Discovered a new type of stellar explosion: micronovae

Discovered a new type of stellar explosion: micronovae

A team of astronomers and astronomers has detected a new type of stellar explosion: a micronova , explosions that take place over a few hours on the surface of white dwarfs. The discovery is published this week in the journal Nature.

“We have discovered and identified for the first time what we call a micronova,” explains Simone Scaringi , a researcher at Durham University (UK) who led the study, “a phenomenon that challenges our understanding of how thermonuclear explosions occur in stars; We thought we knew, but this discovery proposes a totally new way.”

Micronovae are extremely powerful events, but they are small on astronomical scales. They are much less energetic than exploding stars known as novae , a type of explosion astronomers have known about for centuries. Both types occur in white dwarfs, dead stars with a mass close to that of our Sun, but as small as Earth.

A white dwarf in a two-star system can steal material, mostly hydrogen, from its companion star if they are close enough.

As this gas falls onto the very hot surface of the white dwarf star, it triggers hydrogen atoms to explosively fuse into helium. In novae, these thermonuclear explosions occur across the entire stellar surface.

“Those detonations cause the entire surface of the white dwarf to burn and glow for several weeks,” explains co-author Nathalie Degenaar , an astronomer at the University of Amsterdam, the Netherlands.

Micronovae are similar explosions, smaller in scale and faster, lasting only several hours. They occur in some white dwarfs with strong magnetic fields, which funnel material toward the star’s magnetic poles.

Localized fusion of hydrogen

“For the first time, we have now seen that hydrogen fusion can also occur in a localized manner. Hydrogen fuel may be contained at the base of the magnetic poles of some white dwarfs, so fusion only occurs at these magnetic poles,” says co-author Paul Groot , an astronomer at Radboud University in the Netherlands.

“This causes microfusion bombs to explode , which have about one millionth the force of a nova explosion, hence the name micronova,” Groot continues. Although ‘micro’ may imply that these events are small, make no mistake: just one such burst can burn around 20,000,000 trillion kilograms, which is equivalent to about 3.5 billion Cheops pyramids of material.

These new micronovae challenge astronomers’ understanding of exploding stars and may be more abundant than previously thought. “It just shows how dynamic the universe is. These events can actually be quite common , but because they are so fast , they are difficult to detect in action,” explains Scaringi.

TESS satellite next to the Very Large Telescope

The team first found these mysterious microbursts by analyzing data from NASA’s Transiting Exoplanet Survey Satellite ( TESS ). “Looking at their data we discovered something unusual: a bright flash of optical light lasting a few hours, and looking further, we found several similar signals,” confirms Degenaar.

The team observed three micronovae with TESS: two were from known white dwarfs, but the third required further observations with the X-shooter instrument , installed on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) , to confirm its status. White dwarf.

“With the help of the VLT we discovered that all these optical flares were produced by white dwarfs,” says Degenaar. “This observation was crucial for interpreting our result and for the discovery of micronovae,” adds Scaringi.

The discovery of micronovae adds to the repertoire of known stellar explosions. The team now wants to capture more of these elusive events, which requires large-scale surveys and fast follow-up measurements. “The rapid response of telescopes such as the VLT or ESO’s New Technology Telescope and the set of available instruments will allow us to unravel in more detail what these mysterious micronovae are”, concludes Scaringi.

References:

S. Scaringi et al. “Localized thermonuclear bursts from accreting magnetic white dwarfs”. Nature , 2022

(The journal Monthly Notices of the Royal Astronomical Society  has accepted follow-up research on that work, titled Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs )

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