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Blast from the Past — Observations led by the PRL scientist enables breakthrough in centuries-old effort to unravel astronomical mystery

An international team of astronomers led by PRL Scientist using Gemini North’s GNIRS instrument have discovered that CK Vulpeculae, first seen as a bright new star in 1670, is approximately five times farther away than previously thought. This makes the 1670 explosion of CK Vulpeculae much more energetic than previously estimated and puts it into a mysterious class of objects that are too bright to be members of the well-understood type of explosions known as novae, but too faint to be supernovae.

Chart published by the astronomer Hevelius in 1670 of the Cygnus (the Swan) constellation with CK Vul being the star labeled “a” near the bottom left of the figure. It was described as “a new star below the head of the Swan.”

Figure 1:  Chart published by the astronomer Hevelius in 1670 of the Cygnus (the Swan) constellation with CK Vul being the star labeled “a” near the bottom left of the figure. It was described as “a new star below the head of the Swan.”.

350 years ago, the French monk Anthelme Voituret saw a bright new star flare into life in the constellation of Vulpecula. Over the following months, the star became almost as bright as Polaris (The North Star) and was monitored by some of the leading astronomers of the day like Hevelius and Cassini before it faded from view after a year. The new star eventually gained the name CK Vulpeculae, and was long considered to be the first documented example of a nova — a fleeting astronomical event arising from an explosion in a close binary star system in which one member is a white dwarf, the remnant of a Sun-like star. However, a string of recent results have thrown the longstanding classification of CK Vulpeculae as a nova into doubt.

A classical nova eruption results from a thermonuclear reaction on the surface of a white dwarf (WD) that is accreting material from a companion star in a close binary system. The accreted hydrogen rich material gradually forms a layer on the WD’s surface with the mass of the layer growing with time. As the accreted matter is compressed and heated by the gravity of the WD, the critical temperature and pressure needed for the thermonuclear reactions to commence are reached in the degenerate material at the base of the accreted layer thereby starting the nova eruption. Observationally, the nova outburst is accompanied by a dramatic 10000-fold or more increase in the brightness of the star. Astronomers are familiar with and have basic understanding of the two most common types of violent stellar explosions, known as novae and supernovae. But the CK Vul explosion, considered for a long time to be a nova outburst, is one of the very few in our Milky Way Galaxy (Akash Ganga) that does not fit into either category.

In 2015, a team of astronomers suggested that CK Vulpeculae’s appearance in 1670 was the result of two normal stars undergoing a cataclysmic collision. Just over three years later, the same astronomers further proposed that one of the stars was in fact a bloated red giant star, following their discovery of a radioactive isotope of aluminium in the immediate surroundings of the site of the 1670 explosion. Complicating the picture even further, a separate group of astronomers proposed a different interpretation. In their paper, also published in 2018, they suggested that the sudden brightening in 1670 was the result of the merger between a brown dwarf — a failed star too small to shine via thermonuclear fusion that powers the Sun — and a white dwarf.

Now, adding to the ongoing mystery surrounding CK Vulpeculae, new observations from the 8.2m Gemini telescope reveal that this enigmatic astronomical object is much farther away and has ejected gas at much higher speeds than previously reported.

This international team, led by Dipankar Banerjee of the Physical Research Laboratory, Ahmedabad, initially planned to confirm the 2018 detection of radioactive aluminum at the heart of CK Vulpeculae. After realizing that detecting this in the infrared would be far more difficult than they originally thought, the astronomers improvised and obtained infrared observations across the full extent of CK Vulpeculae, including the two wisps of nebulosity at its outermost edges. The key to the discovery was the spectroscopic measurements obtained at the outer edges of the nebula. The signature of red-shifted and blue-shifted iron atoms detected there shows that the nebula is expanding much more rapidly than previous observations had suggested. The gas was found to be traveling at the unexpectedly high speed of about 7 million km/hour – hinting at a different story about CK Vulpeculae than what had been theorized.

By measuring both the speed of the nebula’s expansion and how much the outermost wisps had moved during the last ten years, and accounting for the tilt of the nebula on the night sky, which had been estimated earlier by others, the team determined that CK Vulpeculae lies approximately 10,000 light-years distant from the Sun — about five times as far away as previously thought. That implies that the 1670 explosion was far brighter, releasing roughly 25 times more energy than previously estimated. This much larger estimate of the amount of energy released means that whatever event caused the sudden appearance of CK Vulpeculae in 1670 was far more violent than a simple nova.

The faint hour-glass nebula around CK Vul imaged in the red light of Hydrogen H-alpha emission.  The central star cannot be seen either in the optical or in the infrared as it is enshrouded by dense layers of dust   [courtesy: Gemini Observatory/ NOIRLab/ NSF/ AURA]

Figure 2: The faint hour-glass nebula around CK Vul imaged in the red light of Hydrogen H-alpha emission.  The central star cannot be seen either in the optical or in the infrared as it is enshrouded by dense layers of dust   [courtesy: Gemini Observatory/ NOIRLab/ NSF/ AURA].

In terms of energy released, the finding places CK Vulpeculae roughly midway between a nova and a supernova. It is one of a very few such objects in the Milky Way and the cause — or causes — of the outbursts of this intermediate class of objects remain unknown. It would appear that we all know what CK Vulpeculae isn’t, but no one knows what it is. The visual appearance of the CK Vulpeculae nebula and the high velocities observed by the team could help astronomers to recognize relics of similar events — in our Milky Way or in external galaxies — that have occurred in the past.

Reference: “Near-Infrared Spectroscopy of CK Vulpeculae: Revealing a Remarkably Powerful Blast from the Past”  by D. P. K. BANERJEE, T. R. GEBALLE, A. EVANS,  M. SHAHBANDEH, C. E. WOODWARD, R. D. GEHRZ, S. P. S. EYRES, S. STARRFIELD,  AND A. ZIJLSTRA; 

Astrophysical Journal Letters, in press,  preprint available at  arXiv:2011.02939v1

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