Astronomers have discovered a new type of stellar explosion of a white dwarf – micron. Material extracted from the companion star triggers nuclear fusion and causes a thermonuclear explosion. However, unlike classic novae, this chain reaction is locally limited by strong magnetic fields and therefore cannot propagate across the entire surface of the star, the team reports in the journal Nature.
When sun-like stars reach the end of their life cycle, they become white dwarfs – burnt out stellar remnants that gradually cool down. However, if a white dwarf is part of a binary star system, it can become a cannibal and suck material out of its companion. This often leads to explosions in the accretion disk of “stolen” gas, the so-called. dwarf novae. But material can also be pulled into the remnant of a star and temporarily sparked nuclear hydrogen fusion there. new star.
When a white dwarf sucks material from its companion star, the consequences can be explosive. © ESO/ M. Kornmesser, L. Calzada
Mysterious flashes
However, astronomers have long observed flashes of brightness in some white dwarfs that do not seem to fit into any of these categories. “During these flares, the optical and infrared brightness triples in less than an hour and then decreases again in about ten hours,” said Simone Scaringhi of Durham University in the UK and his team.
The strange thing is, the bursts of brightness are too short, too faint, and too sharp to come from a classical nova. This is because these thermonuclear explosions typically last from a few days to weeks. For a dwarf nova, eruptions are too uneven, sometimes several such eruptions follow each other at short intervals.
A brand new form of explosion
To understand this phenomenon, Scaringi and his colleagues studied three white dwarfs with such short bursts of brightness. They observed the white dwarf TV Columbae at about 1630 light-years away and its cousins EI Ursa Majoris and ASASSN-19bh with NASA’s TESS Space Telescope and the European Southern Observatory (ESO) Very Large Telescope in Chile.
Observations have confirmed that the bursts of brightness come from white dwarfs and do not correspond to any of the previously known types of explosions. According to astronomers, this should be a completely new form of thermonuclear explosion. “For the first time, we have discovered and identified the phenomenon that we call the micronova,” explains Skaringi.
Micronova: local, not global
According to astronomers, such a micronova is also based on a thermonuclear explosion and, therefore, on the short-term ignition of nuclear fusion. However, this explosive chain reaction is only about a millionth of the intensity of the new star. However, the energy released is still huge: one of these eruptions can burn about 20,000 trillion tons of material, which corresponds to the mass of 3.5 billion Cheops pyramids.
“This event challenges our understanding of how thermonuclear explosions occur in stars,” says Skaringi. Because usually such chain reactions quickly spread over the entire surface of a white dwarf. However, this does not seem to be the case for the micron: “Given the short duration and the energy released, this thermonuclear reaction should be limited to a small area of the stellar surface and burn only a limited amount of material.” write Scaringhi and his colleagues.
Chain reaction in a magnetic cage
This raises the question of why and how the micron remains confined to only a small portion of the surface. Astronomers suspect that magnetic fields play a decisive role. Unlike many other white dwarfs, the three micron candidates have strong magnetic fields. Its lines of force, apparently, can form a kind of cage for the extracted material and thus concentrate it on a small area of the surface.
“At the base of the magnetic poles of some white dwarfs, hydrogen fuel can be trapped so that fusion only occurs at those magnetic poles,” explains co-author Paul Groot of Radboud University in the Netherlands. Because the densities and temperatures required for fusion can only be reached in this small “cage”, the fusion chain reaction remains locally limited.
A new type of star formation.© ESO
Not an isolated case
According to astronomers, this may mean that such micron ones are much more common than previously observed. Flashes of brightness of unknown origin have also been observed in some other white dwarfs. “These events can actually be quite frequent, but because they happen so quickly, it’s difficult to observe them,” says Skaringi.
According to the complementary model, microns arise mainly in rather massive white dwarfs with a strong magnetic field. Only they can concentrate enough material in a small space to ignite local fusion. “At the same time, thermonuclear reactions at such objects occur at shallower depths, which allows more radiation to escape,” Scaringi and his colleagues explain. This makes breakouts easier to see.
Astronomers now want to search more microns to learn more about the exact mechanisms and processes of these new types of explosions. (Nature, 2022; doi:10.1038/s41586-022-04495-6)
Source: Durham University, European Southern Observatory (ESO).
April 21, 2022
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