Violent explosions can occur when white dwarfs – remnants of sun-like stars – suck material from a companion. In most cases, the fusion reaction briefly ignites the rest of the star. But now astronomers have discovered a completely new kind of such a thermonuclear explosion. In these “micron” strong magnetic fields concentrate the gas sucked off by the companion star in a very small space, thus creating the conditions for a locally limited fusion reaction lasting only a few hours. Scientists have already observed such microns in three white dwarfs.
White dwarfs are the remnants of low-mass stars, such as our sun, as they eject their outer layers at the end of their life cycle. Since nuclear fusion hardly takes place in the star’s core, it gives way under the pressure of gravity and condenses into an extremely compressed object. A white dwarf is therefore only the size of a planet, but can become as heavy as the sun. When such a white dwarf is part of a tight binary system, its gravity can suck gas from its companion star, effectively breathing new life into the stellar remnant. When the hydrogen heats up and thickens sufficiently on the surface of the white dwarf, an explosive nuclear fusion immediately takes place – a new one appears. This thermonuclear chain reaction covers the entire surface of the star’s remnant and illuminates it so brightly for days to weeks it can outshine any surrounding stars.
The twinkling remains of the stars
Now, astronomers led by Simone Scaringi of Durham University in the UK have discovered another new type of explosion in white dwarfs. In their research, they examined a phenomenon that astronomers first noticed in the TV Columbae white dwarf, about 1,630 light-years away. This remnant of the star shows sudden, brief flashes of light again and again at irregular intervals. “During these outbursts, the optical and infrared brightness triples in less than an hour and then falls back down in about ten hours,” reports Scaringi and his team.
To study this phenomenon, astronomers took a closer look at TV Columbae and two other white dwarfs, EI Ursa Majoris and ASASSN-19bh, with similarly short bursts of brightness using NASA’s Transiting Exoplanet Survey Satellite (TESS). They also observed a third white dwarf with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. “With the Very Large Telescope, we were able to determine that all these optical flashes were produced by white dwarfs,” says co-author Nathalie Degenaar of the University of Amsterdam. The observations also showed that all outbursts were accompanied by rapid outflows of matter: matter shot from the surface of the white dwarf into space at a speed of about 3,500 kilometers per second.
Localized thermonuclear explosion
From the observed characteristics of the flares, astronomers conclude that they cannot be classic new ones – the brightness peaks are too short, too faint and too violent. The explosions in the gas accretion disk around the white dwarf, the so-called dwarf novae, also do not match the timing of the outbursts, some of which occur very close to each other, the team explains. Instead, everything points to a local thermonuclear explosion. “Given the short duration and the energies released, this thermonuclear reaction must be confined to a small area of the star’s surface, burning only a limited amount of material,” write the Scaringi and his colleagues.
Based on these observations and the complementary model, the astronomers conclude that the explosion must be of a new type. “For the first time we discovered and identified what we call a micron phenomenon,” explains Scaringi. The name refers to the fact that these explosions are only about a millionth new intensity. However, the energy released is still huge: one of these eruptions can burn some 20,000 trillion tons of matter, the equivalent of the 3.5 billion mass of the Cheops pyramids. “This event challenges our understanding of how fusion blasts occur in stars,” says Scaringi. Because normally such chain reactions spread very quickly over the entire surface of the white dwarf.
This raises the question of why and how the micron remains confined to only a small fraction of the surface. Astronomers assume that the strong magnetic field of the three white dwarfs plays a decisive role. The magnetic field lines can visibly form a sort 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 become trapped so that fusion only occurs at these magnetic poles,’ explains co-author Paul Groot of Radboud University in the Netherlands. The team therefore suspects that such microns are much more common in space than previously observed. “These events can actually be quite common, but because they happen so quickly, it’s difficult to observe them,” says Scaringi. The team would now like to know more about such events.
Source: Simone Scaringi (Durham University, UKL) et al., Nature, doi: 10.1038 / s41586-022-04495-6