A newly developed “early warning system” could warn astronomers when a massive star is about to explode and end its life in a supernova.
Because supernovae mark the collapse of a stellar core, leaving behind a neutron star or black hole, such a “red alert” could also alert astronomers eager to observe the birth of such an exotic stellar remnant.
A team of astronomers from Liverpool John Moores University and the University of Montpellier found that stars with a mass between 8 and 20 times that of the Sun in the red supergiant phase fade in visible light about a hundred times just before exploding.
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This dimming results from material suddenly accumulating and forming a “pre-explosion cocoon” around the red supergiant, blocking its light.
“The dense material almost completely obscures the star, making it 100 times dimmer in the visible part of the spectrum,” said lead author of the research and scientist at Liverpool John Moores University’s Astrophysics Research Institute, Benjamin Davies, in a Expression. (opens in new tab) “That means that the day before the star explodes, you probably wouldn’t be able to see it there.”
Previously, scientists were unaware of how long this accumulation of material around the dying star would last. Establishing a time frame for this cocoon accretion is key to solving the mystery of how it forms.
Theories surrounding this cocoon formation fall in two categories (opens in new tab) based on how rapidly red supergiants accumulate this material. The “superwind” scenario suggests that the pre-explosion cocoon is formed as a result of a steady flow of material from the red supergiant for a period of up to 100 years before a supernova explosion.
An alternative to this is the “bursting out” scenario, in which the pre-explosion cocoon forms as a result of a mass-loss event for the dying star, which would last less than a year.
Davies and his team modeled both large cocoon formation scenarios, simulating what a red supergiant might look like when embedded in these pre-explosion cocoons. The team found that both models produced large amounts of optically thick circumstellar gas, enough to radically reduce the luminous output of the simulated red supergiants.
However, the “superwind” scenario caused the dying star to be conspicuously red and faint many decades before a supernova. This conflicts with images that exist of stars that eventually erupted in supernova explosions about a year later, as the stars appear normal in these images.
This implies that these stars may not have built their dense, light-blocking cocoon before the explosion, and suggests that this material has less than a year to evolve.
This represents an extremely rapid accumulation of material that supports the breakout scenario and rejects the Superwind model. Astronomers could use this information and a star’s dimming to issue “red alerts” and prepare to observe a supernova within a year or so.
“Until now, we have only been able to make detailed observations of supernovas hours after they occur,” said Davies. “With this early warning system, we can prepare to monitor them [supernovas] in real time to turn the world’s best telescopes on the progenitor stars and watch as they are literally ripped apart before our very eyes.”
The team’s work is discussed in an article published in the journal Monthly Bulletins of the Royal Astronomical Society (opens in new tab).
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