Monday, 16 October 2017

We are all made of stardust but gold, silver and platinum are neutron stardust!

GW170817/GRB170817A/SSS17a was the event many astrophysicists have been waiting for since LIGO made the first detection of gravitational waves in 2015. This time we didn’t just detect the gravitational waves from two merging neutron stars, but have also seen the associated gamma-rays, optical light and radio emission from the possible black hole and from the two exploding stars.

This new discovery shows again the importance of LIGO and VIRGO’s work over decades. The refinement of their highly sensitive work allows us to be able to detect these gravitational waves.

In addition, there have been a large number of dedicated astronomers that have been following up the fading afterglow of the merger. Again showing their practice over many years of following up transient events as quickly as possible have payed off. I offer huge congratulations to everyone that contributed to this stunning discovery. Especially to everyone who wrote and published those papers in the two months since discovery!

Neutron stars are quite different to the black holes that have been detected merging before. They are made of “stuff” so there is material we can see when they do collide. Neutron stars are formed when massive stars (that about 8 times more massive than our Sun) die, in their last gasp these stars crush all the material in their core down into a sphere 10km in radius. This releases a large amount of energy that makes a star explode in a supernova.

If the star was in a binary, and the binary wasn’t destroyed in the two supernovae that much have occurred to make the two neutron stars, then these two neutron stars will slowly orbit closer and closer together as they emit gravitational waves. This can take billions of years until they touch, merge and possibly form a black hole as we’ve seen in GW170817.

We don’t even know the full story yet. The observations are ongoing and only in the coming months and years will we really begin to fully understand how exciting this object is. The fact that we have so much information from so many different sources will allow us to piece together in a way we have never been able to before. It’s going to take a lot of time and a lot of effort!

This event alone has already answered questions on the nature and structure or neutron stars and confirms that merging neutron stars look like we’ve always expected them to, as a short gamma-ray burst and a type of explosion called a “kilonova”.

A short gamma-ray burst is a highly energetic burst of gamma-rays that have the same apparent energy as entire exploding star but spaced over a few seconds. While a kilonova is the explosion powered by the collision, making new heavy elements that are ejected extremely fast, some maybe at close to the speed of light!

The observations of the kilonova, the explosive afterglow, also confirms something else. In the explosion we have seen evidence for large amount of heavy elements. These events mostly create elements such as gold, silver and platinum. In this event it’s likely that 100s or 1000s of Earth masses of gold and other elements were made. If the rate of neutron stars mergers is as high as we now think, these dying stars are now the source of most of these elements in the Universe.
We’re all made of stardust, but gold, silver and platinum are made of neutron stardust!

One funny thought is that in a Doctor Who story called “Revenge of the Cybermen”, there was a planet called “Voga” that was made entirely of gold. Since this is the one element that is lethal to the Cybermen they attempted to destroy the planet. Now after the observations around this event we know that solid-gold planets are not as unlikely as we might have thought before!

Researchers in NZ have not played a role in the detection of this event but we are highly interested in gravitational waves. For example, at the University of Auckland Assoc. Prof. Renate Meyer has worked on the problem of extracting the faint gravitational waves signals from the much larger terrestrial signals that jostle the instrument, a problem that is like trying to hear a whispered conversation in a noisy room.

While in my own group two students PhD student John Bray and MSc student Petra Tang are both working to predict from models what the rate of these events should be, this event is showing that our models need to be improved to match the high rate of such mergers we now expect in the Universe.
The problem comes down to this, for two neutron stars to merge, the binary must have survived two supernovae, these are extremely destructive events. The stars musts also have remained close enough that the two stars only took a few days to orbit around one another. Otherwise the Universe is not old enough for the stars to merge via gravitational radiation. Modelling these in population synthesis depends on many uncertain bits of physics, but now merging neutron stars and black holes will give us a new tool to constrain the evolution of stars. That’s what makes me most excited!

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