With the help of the European Southern Observatory’s (ESO) Very Large Telescope (VLT), a team of astronomers have observed a new type of stellar explosion they’ve coined ‘micronova’. While the event is a smaller cousin of the general nova, they still pack quite a punch and can burn through around 3.5 billion Great Pyramids of Giza of stellar material in only a few hours.
In a classical nova, the accreting white dwarf builds a layer of fresh hydrogen that covers its entire surface. Once this layer reaches high enough temperatures and pressures, the whole layer ignites thermonuclearly. This makes the entire system a lot brighter for weeks or months.
Meanwhile, this micronova explosion is created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. Micronovae are similar explosions but smaller in scale and faster, lasting just a few hours. These smaller explosions occur on some white dwarfs with strong magnetic fields, which funnel material towards the star’s magnetic poles.
“Classical novae are somewhat similar to micronovae,” Simone Scaringi, assistant professor in the Centre for Extragalactic Astronomy at Durham University who led the study on the explosions, told ZME Science. “In micronovae, because of the strong white dwarf magnetic field, we think the accretion is restricted to the magnetic poles of the white dwarf only. This allows the fresh hydrogen to reach high enough temperature and pressure to ignite thermonuclearly much faster. These events burn through about one million times less mass than classical novae, and consequently are about one million times less bright than classical novae.”
The team first came across these mysterious micro-explosions when analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). In their research, they found something startling, a bright flash of optical light lasting for a few hours. They ended up viewing three micronova in all with TESS, two of which originated from white dwarfs, however, the third required further observations with the X-shooter instrument on ESO’s VLT to confirm its white dwarf status.
Although they’ve only been discovered now, researchers suspect micronovae are a common occurrence in our great Universe.
“We think they may be quite common in an accreting magnetic white dwarf, and possibly a phenomenon observe on those which have relatively high white dwarf masses,” Scaringi said. “They are just very difficult to catch in action: you need to be looking at the right place and at the right time or you’ll miss them entirely!”
The discovery of micronovae adds to the repertoire of known stellar explosions. The team now wants to capture more of these elusive events, requiring large-scale surveys and quick follow-up measurements.
“We already have a follow-up paper detailing the theoretical model we have to trigger micronovae. Now that we know, more or less, what to look for, the challenge will be to find more and use these new observations to test and refine our current models,” Scaringi said. “I think this goes to show just how dynamic the Universe is. These micronova are fast flashes of light, that may be quite common out there. It also goes to show that thermonuclear explosions can occur on localized areas (as opposed to the entire surface) of white dwarfs, something that was unexpected and surprising.”
The discovery was published in Nature.