Astronomers discover new ‘strange radio circle’ near the center of our galaxy
A mysterious ring of radio light may have been created by a massive star whose outer layers were blown away by a powerful radiation wind, say astronomers who made the discovery using the MeerKAT radio telescope in South Africa.
In 2019, astronomers conducting a survey with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) noticed several strange rings of radio light that were not detectable at any other wavelength of light and had no obvious source. The astronomers called them “strange radio circles” or ORCs in short.
Only a handful of them are known so far, but now a new ORC has been discovered that breaks all the rules.
ASKAP is a technological precursor of Square kilometer array (SKA), a vast network of radio dishes and antennas that will be spread between Australia and South Africa. It is fitting, then, that South Africa also has its own SKA precursor observatory, in the form of MeerKAT, originally the Karou Radio Telescope, which is located in the country’s Meerkat National Park.
During observations with MeerKAT in November 2022, astronomers led by Cristobal Bordiu of the Catania Observatory in Italy discovered something unusual. It was an ORC, but it was not where it was supposed to be.
Before this discovery, all previous ORCs were found at high galactic latitudes. In other words, they lie high above the plane of our Milky Waywhich means that they are either very close to us in our galaxy or are extragalactic. In fact, several ORCs contain a galaxy in the middle of the ring, and these ORCs are thought to have been created by an eruption in this galaxy, perhaps a starburst event that led to many supernovasor a merger of two supermassive black holes which leads to an energy pulse.
However, this new ORC is located just six degrees above the plane of our galaxy, exactly in the Milky Way as it appears in the sky. It also appears to be quite close to the galactic center from our perspective. However, this could just be a coincidence – it could be much closer or much farther away than the center of our galaxy, which is 26,000 Light years away.
The ORC, catalogued as J1802–3353, was named Kýklos by its discoverers, a word that means circle in Greek. Kýklos stretches across 80 arcseconds in the sky—an arcsecond is 1/3,600 of a degree. The ring itself is visible only at radio wavelengths, where it is faint, blotchy, thin (just 6 arcseconds thick), and almost a perfect circle. Its radio spectrum is surprisingly flat, meaning that unlike previous ORCs, it does not exhibit any notable spectral lines.
Bordius’ team was aware that this ORC might be something new, but before they could determine that, they first had to rule out other possibilities.
The European Space Agency Gaia The mission has catalogued three galaxies, all of which happen to lie within Kýklos in the sky. One galaxy in particular is only 3 arcseconds from the ring’s center, but if Kýklos was created by this galaxy, it would be a challenge to explain why it has such a flat spectrum compared to other ORCs associated with galaxies.
If Kýklos is not extragalactic, then it must be in our Milky Way, which suggests a stellar origin. Supernova The remnants are often round nebula structures created by the shock wave of an exploding star colliding with the gas and dust in the interstellar medium.
However, supernova remnants usually also produce X-rays, and no X-rays have been detected from Kýklos so far. Although several Pulsarsthat rotate Neutron stars For example, some supernovae have been discovered inside the ring. From our point of view, we have no distance information about them that could tell us whether they are connected to the ring or whether their position is just a coincidence in the line of sight.
Maybe Kýklos is a planet Fogthe discarded shell of a dying Sun-like star. A planetary nebula usually becomes about 3 light-years wide before it dissipates; if Kýklos were a planetary nebula, it would have to be unusually large or quite close to us to appear 80 arcseconds wide in the sky. (For comparison, perhaps the most famous planetary nebula is the Ring Nebula in the constellation Lyra, which is 230 arcseconds wide and 2,200 light-years away.) However, a planetary nebula produces optical emissions, particularly in hydrogen-alpha light, but no such light has been detected.
Instead, the ring may have been created by a massive, unstable star, specifically a Wolf-Rayet star. Because a Wolf-Rayet star is so massive, it can generate a powerful radiation wind that blows its outer layers deep into space, decreasing its mass and making it more stable. Wolf-Rayet stars are often identified either by the nebula they expel or by their composition – as their hydrogen shell is blown away, the heavier elements like helium and oxygen in their deeper layers are exposed.
Although no Wolf-Rayet nebula is seen within Kýklos, this is not a problem because the fast radiation wind from a Wolf-Rayet star quickly destroys the dust nebula in its vicinity. Colder dust further out would survive and radiate at 24 micrometers, but Bordiu’s team points out that there is no data available at this wavelength to say whether or not this colder dust is present. However, there is other evidence that supports the Wolf-Rayet star scenario, namely the flat spectrum that is very typical of this type of mass-shedding star.
“Based on the limited data currently available, the morphological and spectral properties of Kýklos appear to be more consistent with those of a Wolf-Rayet shell,” Bordiu’s team wrote in its research report.
It is true that no Wolf-Rayet star has yet been identified inside Kyklos. While there is a bright star, HD 164455, it is not a Wolf-Rayet star. Gaia has identified three other stellar candidates, including a bluish star 24,500 light-years away. If this is the source of Kyklos, then the ring would be about 10 light-years across at that distance. However, there is no spectral data on this star to say whether it is a Wolf-Rayet star or not.
The next step will therefore be to continue the James Webb Space Telescope and to obtain this data in order to possibly solve the mystery of Kýklos.
The results have been accepted for publication in the journal Astronomy & Astrophysics, and a form Version is available on arXiv.
