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Stars are not static. Not only are they the site of high-energy reactions, but during their long life most change in size and appearance, from a massive mass of gas to their turbulent end of life to their explosive death (the supernova) or by extinction. But a new phase has been discovered. For the first time, astronomers have discovered a micronova. Micronovas would result from an explosion on the surface of certain stars – which would be a million times less powerful than the explosions of classic novas and of smaller size, lasting only a few hours. Micronovas are therefore more difficult to detect, but could be very numerous. The mini-explosions from which they result will deepen knowledge about the life cycle of stars and how stellar flares occur.
Stars are born by gas condensation within nebulae. Those that are massive enough to initiate and sustain thermonuclear reactions then glow by fusing helium and hydrogen. The rest of their history is determined by their mass. Simply put, the larger a star, the shorter its life. This is how a star of the mass of the Sun, after about ten billion years, is transformed into a very unstable red giant, before losing a large part of its mass, then forming a white dwarf of low light.
In a few million years, the most massive stars become supergiants which implode into supernovae, the remnants of which generate a neutron star or one black hole. These extremely violent events are rare. With the exception of a supernova observed in 1987 in a galaxy dwarf near us, no supernova has occurred in our galaxy, to our knowledge, since the beginnings of modern astronomy.
However, before arriving at this final implosion, white dwarfs experience what are called classical novas. It is a stellar phase triggered by a powerful explosion that can occur in binary star systems — composed of two stars orbiting around a common center of gravity. In these systems, the more massive partner can “steal” matter, mainly hydrogen, from its companion star. We then speak of a cannibal star. When this gas falls on the very hot surface of the white dwarf star, it triggers the explosive fusion of hydrogen atoms into helium. In novas, these thermonuclear explosions occur over the entire stellar surface, but do not destroy it. Classical novas appear as flashes of intense light that can be detected on Earth using advanced telescopes; these flashes can persist for several weeks or even months.
Recently, a team of astronomers from Durham University, with the help of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), located in the Atacama Desert in northern Chile, has witnessed a phenomenon resembling a classic nova, but smaller. A shorter, more modest, more elusive explosion, which they named micronova. The discovery is described in the journal Nature.
Localized and brief explosions in the constellation of the Dove
The researchers, led by Simone Scaringi at Durham University’s Center for Extragalactic Astronomy, looked at data from NASA’s Transiting Exoplanet Survey Satellite, TESS. The latter is used to search for planets around other stars, by closely examining the light from those stars, dips in brightness potentially being caused by other planets passing in front.
Specifically, the team focused on the Gamma Columbae (γ Col/γ Columbae) binary system of the southern constellation of the Dove. This system is composed of a white dwarf and a companion star, visible to the naked eye as the 6th brightest star in the constellation.
It was then that they detected a much shorter and less intense flash coming from a binary system, which lasted only 10 hours before dying out. After this observation, the team detected two more similar flashes and a fourth from previous studies. Astronomers assumed they were much fainter versions of classic novas, but they had no way of explaining how or why. Scaringi, lead author, explains in a communicated : “ We couldn’t explain it until we finally made the connection that it might be thermonuclear explosions happening on accreting white dwarfs “.
By the way, scientists believe that micronovas and classical novas only occur in binary systems where the most massive cannibalistic star is a white dwarf. Scaringi says: In classical novas, the accreting white dwarf builds up a layer of fresh hydrogen that covers the entire star. Once this layer reaches high enough temperatures and pressures, the entire layer ignites “. However, computer models created by the researchers revealed that during micronovas, hydrogen accretion likely only occurs around the magnetic poles of the star.
Indeed, what can usually happen is that a white dwarf attracts enough material from its companion to grow. And when it reaches critical mass, it collapses in on itself, causing a thermonuclear supernova implosion. But this is not what happens in the Constellation of the Dove. The star accretes material from its companion well, but instead of continuing to accumulate mass until it exceeds the limit, it produces smaller explosions on its surface which the team has therefore dubbed “micronovas”. Scaringi points out: It was a real surprise, this type of explosions do not take place on white dwarfs, and it took us a whole year to understand what it was “.
A magnetic field that channels the explosions
This discovery raises questions as to why this white dwarf is not completely consumed. Limited accretion means that a micronova needs much less hydrogen to reach the temperature and pressure needed for detonation. This is why the explosions are much smaller than those of classic novas and last less time. The study’s researchers were initially puzzled as to why accreting white dwarfs that produce micronovae only collect hydrogen at their poles. But they now suspect that such accretion is determined by the strength of the stars’ magnetic fields.
According to the authors, this could be due to hydrogen fusion. On a classic supernova, the hydrogen comes from the companion: it is attracted to the white dwarf, then arriving on the extremely hot surface, the atoms are excited, fuse to form helium, and explode. But some white dwarfs act differently: the strength of their magnetic field channels the hydrogen at the magnetic poles, as if the atoms were passing through a funnel to come together at a single point. Thus, these areas are the only ones to be affected by the explosions. Instead of completely burning out, the star only undergoes a few “mini” thermonuclear explosions.
Indeed, Paul Groot, an astronomer at Radboud University in the Netherlands and co-author of the study, explains: For the first time we have now 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 “. Scaringi adds: We believe that the powerful magnetic field of the white dwarf keeps the accreted flow of matter confined to the magnetic poles and prevents this flow from spreading over the entire surface of the white dwarf. “. But to do this, the magnetic field must be extremely powerful, the authors estimate it between 1 and 10 million Gauss. Note that the Earth’s magnetic field is between 0.25 and 0.65 Gauss, and the most powerful observed to date is that of the sun, around 350 Gauss.
White dwarfs much more restless than expected?
However, most white dwarfs have magnetic fields much more limited than that necessary for these micro-explosions and therefore only produce classical novas. Despite these limitations, micronovas challenge astronomers’ understanding of stellar explosions, and may be more abundant than previously thought. Scaringi explains: This shows how dynamic the Universe is. These events can actually be quite common, but because they’re so fast-paced, they’re hard to capture in action. “.
Finally, even though these types of stellar explosions (micronova explosions) are about one millionth the power of a nova explosion, the term “micro” should not mislead us. These are not small-scale events: just one of these explosions can burn about 20,000,000 billion kg of material, or about 3.5 billion Great Pyramids of Giza, according to the authors.
The team now wants to capture more of these elusive events, requiring large-scale investigations and rapid follow-up action. Scaringi concludes: The rapid response of telescopes such as the VLT or ESO’s New Technology Telescope, and the suite of instruments available, will allow us to discover in more detail what these mysterious micronovae are. […] When scaled, micronovae and Type 1 X-ray bursts [indices des étoiles à neutrons] look remarkably similar “. This suggests that by finding and studying more micronovae, researchers could also learn more about neutron stars.
Source : Nature