Solving the mystery of the wimpy supernova
Posted by admin on 11th October 2018
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A spectacular supernova explosion, more than a billion times brighter than our sun, marked the birth of a neutron star orbiting its hot and dense companion. Now these two dense remnants are destined to spiral into each other in about a billion years, eventually merging and yielding some of the heaviest known elements in the universe.

The explosion occurred in a galaxy similar to our own Milky Way, nearly 920 million light years away. A small telescope at Palomar observatory in California detected the first photons from the supernova – named “iPTF 14gqr” – just hours after the explosion, when it was more than 10 times hotter than the surface of our sun. As the brightness of the supernova evolved during the next two weeks, an international team of astronomers used the data to trace the origin of the explosion to a massive star with a radius 500 times that of the sun.

But it wasn’t just the giant size of the star that made this discovery particularly noteworthy. What was unusual was that the star also seemed to be the lightest of all known exploding giant stars. This massive star had been robbed of nearly all of its mass, perhaps by a dense orbiting partner. When it exploded, it left behind a newborn neutron star that continued to orbit its companion.

The three panels represent moments before, during, and after the faint supernova iPTF14gqr, visible in the middle panel, appeared in the outskirts of a spiral galaxy located 920 million light years away. The massive star that died in the supernova left behind a neutron star in a very tight binary system. These dense stellar remnants will ultimately spiral into each other and merge in a spectacular explosion, giving off gravitational and electromagnetic waves.
SDSS/Caltech/Keck, CC BY-SA

Understanding the formation of binary star systems in which two super dense stars orbit each other has always been a puzzle. These fleeting supernovae that yield these dense binary star systems are both rare and difficult to find, because they quickly appear and disappear in the sky – about five times faster than a typical supernova.

This first observation of a “ultra-stripped” supernova, which my colleagues and I detail in a new study, not only provides insights into the formation of these systems but also reveals the final stages in the lives of these unique massive stars that have been plundered of all of their mass before they die.

Solving a longstanding mystery

Stars born with more than eight times the mass of the sun quickly run out of fuel and succumb to gravity at the end of their lives – collapsing in on themselves and exploding in a supernova. When this happens, all of the star’s outer layers – a few times the mass of the sun – are scattered.

A binary star system is composed of two stars orbiting each other. Here the larger blue star is absorbing the other smaller secondary star.
Catmando / Shutterstock.com

When I started working with my advisor, Mansi Kasliwal, as a new graduate student, I decided to study supernovae that quickly fade in brightness. Mining the database of events discovered by iPTF, I came across iPTF 14gqr, a quickly fading supernova that was discovered more than a year before but whose true physical nature remained mysterious.

The data were puzzling because our preliminary models suggested this supernova was caused by the death of a giant massive star, yet the explosion in itself was quite wimpy. It ejected only a fifth of the mass of the sun, while its energy was only a tenth of a typical supernova. Where was all the missing matter and energy?

The clues indicated that the exploding star must have been stripped of nearly all of its original mass before the explosion. But what could have stolen so much matter from this giant star? Perhaps an unseen binary companion?

I started reading up about rare binary star scenarios, when I first came across the idea of “ultra-stripped supernovae.”

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