A new type of thermonuclear stellar explosion

A team of astronomers, with the help of the European Southern Observatory very large telescope (THAT‘s VLT), have observed a new type of stellar explosion: a micronova. These outbursts occur on the surface of certain stars, and each one can burn up around 3.5 billion Great Pyramids of Giza worth of stellar material in just a few hours.

“We have discovered and identified for the first time what we call a micronova,” explains Simone Scaringi, an astronomer at the University of Durham in the United Kingdom who led the study on these explosions published today in Nature. “The phenomenon challenges our understanding of how thermonuclear explosions occur in stars. We thought we knew this, but this discovery proposes a totally new way to achieve them,” she adds.

Astronomers have discovered a new type of explosion that occurs in white dwarf stars in two-star systems. This video summarizes the discovery.

Micronovae are extremely powerful events, but they are small on astronomical scales; they are far less energetic than exploding stars known as novae, which astronomers have known about for centuries. Both types of explosions occur in white dwarfs, dead stars with a mass similar 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 causes hydrogen atoms to explosively fuse into helium. In novae, these thermonuclear explosions occur over the entire stellar surface. “Such 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.

Artist impression Micronova

This artist’s impression shows a two-star system where micronovae can occur. The blue disk swirling around the bright white dwarf in the image center is made of material, mostly hydrogen, stolen from its companion star. Towards the center of the disk, the white dwarf uses its strong magnetic fields to funnel hydrogen toward its poles. As material falls onto the star’s hot surface, it triggers a micronova explosion, contained by magnetic fields at one of the white dwarf’s poles. Credit: ESO/M. Kornmesser, L. Calçada

Micronovae are similar smaller-scale and faster explosions, lasting only a few hours. They occur in some white dwarfs with strong magnetic fields, which funnel material toward the star’s magnetic poles. “For the first time, we have 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 Paul Groot, an astronomer at Radboud University in the Netherlands and co-author of the study. .

“This leads to the explosion of microfusion bombs, which are about a millionth of 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 kg, or around 3.5 billion Great Pyramids of Giza, of material.[1]

Artist's impression Micronova (foreground)

This artist’s impression shows a two-star system, with a white dwarf (foreground) and a companion star (background), where micronovae can occur. The white dwarf steals materials from her companion, which is funneled toward her poles. When the material falls onto the hot surface of the white dwarf, it triggers a micronova explosion, contained in one of the star’s poles. Credit: Mark Garlick

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. In reality, these events can be quite common, but because they are so fast, it is difficult to capture them in action”, explains Scaringi.

The team first came across these mysterious microbursts by analyzing data from POTTransiting Exoplanet Survey Satellite (tess). “Looking at astronomical data collected by NASA’s TESS, we discovered something unusual: a bright flash of optical light that lasts a few hours. Looking further, we found several similar signals,” says Degenaar.

This video shows an animation of a micronova explosion. The blue disk swirling around the bright white dwarf in the image center is made of material, mostly hydrogen, stolen from its companion star. Towards the center of the disk, the white dwarf uses its strong magnetic fields to funnel hydrogen toward its poles. As material falls onto the star’s hot surface, it triggers a micronova explosion, contained by magnetic fields at one of the white dwarf’s poles. Credit: ESO/L. Calçada, M. Kornmesser

The team observed three micronovae with TESS: two were from known white dwarfs, but the third required further observations with the X-shooter instrument on ESO’s VLT to confirm its white dwarf status.

“With the help of ESO’s Very Large Telescope, 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.

This artist’s animation shows a two-star system where one of the components is a normal star and the other is a white dwarf, which appears surrounded by a disk of gas and dust. A white dwarf in a two-star system can steal material, mostly hydrogen, from its companion star if they are close enough. Credit: ESO/M. kornmesser

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 rapid 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.

Reference: “Localized Thermonuclear Outbursts of Cumulative Magnetic White Dwarfs” by S. Scaringi, PJ Groot, C. Knigge, AJ Bird, E. Breedt, DAH Buckley, Y. Cavecchi, N.D. Degenaar, D. de Martino, C. Done, M. Fratta, K. Iłkiewicz, E. Koerding, J.-P. Lasota, C. Littlefield, CF Manara, M. O’Brien, P. Szkody, and FX Timmes, April 20, 2022, Nature.
DOI: 10.1038/s41586-022-04495-6

grades

  1. We use trillions to mean a million million (1,000,000,000,000 or 1012) and trillions to mean billion (1,000,000,000 or 109). The weight of the Great Pyramid of Giza in Cairo, Egypt (also known as the Pyramid of Cheops or Cheops Pyramid) is about 5,900,000,000 kg.

More information

This research was featured in a paper titled “Localized Thermonuclear Outbursts of Cumulative Magnetic White Dwarfs” to appear in Nature. A follow-up letter, titled “Activation of Micronovae via Magnetically Confined Accretionary Fluxes in White Dwarf Accretion” has been accepted for publication at Royal Astronomical Society Monthly Notices.

the team in the Nature the article is composed by S. Scaringi (Centre for Extragalactic Astronomy, Department of Physics, University of Durham, UK) [CEA]), PJ Groot (Department of Astrophysics, Radboud University, N?megen, The Netherlands [IMAPP] and South African Astronomical Observatory, Cape Town, South Africa [SAAO] and Department of Astronomy, University of Cape Town, South Africa [Cape Town]), C. Knigge (School of Physics and Astronomy, University of Southampton, Southampton, UK [Southampton]), AJ Bird (Southampton), E. Breedt (Institute of Astronomy, University of Cambridge, UK), DAH Buckley (SAAO, Cape Town, Department of Physics, University of the Free State, Bloemfontein, South Africa), Y. Cavecchi (Institute of Astronomy, National Autonomous University of Mexico, Mexico City, Mexico), ND Degenaar (Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, the Netherlands), D. de Martino (INAF-Osservatorio Astronomico di Capodimonte, Naples , Italy), C. Done (CEA), M. Fratta (CEA), K. Ilkiewicz (CEA), E. Koerding (IMAPP), J.-P. Lasota (Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw , Poland and Institut d’Astrophysique de Paris, CNRS et Sorbonne Universités, Paris, France), C. Littlefield (Department of Physics, University of Notre Dame, USA and Department of Astronomy, University of WashingtonSeattle, United States [UW]), CF Manara (European Southern Observatory, Garching, Germany [ESO]), M. O’Brien (CEA), P. Szkody (UW), FX Timmes (School of Earth and Space Exploration, Arizona State University, Arizona, USA, Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements, UNITED STATES).

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