A few dozen supernovae have occurred relatively “close” to Earth (less than 30 megaparsecs). This allowed scientists to identify their progenitor star. For this, they analyzed high-resolution images taken between one and ten years before the eruption. In the most common type of core-collapse supernova, the researchers concluded that the star in question was a red supergiant. They simulated how these massive stars die as they enter their pre-explosion phase. They have succeeded in determining the signs that indicate a star is about to end its life. Their findings are the subject of an article Inside Monthly Notices of the Royal Astronomical Society.
Signs of an Oncoming Supernova
Red supergiants like the well-known ones Betelgeuse and Antares, massive stars (about 8 to 35 solar masses). They consumed all the hydrogen in their hearts. Nuclear reactions no longer occur. Internal pressure cannot compensate for external gravity. The core of the star collapses onto it. This event releases a critical level of energy that causes the star’s envelope to expand. It expands until it disappears in a violent explosion.
Several recent studies have shown that at the time of their outbreak, Red supergiants Surrounded by a large amount of contextual material. However, this sudden and particularly dense accumulation of material, like a shell around the star, acts as a screen to obscure the light it emits. ” This dense material completely obscures the star. It dims by 100 times in the visible part of the spectrum. That is, the day before the star explodes, you can’t see it’s existence. », Dr Ben Davies explainsJohn Moores is an astrophysicist at the University and first author of the study.
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Until now, scientists did not know how long it took a red supergiant to form this material. Did it evolve via a “super-wind” over the decades? Or did it cause the loss of a tenth of the solar mass in a brief burst in less than a year? To find out, the researchers simulated the appearance of red supergiants in the pre-explosion phase for each of these scenarios.
“Dramatic” luminosity changes for supernovae
So the team simulated the emission spectra of red supergiants undergoing such mass-loss events. They observed that the presence of ambient material reduces the optical flux by a factor of more than 100 and the infrared by a factor of 10, regardless of the conditions considered.
However, the “super-wind” model can be excluded according to them. In fact, the accumulation of material in this case is very slow. This star glowed red and dim for decades before exploding. ” This contrasts sharply with observations of nearby supernovae. These always seemed to have normal colors and sizes “, the researchers note.
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In addition, the scientists explored archives of old telescopes. They discovered images that were accidentally taken a year before the stars exploded. However, in these images, the stars appear normal. This means that they have not yet developed the surface of the material. This suggests that the material has accumulated over a period of less than a year.
” We therefore predict that red supergiants experience dramatic optical and infrared luminosity variability. These variations take place in the weeks or months preceding the collapse of their core “, the researchers conclude. This discovery will help astronomers design an early warning system. It will make it possible to direct the world’s best telescopes in advance of an imminent eruption for real-time observations.
Real-time monitoring thanks to VRO
In short, just before their core collapse, red supergiants experience a spectacular mass-loss event. It radically changes their appearance. But scientists aren’t sure what caused this sudden mass loss. ” Our best guess is that the later stages of nuclear combustion trigger gravity/pressure waves that propagate across the surface of the collapsing star. », Davis said Direct science. Only real-time monitoring of the phenomenon will allow us to know more.
No live supernova has been observed to date. A “closer” observation is SN 2013fs, a supernova located in the spiral galaxy NGC 7610. Three hours of discovery Only after its explosion. ” When you see a supernova this early, you see the outer edge of the “bulb.” It burns from within due to the ongoing explosion Davis says. The event confirmed that the red supergiant at the origin of the explosion ejected material at a very high rate just before it disappeared.
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To observe a supernova directly, astronomers need highly sensitive telescopes. They can detect a star 100 times fainter in the visible spectrum. For this, the team is particularly dependent Vera-C.-Rubin Lab (VRO), which should be launched next year. The telescope can photograph the entire southern sky in three days. It has a sensitivity that can capture objects whose apparent magnitude is less than or equal to 24.
” VRO can see about half of the red supergiant stars within 10 million light-years. […] “If any of them suddenly start to darken dramatically, it could signal that the countdown to a supernova has begun,” Davis said.