August 18, 2016

White dwarf star Nova caught in the act which occurs every 10,000 to 1 million years

A team of researchers affiliated with the Warsaw University Observatory has captured for the first time the events that led to a classical nova exploding, the explosion itself and then what happened afterwards. In their paper published in the journal Nature, the team describes how they happened to capture the star activity and why they believe it may help bolster the theory of star hibernation.

In this type of binary system, a white dwarf sucks gas from a much bigger partner star until it blows up - about every 10,000 to one million years.

The consistent stream of images snapped for that project, the Optical Gravitational Lensing Experiment, allowed the researchers to go back and see what the star system looked like before the explosion brought it to their attention in May 2009.

Even though it is 20,000 light-years away - a terribly faint pinprick of light barely visible among brighter stars, even in magnified images - this was a rare opportunity to study the build-up and aftermath of a classical nova.

"Thanks to our long-term observations, we observed the nova a few years before and a few years after the explosion," Przemek Mróz, the study's first author and a PhD student at the Warsaw University Astronomical Observatory

"The distance between those two stars is very small - actually one solar radius," Mr Mróz said. "Imagine that inside the Sun, you have two stars that are orbiting each other."

So tight is their orbit, which in this case takes just five hours, that the dwarf steadily steals gas from its larger companion.

That extra matter builds up on the surface of the white dwarf until it kicks off a runaway, explosive thermonuclear reaction. Crucially, however, this blast only rips off the extra material; the white dwarf is left behind.

"The entire system survives the nova explosion... so the whole process starts again," said Mr Mróz. "After thousands of years, our nova will awake and explode again but no one will be able to see it."

Nature - The awakening of a classical nova from hibernation

Cataclysmic variable stars—novae, dwarf novae, and nova-likes—are close binary systems consisting of a white dwarf star (the primary) that is accreting matter from a low-mass companion star (the secondary). From time to time such systems undergo large-amplitude brightenings. The most spectacular eruptions, with a ten-thousandfold increase in brightness, occur in classical novae and are caused by a thermonuclear runaway on the surface of the white dwarf. Such eruptions are thought to recur on timescales of ten thousand to a million years. In between, the system’s properties depend primarily on the mass-transfer rate: if it is lower than a billionth of a solar mass per year, the accretion becomes unstable and the matter is dumped onto the white dwarf during quasi-periodic dwarf nova outbursts. The hibernation hypothesis predicts that nova eruptions strongly affect the mass-transfer rate in the binary, keeping it high for centuries after the event. Subsequently, the mass-transfer rate should significantly decrease for a thousand to a million years, starting the hibernation phase. After that the nova awakes again—with accretion returning to the pre-eruption level and leading to a new nova explosion. The hibernation model predicts cyclical evolution of cataclysmic variables through phases of high and low mass-transfer. The theory gained some support from the discovery of ancient nova shells around the dwarf novae Z Camelopardalis and AT Cancri, but direct evidence for considerable mass-transfer changes prior, during and after nova eruptions has not hitherto been found. Here we report long-term observations of the classical nova V1213 Cen (Nova Centauri 2009) covering its pre- and post-eruption phases and precisely documenting its evolution. Within the six years before the explosion, the system revealed dwarf nova outbursts indicative of a low mass-transfer rate. The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion.

SOURCES- Nature, BBC News

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