APEX and other telescopes observed the remnants of the 1670 stellar collision that was seen easily with the naked eye during its first eruption, according to a press release from the European Southern Observatory (ESO), but is now so faint submillimeter telescopes were needed for observations 340 years later.
The results of the newest observations appear online in the journal Nature on March 23.
The initial outburst was documented by the 17th century's astronomy greats, including the father of lunar cartography, Hevelius, and Cassini. 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 Vulpeculae 1670. Nova were not a commonly documented occurrence in history. Nova Vul 1670 is now considered the oldest recorded nova and the faintest nova when later rediscovered.
"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 study lead author Tomasz Kaminski, now with ESO, of Max Planck Institute for Radio Astronomy (MPIfR) at the time of discovery, according to MPIfR's press release.
At first, Nova Vul 1670 was easily visible and its brightness varied over the following two years. Despite the meticulous documentation, early astronomers did not have the equipment needed to unravel the mystery of the nova's performance.
Nova Vul 1690 did not follow the typical binary model of most novae. As telescopic power increased, researchers began to believe that no trace remained - until the 1980s.
"We have now probed the area with submillimeter and radio wavelengths," said Kaminski, according to the press releases. "We have found that the surroundings of the remnant are bathed in a cool gas rich in molecules, with a very unusual chemical composition." The team discovered the molecules CO, CN, HCN, HNC, NH3, SiO, the ionized molecules N2H+, HCO+ and the organic H2CO (formaldehyde).
APEX, Submillimeter Array (SMA) and the Effelsberg radio telescope were used to uncover the chemical composition and measure the ratios of the various isotopes, but the team found that the mass of the cool material was too great to have come from a nova explosion.
What was it then?
"A spectacular collision between two stars, more brilliant than a nova, but less so than a supernova, which produces something called a red transient," according to the press releases. A red transient is a super rare event in which stars merge with another star, causing an explosion that throws stellar "guts" into space. The upheaval of star interiors leaves a cool, molecule and dust-rich environment. According to the press release, Nova Vul 1670 fits into the newly recognized class of explosive stars almost perfectly.
Co-author Karl Menten from Max Planck Institute for Radio Astronomy concluded: "This kind of discovery is the most fun: something that is completely unexpected!"
The rest of the team includes Romuald Tylenda (N. Copernicus Astronomical Center, Torun, Poland), Marcin Hajduk (N. Copernicus Astronomical Center), Nimesh A. Patel (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., USA) and Alexander Kraus (MPIfR).
