When white dwarfs – the remnants of sun-like stars – siphon material from a companion, violent explosions can result. Most of the time, a fusion reaction ignites briefly on the entire rest of the star. But now astronomers have discovered a whole new type of such thermonuclear explosion. In the case of these “micronovae”, 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. The researchers have now observed such micronovae in three white dwarfs.
White dwarfs are what remains of low-mass stars like our Sun after they have ejected their outer layers at the end of their life cycle. Because nuclear fusion hardly takes place in the core of a star, it gives way under the pressure of gravity and condenses into an extremely compressed object. A white dwarf is therefore only planet-sized, but can become as massive as the sun. When such a white dwarf is part of a tight binary, its gravity can siphon gas from its companion star, effectively breathing new life into the stellar remnant. When that hydrogen gets hot and dense enough on the white dwarf’s surface, explosive nuclear fusion sets in momentarily—a nova occurs. This thermonuclear chain reaction engulfs the entire surface of the stellar remnant and, for several days to weeks, lights it up so brightly that it can outshine all surrounding stars.
Flickering Star Remnants
Now astronomers led by Simone Scaringi from Durham University in Great Britain have discovered another new type of explosion in white dwarfs. In their study, they investigated a phenomenon that astronomers first noticed in the white dwarf TV Columbae, around 1,630 light-years away. This remnant of the star shows abrupt, short bursts of brightness again and again at irregular intervals. “During these outbursts, the optical and infrared brightness increases threefold in less than an hour and then decreases again in about ten hours,” report Scaringi and his team.
To investigate 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 the third white dwarf with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. “Using the Very Large Telescope, we were able to determine that all of these optical flashes were produced by white dwarfs,” says co-author Nathalie Degenaar from the University of Amsterdam. The observations also showed that all outbursts were accompanied by rapid outflows of material: The material shot from the surface of the white dwarf into space at around 3500 kilometers per second.
Localized thermonuclear explosion
From the observed characteristics of the outbursts, the astronomers conclude that they cannot be classic novae – the brightness peaks are too short, too weak and too abrupt. Outbursts in the gas accretion disk around the white dwarf, so-called dwarf novae, also do not match the timing of the outbursts, some of which follow each other very closely, as the team explains. Instead, everything points to a local thermonuclear explosion. “Given the short duration and energies released, this thermonuclear reaction must be confined to a small area of the stellar surface, burning only a limited amount of material,” Scaringi and his colleagues write.
Based on these observations and a supplementary model, the astronomers conclude that the explosion must be of a new type. “For the first time, we have discovered and identified a phenomenon that we call a micronova,” explains Scaringi. The name refers to the fact that these explosions are only about a millionth the intensity of a nova. However, the energy released is still enormous: A single one of these eruptions can burn around 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 Scaringi. Because normally such chain reactions spread very quickly over the entire surface of a white dwarf.
(Video: ESO)
This raises the question of why and how the micronova remains confined to just a small portion of the surface. The astronomers assume that the strong magnetic field of the three white dwarfs plays a decisive role. The magnetic field lines can apparently form a kind of cage for the extracted material and thus concentrate it in a small area of the surface. “The hydrogen fuel can be trapped at the base of the magnetic poles of some white dwarfs, 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 micronovae are much more common in the cosmos than previously observed. “These events can actually be quite common, but because they happen so quickly, observing them is difficult,” says Scaringi. The team would now like to find out more such events.
Source: Simone Scaringi (Durham University, UKL) et al., Nature, doi: 10.1038/s41586-022-04495-6