According to an international group of astronomers led by Dr Tomasz Kamiński of ESO and the Max Planck Institute for Radio Astronomy in Bonn, Germany, CK Vulpeculae – a nova that erupted 345 years ago – was not an ordinary nova, but a much rarer event called luminous red nova.
This image shows the remnant of CK Vulpeculae; the image was created from a combination of visible-light images from the Gemini telescope, blue, a submillimeter map showing the dust from the SMA, green, and finally a map of the molecular emission from APEX and the SMA, red. Image credit: ESO / T. Kamiński.
CK Vulpeculae, also often referred to as Nova Vulpeculae 1670 and Nova Vul 1670, lies within the boundaries of the constellation Vulpecula, just across the border from Cygnus.
It was first observed by Pere Dom Voiture Anthelme and was discovered independently by Johannes Hevelius in June/July 1670.
The observed eruption lasted for two years, during 1670-1672. It was discovered at 3rd magnitude, and faded over 100 days to below visual limits.
The following year it was recovered during rebrightening, reaching magnitude 2.6, about 300 days after the first maximum. It again faded on the same time scale of 100 days to below observational limits.
Although well documented for its time, the intrepid astronomers of the day lacked the equipment needed to solve the riddle of the apparent nova’s peculiar performance.
“For many years this object was thought to be a nova, but the more it was studied the less it looked like an ordinary nova – or indeed any other kind of exploding star,” said Dr Kamiński, who is the lead author of the paper published online in the journal Nature.
During the 20th century, astronomers came to understand that most novae could be explained by the runaway explosive behavior of close binary stars. But CK Vulpeculae did not fit this model well at all and remained a mystery.
Even with ever-increasing telescopic power, the event was believed for a long time to have left no trace, and it was not until the 1980s that astronomers detected a faint nebula surrounding the suspected location of what was left of the star.
While these observations offered a tantalizing link to the sighting of 1670, they failed to shed any new light on the true nature of the event.
“We have now probed the area with submillimeter and radio wavelengths. We have found that the surroundings of the remnant are bathed in a cool gas rich in molecules, with a very unusual chemical composition,” Dr Kamiński said.
Dr Kamiński and his colleagues used three ground-based telescopes – the Atacama Pathfinder Experiment telescope (APEX), the Submillimeter Array (SMA), and the Effelsberg radio telescope – to discover the chemical composition and measure the ratios of different isotopes in the gas. Together, this created an extremely detailed account of the makeup of the area, which allowed an evaluation of where this material might have come from.
What the astronomers discovered was that the mass of the cool material was too great to be the product of a nova explosion, and in addition the isotope ratios the team measured around CK Vulpeculae were different to those expected from a nova.
But if it wasn’t a nova, then what was it? The answer is the luminous red nova – a spectacular collision between two stars, more brilliant than a nova, but less so than a supernova.
These are a very rare events in which stars explode due to a merger with another star, spewing material from the stellar interiors into space, eventually leaving behind only a faint remnant embedded in a cool environment, rich in molecules and dust.
This recently recognized class of eruptive stars fits the profile of CK Vulpeculae almost exactly.
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T. Kamiński et al. Nuclear ashes and outflow in the oldest known eruptive star Nova Vul 1670. Nature, published online March 23, 2015;
