When the white dwarf meets the brown dwarf, pow! | Space

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In the 1600s, Western astronomers were barely born of centuries of medieval thought, while the sky was supposed to remain unchangeable. You can imagine their astonishment when – in July 1670, in what was an empty, dark sky – some observers witnessed a swarm of bright light that appeared, faded, reappeared, and then completely disappeared from view. At that time, astronomers called such an event nova or new star. This one was located in front of the constellation Cygnus the Swan and therefore received the name Nova sub Capite Cygni (a new star under the head of the swan). Modern astronomers have learned that it was not a new star. As announced in 2015, it was not even a spectacular collision between two stars in the main sequence. Instead of using data from the ALMA telescope in Chile, astronomers now think that it was a collision between an aging white dwarf star and a brown dwarf (star with too little mass to trigger thermonuclear fusion and thus shine as most stars do).

The object in question is now called CK Vulpeculae.

The new work is based on observations made with Atacama's large millimeter / submillimeter Array (ALMA) system in northern Chile. Astronomers have studied the debris of this explosion, which came in the form of two rings of dust and hourglass-like gas with a compact central object (see image above). Sumner Starrfield of Arizona State University is co-author of an article published in a peer-reviewed journal Monthly Notices from the Royal Astronomical Society. He said in a statement:

It now seems that what was observed centuries ago is not what we would describe today as a classical "nova". It was rather the fusion of two stellar objects, a white dwarf and a brown dwarf. When these two objects met, they created a cocktail of unusual molecules and isotopes, which allowed us to better understand the nature of this object.

How did they conclude that the collision occurred between a white dwarf and a brown dwarf and not two stars in the main sequence, as previously reported? They did it by studying the light of two more distant stars, shining behind the dusty remains of the melting. These studies allowed astronomers to detect the revealing signature of the lithium element, which is easily destroyed inside a star of the main sequence, but not inside of it. 39, a brown dwarf. The astronomer Stewart Eyres of the University of South Wales (@astrostewey on Twitter) and the main author of the document explained:

The presence of lithium, as well as unusual isotopic ratios of carbon, nitrogen and oxygen elements suggest a material from a brown dwarf star deposited on the surface of a white dwarf. The thermonuclear "burning" and the eruption of this material gave the hourglass that we see today.

According to the researchers, the white dwarf would have been about 10 times more massive than the brown dwarf, although of much smaller size. When the brown dwarf made an inner spiral, the intense tidal forces exerted by the white dwarf would have torn him apart. Starrfield remarked:

This is the first time that such an event is conclusively identified.

Conclusion: In 1670, observers saw a nova, a star that did not appear anywhere before. Today's astronomers have learned that it was a collision between an old white dwarf star and a less massive brown dwarf.

Source: ALMA reveals the consequences of a merger between white dwarves and brown dwarves at CK Vulpeculae

Via NRAO