White dwarf start caught exploding out of hibernation

Artist's portrayal of a classical nova explosion
Artist’s portrayal of a classical nova explosion

K. Ulaczyk / Warsaw University Observator

White dwarf stars have been known to erupt into explosions known as classic nova. But a new study has shed light on the evolution of this phenomenon after a star was observed over a 12-year period.

Classical nova explosions attract attention during eruptions when they are bright and easy to observe. However, because of the unpredictable nature of knowing when an eruption is going to happen, very little is known about the pre-eruption behavior of novae.

“This study is the first case that the evolution of a classical nova can be investigated so precisely with long-term pre- and post-eruption observations,” Przemek Mróz, the study’s author, told WIRED.

A classical nova explosion occurs in a binary system, when two stars are orbiting each other. The distance between the components is usually smaller than one solar radius so the strong gravitational interaction distorts one star, (the “secondary” red dwarf star) which is transferring matter onto the second star, in this case the white dwarf star. The matter piles up on the white dwarf surface, and when its mass reaches a critical value, it ignites, triggering a thermonuclear runway.

One such star, known as Nova Centauri 2009, erupted in the sky in May 2009. At the time it had been monitored by the Optical Gravitational Lensing Experiment (OGLE) sky survey since 2003. Using the data from before the eruption, Mróz and his team from the Warsaw University Observatory, were able to find evidence of dwarf nova outbursts, or periodic brightening, in the six years leading up to the explosion. This implied that a low-transfer rate between the two stars was occurring, the process which is known to lead up to a classical nova explosion.

The team found that the classical nova eruption occurred within six days of the last outburst, which suggests that the matter dumped onto the white dwarf at the time triggered the runaway thermonuclear reaction that led to the explosion.

Upper panels: Snapshots of a nova lifecycle. Lower panel: The Milky Way over the Warsaw Telescope dome, Las Campanas Observatory
Upper panels: Snapshots of a nova lifecycle. Lower panel: The Milky Way over the Warsaw Telescope dome, Las Campanas Observatory

J. Skowron, K. Ulaczyk / Warsaw University Observatory

The results point to evidence for mass-transfer changes before, during and after nova eruptions. In addition, this supports the “nova hibernation hypothesis”.

“The hibernation hypothesis says that decades after the nova eruption, the mass-transfer rate in the binary is increased as a result of a nova eruption, possibly owing to the extreme irradiation of the secondary star. Subsequently, the mass-transfer should drop significantly or even cease. This state is called hibernation,” said Mróz.

Mróz predicts that in the next decades the mass-transfer rate in the white dwarf star should gradually decrease and the star will be slowly fading.

“The star will again transform into a dwarf nova, this may take a few years or even several decades, and possibly fall into hibernation for thousands of years, until it will awake again and explode as a classical nova,” said Mróz.

The study was published in Nature.

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17 August 2016 | 5:00 pm – Source: wired.co.uk


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