A new form of stellar explosion may aid our understanding of thermonuclear outbursts on dead stars.
Micronovae are novel phenomena that occur on the surface of white dwarf stars that are actively sucking material from a nearby binary partner. The micronova is caused by the buildup of material on the white dwarf, which results in a localized thermonuclear burst.
According to scientists, these explosions have been witnessed to burn up tens to hundreds of quintillions of kilos of star material in hours.
If it's hard to believe, the experts estimate that there are several billion Great Pyramids of Giza. Or, to put it another way, about a thousandth of the mass of the Moon.
"We have discovered and recognized what we are calling a micronova for the first time," explains Durham University astronomer Simone Scaringi.
"The occurrence casts doubt on our knowledge of how stars undergo thermonuclear explosions. We assumed we already understood, but this revelation suggests a completely different approach to achieving them."
Close binary systems of white dwarfs can act as thermonuclear explosion machines. A white dwarf is a 'dead' star, or the collapsed core that remains when a main sequence star runs out of fuel and ejects its outer material. Neutron stars and black holes are examples of this type of star in various mass classes.
This core has collapsed and is quite dense. White dwarf stars have up to 1.4 times the mass of the Sun and are compressed into a sphere the size of the Earth. In binary systems, several of these may be discovered.
The binary systems are near enough in certain rare circumstances – around ten have been discovered in the Milky Way – that the white dwarf removes material from the partner, resulting in a recurring nova.
The smaller, denser, and more massive white dwarf siphons material – principally hydrogen – from the companion as the two stars spiral around one other. This hydrogen condenses on the surface of the white dwarf, where it warms up.
Periodically, the mass accumulates to the point that the pressure and temperature at the layer's bottom are high enough to cause a thermonuclear explosion, ejecting the extra material into space forcefully. That's where the nova comes in.
Scaringi and his colleagues discovered that a micronova is a tiny form of this explosion.
In data from the TESS exoplanet-hunting telescope, the researchers discovered a white dwarf releasing a micronova for the first time. TESS is designed to detect extremely slight brightness fluctuations in stars with orbiting exoplanets, when the exoplanet passes in front of the star and produces a little dimming.
Micronovae were detected in TESS data when the researchers noticed a short flash of light from a white dwarf star rather than a fading. This sparked a hunt for comparable occurrences in other white dwarfs. They discovered three bursts in all, the third of which led to the identification of a previously undiscovered white dwarf star after more studies.
The flashes, however, were too little to constitute a nova, which are far more powerful and remain much longer. As a result, the researchers set out to establish a scenario that may explain the findings. Micronovae were discovered to be the most plausible reason.
In a near binary, a white dwarf with a strong magnetic field can suck material from its partner. This material is channeled to the white dwarf's poles by the magnetic field, where it accumulates to generate an outburst comparable to (though smaller in magnitude than) a normal white dwarf nova.
"For the first time," says astronomer Paul Groot of Radboud University in the Netherlands, "we have now observed that hydrogen fusion may also happen in a confined form."
"Some white dwarfs' hydrogen fuel can be trapped at the base of their magnetic poles, allowing fusion to occur exclusively there. This results in the detonation of micro-fusion bombs, which are one millionth the intensity of a nova explosion, thus the term micronova."
The discovery might help to answer a long-standing enigma. Over the last 40 years or so, one of the white dwarfs in the binary system TV Columbae has been spotted producing comparable flashes. Over the years, similar bursts have been observed on additional highly magnetic white dwarfs. This explanation could be the key to figuring out why.
The discovery shows that the bursts are relatively common, but astronomers will need to collect additional data to have a better understanding of them.
Scaringi says, "It only goes to demonstrate how dynamic the Universe is." "These occurrences may be extremely common, yet they are difficult to catch in action due to their speed."
