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Nova Vul of 1670 Was Not a Nova, But Rather Two Colliding Stars

03/23/2015 03:15PM

Nova Vul of 1670 Was Not a Nova, But Rather Two Colliding Stars
Discoveries made with the APEX telescope reveal that the "new" star that European astronomers saw appear in the sky in 1670 was not a nova, but rather a much rarer stellar collision. It was spectacular enough to be easily seen with the naked eye during its first outburst, but the traces it left were so faint that very careful analyses using modern sub-millimeter telescopes were needed to finally unravel the mystery more than 340 years later. A team led by scientists from the Max Planck Institut fur Radioastronomie in Bonn, Germany found emission from a fascinating variety of molecules that provided the required tell-tale evidence.

Some of the greatest astronomers of the 1600s, including Hevelius - the father of lunar cartography - and Cassini, carefully documented the appearance of a new star in the skies in 1670. Hevelius described it as "Nova sub capite Cygni," a new star below the head of the Swan. But astronomers now know it by the name Nova Vul 1670.

This object lies within the boundaries of the modern constellation of Vulpecula (The Fox), just across the border from Cygnus (The Swan) and is also referred to as CK Vulpeculae (its designation as a variable star). Historical accounts of novae are rare and of great interest to modern astronomers.

Tomasz Kaminski (at the time of the observations at the Max Planck Institut fur Radioastronomie and now at ESO in Chile) explains: "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."

When it first appeared, Nova Vul 1670 was easily visible with the naked eye and varied in brightness significantly over the course of two years. It then disappeared and reappeared twice before vanishing for good. 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.

During the twentieth century, astronomers came to understand that most novae could be explained as the runaway explosive behavior of close binary stars. But Nova Vul 1670 did not fit this model at all and remained a mystery.

Even with ever-increasing telescopic power, for a long time the event was believed to have left no trace, and it was not until the 1980s that a team of 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 witnessed over the skies of Europe over three hundred years ago.

Tomasz Kaminski continues the story: "We have now probed the area in the sub-millimeter 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." Among others, the team discovered molecules of CO, CN, HCN, HNC, NH3, SiO, the ionized molecules N2H+, HCO+, and even the organic H2CO (formaldehyde).

In addition to APEX, the team also used 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 team discovered was that the mass of the cool material was too great to be the product of a nova explosion. In addition, the isotope ratios the team measured around Nova Vul 1670 were different to those expected to be produced by a nova. But if it wasn't a nova, then what was it?

The answer is a rare and spectacular collision between two stars -- more brilliant than a nova, but less so than a supernova -- which produces a so-called red transient. These are very rare events in which a star explodes due to a merger with another star, spewing nuclear material into space and eventually leaving behind only a faint remnant embedded in a cool environment that is rich in molecules and dust. This newly recognized class of eruptive stars fits the profile of Nova Vul 1670 almost exactly.

Karl Menten of the Max Planck Institut fur Radioastronomie and the principal investigator of APEX concludes: "This kind of discovery is the most fun -- Something that is completely unexpected!"

The Atacama Pathfinder Experiment (APEX) is a collaboration between the Max Planck Institut fur Radioastronomie (MPIfR), the Onsala Space Observatory (OSO), and the European Southern Observatory (ESO). APEX operates a modified prototype antenna of ALMA (Atacama Large Millimetre Array) as a single dish on the Chajnantor plateau at an altitude of 5100 meters above sea level (in the Atacama Desert, Chile). The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. The operation of the telescope is entrusted to ESO.

The 100m Radio Telescope Effelsberg is one of the largest fully steerable radio telescopes on earth. It is operated by the Max Planck Institut fur Radioastronomie in Bonn, Germany and is located in a valley approximately 40 km southwest of Bonn. It was used by the team to observe a number of radio transitions of different molecules, namely SiO, OH, H2O (water), and NH3 (ammonia).

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