizza oven at around 420 °C (788 °F), which is quite cold for a star but quite hot by human standards.
It is more difficult for radio astronomers to locate stars like WISE J0623 because these chilly brown dwarfs are incapable of maintaining the amounts of atmosphere activity that produce radio emission in hotter stars.
How did we find the coolest radio star?
Here comes the new SKA Pathfinder radio telescope from Australia. This has an array of 36 antennas, each measuring 12 meters in diameter, and is situated in Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory in Western Australia.
The telescope has already examined about 90% of the sky and can take in enormous areas of it in a single observation. We have located about three million radio emitters from our scan, the majority of which are active galactic nuclei, or black holes in the centers of far-off galaxies.
So how can we identify the radio stars among these millions of sources? Searching for “circularly polarized radio emission” is one approach.
Like other electromagnetic waves, radio waves oscillate as they travel through space. When a wave’s electric field turns in a spiraling or corkscrew motion as it travels across space, this is known as circular polarization.
We employed the fact that only stars and pulsars (spinning neutron stars) are known to emit a significant portion of circularly polarized light in our search.
We discovered WISE J0623 by limiting our search to radio emitters with a high degree of circular polarization. With the help of the slider, you can see that there is only one object visible once the light is polarized.
What does this discovery mean?
Was this star’s radio emission an unusual one-off occurrence that occurred during our 15-minute observation? Or may we still find it?
According to earlier studies, radio emissions from other cool brown dwarfs were correlated with their magnetic fields and typically repeated at the same rate as the star rotates.
We conducted follow-up observations using the MeerKAT telescope run by the South African Radio Astronomy Observatory and the Australian Telescope Compact Array operated by CSIRO in order to look into this.
These new observations revealed that WISE J0623 produced two brilliant, circularly polarized bursts every 1.9 hours, followed by a pause of half an hour before the subsequent set of bursts.
The first instance of persistent radio pulsations was found in WISE J0623, the coolest brown dwarf yet discovered using radio waves. We anticipate that future surveys will find even cooler brown dwarfs using the same search strategy.
We can learn more about stellar evolution and the formation of massive exoplanets’ magnetic fields by investigating these missing link dwarf stars.