"Zombie" Supernova gives up puzzles

Stern explodes a good 50 years after his last supernova

In a supernova, the star core normally collapses and only a neutron star or black hole remains - not so with iPTF14hls. (Illustration) © ESO / M. Kornmesser
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Cosmic Revenant: An ordinary supernova at first sight astounds astronomers. Because the glow of the star explosion lasted six times as long as usual and wavered instead of fading. Even stranger though: The same star exploded in 1954 in a supernova. Why he survived his death and has now exploded again, can not be explained with the current models, as the researchers report in the journal "Nature".

Normally, a so-called type IIP supernova follows clear rules: the core of a massive star collapses, causing a violent explosion. In this case, the hydrogen-rich star sheath is blown off and in the center only a neutron star remains - an extremely dense, consisting of exotic matter object. The shockwave of the explosion causes the ejected gases to light up, producing a glow of this supernova that usually lasts around 100 days.

"Impossible" long glow

Now, however, astronomers around Iair Arcavi at the Las Cumbres Observatory in California have discovered a supernova that does not fit into this scheme, or any other type of supernova. When they registered the flare of a star 800 million light-years away in September 2014, everything seemed quite normal: the spectrum of iPTF14hls light was exactly the same as that of an IIP supernova, and the temperature of around 5, 000 to 6, 000 Kelvin was just right.

But when the astronomers re-targeted the same position in January 2015, the astonishing ones discovered that the supernova was still shining - and did not seem to stop. "Instead of a 100-day plateau, the light curve of iPTF14hls lasted more than 600 days, " report Arcavi and his colleagues. "It almost seems as if everything is ten times slower than normal in this blast."

The light curve of the Supernova iPTF14hls is completely different from all other star explosions. © LCO / S. Wilkinson

First explosion in 1954

Unusually also: The brightness of the explosion did not decrease after the initial summit, as usual with such a supernova. Instead, iPTF14hls has brightened and darkened at least five times over the past three years, astronomers say. According to the spectral lines, the elements ejected or exploded in the explosion move differently than usual. display

But it becomes even more puzzling: As the researchers looked in archive data for clues to the history of the star, they came as a surprise: In 1954, had occurred in exactly the same position ever before a supernova. "All this data suggests that there must have been a high-energy Pr supernova outbreak at that time, " the astronomers report.

The star death survived

Specifically, PTF14hls' forerunner exploded for the first time more than 50 years ago, but somehow managed to retain enough bulk material to survive this normally safe star death. In 2014, this "zombie" exploded for the second time in a supernova. This in turn does not fit any of the common models.

This 1954 Palomar Observatory Sky Survey shows a supernova at exactly the same sky position POSS / DSS / LCO / S. Wilkinson

"This supernova contradicts everything we thought we knew about these events, " says Arcavi. "It's the biggest riddle I've encountered in research on stellar explosions in nearly a decade." Astronomers suggest that the star of origin must have been at least 50 times more massive than the Sun, maybe even heavy up to 130 solar masses. Because only such giants under the stars could have survived the first supernova.

A "Dinosaur of the Cosmos"?

It may be hypothesized that iPTF14hls is the first known member of a supernova type that has been postulated theoretically. In these "Pulsational Pair Instability" supernovae, the core of massive stars becomes so hot that energy inside is transformed into antimatter and matter. This causes an explosion that blows away the outer hulls of the star but leaves the core intact. Only after repeated repetitions then collapses the core and a black hole is formed.

"These explosions were only typical for the early Universe they should not actually occur today, " says Arcavis colleague Andy Howell. "It's like finding a live dinosaur today." In addition, some of the features of this supernova do not fit the theory of this supernova type, including the continued presence of hydrogen or the enormously high levels of hydrogen Energy of the explosion.

So far only one thing seems clear: The Supernova iPTF14hls does not fit into any of the common models and could thus represent a completely new type of star explosion. Which mechanisms are behind this enigmatic event, astronomers can only puzzle over it. (Nature, 2017; doi: 10.1038 / nature24030)

(Las Cumbres Observatory, Carnegie Institution, Nature, Nov. 9, 2017 - NPO)