Astronomers devised an early warning system for supernova explosions

It turns out stars look dimmer before they are about to explode.
Loukia Papadopoulos
An artist's impression of Betelgeuse's supernova..jpg
An artist's impression of Betelgeuse's supernova.

European Southern Observatory/L. Calçada 

Experts from the Astrophysics Research Institute at LJMU, with help from colleagues from the University of Montpellier, devised an "early warning" system to alert when a massive star is about to end its life in a supernova explosion, according to a press statement published by the LJMU institution on Thursday.

A spectacular event

A supernova has been described as one of the most spectacular fireworks displays in the universe. The last gasp of a sufficiently large star in its death throes, the star's explosive finale often becomes the brightest point of light in an entire galaxy in less than a second and can remain so for several weeks afterward before fading away into a spectacular nebula spread across dozens if not hundreds of light-years.

For the first time ever, astronomers have simulated how these massive stars seemingly fade away and disappear when they enter their pre-explosion phase. They have concluded that massive stars in the last phase of their lives, the so-called 'red supergiant' phase, will suddenly become around a hundred times fainter at visible wavelengths in the few months before they die.

This dimming is caused by the accumulation of material around the star, thwarting the light coming from it from being visible. Right before red supergiants explode, they are surrounded by a dense shell of celestial material.

Astronomers devised an early warning system for supernova explosions
Illustration of a supernova

"The dense material almost completely obscures the star, making it 100 times fainter in the visible part of the spectrum. This means that the day before the star explodes, you likely wouldn't be able to see it was there," said Dr. Ben Davies of LJMU's Astrophysics Research Institute and lead researcher of the new study.

Accumulating celestial material

Up until now, researchers had little information on how long it took the dying stars to develop this material. Now, for the first time, researchers have simulated how red supergiants look when they are embedded within these pre-explosion "cocoons."

The astronomers came to their conclusion by searching through old telescope archives. They found images of stars taken incidentally around a year before they exploded and noticed that they appeared normal in these images, meaning they had yet built up this celestial cocoon. This data suggests that the cocoon is therefore assembled in less than a year, a blink of an eye considering the star was tens of millions of years old.

"Until now, we've only been able to get detailed observations of supernovae hours after they've already happened. With this early-warning system, we can get ready to observe it in real-time, to point the world's best telescopes at it, and watch the surface of the star getting literally ripped apart in front of our eyes," Davies added.

Last month, a team of astronomers declared they had found an effective method for predicting a supernova a few years in advance. They also highlighted a few tell-tale signs for a supernova — namely, if a star is giant and red and surrounded by a thick shroud of material.

The new study is published in the journal Monthly Notices of the Royal Astronomical Society.

Abstract:

From the early radiation of type II-P supernovae (SNe), it has been claimed that the majority of their red supergiant (RSG) progenitors are enshrouded by large amounts of circumstellar material (CSM) at the point of explosion. The inferred density of this CSM is orders of magnitude above that seen around RSGs in the field, and is therefore indicative of a short phase of elevated mass-loss prior to explosion. It is not known over what time-scale this material gets there: is it formed over several decades by a 'superwind' with mass-loss rate M˙∼10−3M⊙yr−1⁠; or is it formed in less than a year by a brief 'outburst' with M˙∼10−1M⊙yr−1? In this paper, we simulate spectra for RSGs undergoing such mass-loss events, and demonstrate that in either scenario, the CSM suppresses the optical flux by over a factor of 100, and that of the near-IR by a factor of 10. We argue that the 'superwind' model can be excluded as it causes the progenitor to be heavily obscured for decades before explosion, and is strongly at odds with observations of II-P progenitors taken within 10 yr of core-collapse. Instead, our results favour abrupt outbursts < 1 yr before explosion as the explanation for the early optical radiation of II-P SNe. We therefore predict that RSGs will undergo dramatic photometric variability in the optical and infrared in the weeks-to-months before core-collapse.

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