Breakthrough in centuries-old effort to unravel the astronomical mystery

An international team of astronomers using the GNIRS instrument from Gemini North have found that CK Vulpeculae, first seen as a bright new star in 1670, is about five times farther away than previously thought. This makes CK Vulpeculae’s 1670 explosion much more energetic than previously estimated and places it in a mysterious class of objects that are too bright to be members of the well-known type of explosions known as de novae, but too weak to be supernovae.

350 years ago, the French monk Anthelme Voituret saw a new shining star come to life in the constellation Vulpecula. Over the next several months, the star grew almost as bright as Polaris (the North Star) and was watched by some of the leading astronomers of the day before it disappeared from view after a year.^[1] The new star eventually earned the name CK Vulpeculae and has long been considered the first documented example of a nova – a fleeting astronomical event resulting from an explosion in a nearby binary star system in which a limb is a white dwarf, the rest of a star similar to the sun. However, a series of recent results have cast doubt on the long-held classification of CK Vulpeculae as nova.

In 2015, a team of astronomers suggested that the appearance of CK Vulpeculae in 1670 was the result of two normal stars experiencing a cataclysmic collision. A little over three years later, the same astronomers further proposed that one of the stars was in fact a swollen red giant star, following their discovery of a radioactive isotope of aluminum in the immediate vicinity. from the site of the 1670 explosion. To complicate the picture even further, a separate group of astronomers have come up with a different interpretation. In their article, also published in 2018, they suggested that the sudden brightening in 1670 was the result of the fusion between a brown dwarf – a failed star too small to glow via the thermonuclear fusion that powers the Sun – and a white dwarf. .

Now, adding to the mystery surrounding CK Vulpeculae, new observations from the Gemini International Observatory, a program of NSF’s NOIRLab, reveal that this enigmatic astronomical object is much more distant and has ejected gas at much faster speeds. higher than previously reported.

This team, led by Dipankar Banerjee from Physical Research Laboratory Ahmedabad, India, Tom Geballe from Gemini Observatory and Nye Evans from Keele University in the UK, originally planned to use the Gemini Near-Infrared Spectrograph (GNIRS) instrument on Gemini North on Maunakea of ​​Hawaii to confirm the detection in 2018 of radioactive aluminum in the heart of CK Vulpeculae.^[2] After realizing that detecting this in the infrared would be much more difficult than they originally thought, astronomers improvised and obtained infrared observations over the full extent of the CK vulpecules, including the two volutes of cloudiness at its outermost edges.

“The key to our discovery was the GNIRS measurements obtained on the outer edges of the nebula,” Geballe explained. “The signature of the red-shifted and blue-shifted iron atoms detected there shows that the nebula is growing much faster than previous observations suggested.^[3]

As lead author and astronomer Banerjee explains, “We didn’t think this was what we would find. It was exciting when we found gas traveling at the surprisingly high speed of around 7 million km / hour. This hinted at a different story of CK Vulpeculae than what had been theorized.

By measuring both the speed of the nebula’s expansion and the momentum of the outermost wicks over the past ten years, and taking into account the tilt of the nebula in the night sky, which had been estimated earlier by others, the team determined that CK Vulpeculae is about 10,000 light years from the Sun – about five times farther away than previously thought. This implies that the explosion of 1670 was much brighter, releasing about 25 times more energy than previously estimated.^[4] This much larger estimate of the amount of energy released means that whatever event caused the sudden appearance of CK Vulpeculae in 1670 was much more violent than a simple nova.

“In terms of the energy released, our discovery places CK Vulpeculae roughly halfway between a nova and a supernova,” Evans commented. “It is one of the rare objects of this type in the Milky Way and the cause – or causes – of 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.

The visual appearance of the CK Vulpeculae nebula and the high speeds the team observed could help astronomers recognize relics of similar events – in our Milky Way or in external galaxies – that have occurred in the past.

“It is difficult at this point to offer a definitive or convincing explanation for the origin of the 1670 eruption of CK Vulpeculae,” Banerjee concluded. “Even 350 years after the discovery of Voituret, the nature of the explosion remains a mystery. “

Notes

[1] 17th-century astronomers who observed the bright new star CK Vulpeculae included prominent Polish mayor, brewer, and astronomer Johannes Hevelius and Franco-Italian astronomer Giovanni Domenico. Cassini, who discovered four of the Saturnthe moons. After it disappeared from sight in 1671, there were numerous unsuccessful attempts over the centuries to recover it, some by renowned astronomers such as Halley, Pickering and Humason.

[2] A spectrograph is an instrument that divides the light of an astronomical object into its wavelengths, which makes it possible to measure the composition of the gas emitting the light, its speed, and other traits.

[3] Just as the height of an ambulance siren changes depending on whether the vehicle is approaching or moving away from you, astronomical objects change color depending on whether they are approaching or moving away from an observer. Objects moving away from Earth become redder (called redshift) and objects approaching become more blue (called blueshift).

[4] The brightness of an object is inversely proportional to the square of the distance from an observer. In the case of CK Vulpeculae, if the explosion of 1670 occurred five times further, it must have been 5² = 25 times brighter.

More information

This research is presented in the article Near infrared spectroscopy of CK vulpecules: revealing a remarkably powerful explosion from the past appear in the Letters from the Astrophysical Journal.

Reference: “Infrared spectroscopy of CK vulpecules: revealing a remarkably powerful explosion from the past” by DPK Banerjee (PRL, India), TR Geballe (Gemini Observatory), A. Evans (Keele University), M. Shahbandeh (Florida State University), CE Woodward, RD Gehrz (Minnesota Institute for Astrophysics), SPS Eyres (University of South Wales), S. Starrfield (Arizona State University), A. Zijlstra (University of Manchester), accepted, Letters from the Astrophysical Journal.
arXiv: 2011.02939

The team consisted of DPK Banerjee (Division of Astronomy and Astrophysics, Ahmedabad Physical Research Laboratory), TR Geballe (NSF Gemini / NOIRLab Observatory), A. Evans (Lennard Jones Laboratories, Keele University) , M. Shahbandeh (Department of Physics, Florida State University),

CE Woodward (Minnesota Institute for Astrophysics, University of Minnesota), RD Gehrz (Minnesota Institute for Astrophysics, University of Minnesota), SPS Eyres (Faculty of Computing, Engineering, and Science, University of South Wales), S. Starrfield (School of Earth and Space Exploration, Arizona State University), and A. Zijlstra (Jodrell Bank Center for Astrophysics, University of Manchester).