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In June 1670 – almost exactly 350 years ago – the French astronomer Voituret Anthelme recorded a “new” star in the sky. In the northern constellation of Vulpecula, a point of light flared up in brightness before gradually fading from the naked eye over a year later.
The event has been classified as a nova, a cataclysmic stellar event, and in recent years astronomers have been working to find out what caused it. But a new discovery threw a key in the work.
According to observations made with the Gemini Observatory, the star, CK Vulpeculae, is actually five times further away than astronomers thought. This means that the explosion was also much more powerful than astronomers thought, up to 25 times more energetic. And the cloud of ejected incandescent material surrounding it is also expanding into space at a much faster rate.
Since 2015, astronomers have been exploring the possibility that CK Vulpeculae nova was the result of the collision of two stars. The first test, led by ESO’s Tomasz Kamiński and the Max Planck Institute for Radio Astronomy in Germany, was the unusually high mass and chemical composition of the gas surrounding the star, which scientists found to be consistent with a cataclysmic meltdown. of two sequence stars.
Two follow-up studies emerged in 2018. Kamiński and his team found a radioactive isotope of aluminum that had long been researched in interstellar space, linking its production to stellar fusions.
And another team, led by astronomer Stewart Eyres of the University of New South Wales in Australia, examined the chemical makeup of the CK Vulpeculae nebula and found molecules that could not have survived a collision between two main sequence stars. This, they said, suggested a different event: a collision between a white dwarf star and a brown dwarf.
That aluminum isotope required more careful inspection, so an international team of astronomers, co-led by Dipankar Banerjee of the Physical Research Laboratory in India, Tom Geballe of the Gemini Observatory, and Nye Evans of Keele University in the UK used the spectroscope. in the near infrared of Gemini North to get a better look, imagining the entire nebula in infrared.
It was then that things started to look a little strange. In particular, the outer edges of the CK Vulpeculae nebula, a bi-lobed gaseous structure that explodes away from the faint radio source (the star, or what remains of it) inside. The iron atoms in these edges showed red and blue shifting, the apparent lengthening or shortening of the wavelengths of light as it travels away from or towards the viewer.
These changes were much more pronounced than expected, suggesting that the material cloud was expanding much faster than previous measurements.
“We didn’t suspect this was what we would find,” Banerjee said. “It was thrilling when we found some gas traveling at an unexpectedly high speed of about 7 million kilometers (4.3 million miles) per hour. This hinted at a different story about CK Vulpeculae than it had been. theorized “.
If the material is moving faster than thought, it means that the object should also be larger than we thought, so the team got to work. They carefully studied the nebula’s velocity and 10-year baseline expansion rate and position in the sky and determined that the object is approximately 10,000 light-years away.
It is much further away than previous calculations that placed it about 1,630 light-years away. And if the object is much farther away, it would have had to be much more energetic to produce visible light from Earth in 1670; about 25 times more energetic than previous estimates, according to the team’s calculations.
That’s more energy than a nova can produce. Which means he’s back at the drawing board to figure out what produced the explosion.
“In terms of the energy released, our discovery places CK Vulpeculae roughly halfway between a nova and a supernova,” Evans said. “It is one of the very few objects of this type in the Milky Way and the cause – or causes – of the explosions of this intermediate class of objects remain unknown. I think we all know what CK Vulpeculae is not, but no one knows what it is.”
These rare objects are known as intermediate brightness optical transients (ILOTs) and it is not clear what causes them. Several studies have suggested that they are produced by binary systems, where at least one of the stars is a giant, but this hypothesis has by no means been confirmed.
The researchers did not delve into the possible origins of CK Vulpeculae. This will require further study, but doing so could help solve the ILOT mystery.
“It is difficult at this stage to offer a definitive or convincing explanation for the origin of the 1670 eruption of CK Vulpeculae,” Banerjee said. “Even 350 years after Voituret’s discovery, the nature of the explosion remains a mystery.”
The research was accepted in The Astrophysical Journal Letters, and is available on arXiv.
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