3-D Scanning A Dying Star - With Optical Light!

If astronomy was a beauty contest, supernova remnants would almost certainly win first prize. These beautiful objects are actually the remains of a dying star. With careful study, it’s possible for astronomers to use their own forensic tools to figure out what kind of star died, and why.

The glowing remains of a core-collapse supernova in the Large Magellanic Cloud (source: ESO/https://apod.nasa.gov/apod/ap180930.html)

Just as our understanding of life and death has evolved here on Earth with the development of sophisticated tools, so has our understanding of the lives and deaths of stars. One of the key tools used by doctors and forensic scientists today is ‘Computer-Aided Tomography’ or ‘CAT’ scans (sometimes also referred to as ‘CT’ scans).

Archeologists have begun to use this technology to study the mummified remains preserved by the Ancient Egyptians. Computer-aided tomography pieces together a three-dimensional image of the inside of the mummified body using X-rays, meaning [TW/CW: link contains images of a 3D scan of mummified human remains] the internal structure of the preserved remains can be studied without harming them. This kind of technology is important as it allows us to study irreplaceable artifacts and human remains with the respect and care they deserve. The same technology can be used to rapidly scan the human body to detect internal injuries, such as bleeding on the brain or in the abdomen.

But what about the thousands of years old remains of a dying star?

When a star dies, it hurls material outwards into the interstellar medium, along with powerful shock waves, moving at tens of thousands of kilometres per second, forming a supernova remnant. When the outgoing shockwave slams into the interstellar medium, a ‘reverse shock’ is generated. This shockwave travels inwards with respect to the expansion of the supernova remnant.

As the reverse shock travels inwards, it heats and ionises the supernova ejecta to millions of degrees centigrade. When supernova ejecta material, which contains a lot of iron, is heated to these temperatures, x-ray emission lines are produced. The doppler shift of these emission lines tells you information about the speed at which the supernova ejecta is moving, and hence you can map where the supernova ejecta is in space, and how fast it is moving. Combining all of this information together using a computer, you can then build an internal map of the structure of the supernova ejecta, along with its temperature and velocity. This is another example of computer-aided tomography!

However, this gets very tricky with x-rays because x-rays are very hard to focus accurately. Most supernova remnants are so far away that they appear as little more than point sources to X-ray telescopes, so while we can measure the amount of x-ray emitting material present, and its average temperature, it is hard or impossible to map exactly where it is, or how fast it is moving.

Unlike x-rays, optical light is easy to focus (that’s why we can see so well!). So can we use optical light emitted by the shocked supernova ejecta to map where the ejecta is and how fast it’s moving?

Highly ionised iron will happily emit a lot of x-rays, but can they emit optical light too? They can, but they do so at a much lower rate. The optical light emitted by Fe XIV ions (that’s an iron atom with 13 of its electrons stripped away by the reverse shock) comes from a mechanism called ‘forbidden emission’. This means that if we considered the types of electron transitions allowed by the standard rules of quantum mechanics, the transition just wouldn’t happen. However, if we include a bit of extra physics, such as magnetic dipoles, then the emission can happen, but at a much lower rate than a non-forbidden transition. We usually write the name of the ion producing the emission line in square brackets if the emission line is forbidden - i.e. [Fe XIV].

Because [Fe XIV] emission is so faint, it is very hard to see - you need to use a very big telescope, looking at the supernova remnant for a very long time. Fortunately, astronomers have a Very Large Telescope (the VLT, because in astronomy acronyms either lack imagination completely or use far too much imagination). The VLT is made up of four telescopes, each with a mirror eight meters in diameter - I don’t think Australian Geographic have got around to stocking those yet!

The three supernova remnants studied by Dr Seitenzahl and his colleagues in this paper. Blue represents interstellar gas (hydrogen) heated by the outgoing shockwave, red the x-rays emitted by iron heated by the ingoing reverse shock, and in green the [Fe XIV] emission from the same iron-rich ejecta, observed using the MUSE instrument. This is the first time optical [Fe XIV] emission assoicated with the reverse shock in a Type Ia supernova remnant has been observed

Dr Ivo Seitenzahl and his team of researchers used one of the VLT telescopes, equipped with an instrument called MUSE, to look for [Fe XIV] in several supernova remnants (free downloadable version here) . MUSE is able to take not only a picture of the supernova remnant, but is able to extract a spectrum from each pixel. That way, the scientists can study which parts of the supernova emits different colours of light. Because the VLT is so big, and MUSE is so sensitive, this work is the first time optical light has been used to map the structure of the reverse-shocked ejecta from a type Ia supernova by observing the green light coming from [Fe XIV] emission.

By studying how fast this iron-rich ejecta is moving in three different supernova remnants, the scientists were able to determine just what kind of stars might have exploded - one of the great mysteries of these kinds of exploding star is that we don’t know what the stars looked like before they went supernova.

So just like x-ray tomography on Earth allows us to peer inside the human body and ancient artefacts to learn more about them, this optical tomography allows us to peer inside and reveal the structures left behind by cataclysmic cosmic explosions, and Dr Seitenzahl and his team have created a brand new tool that will allow us to better understand just what makes a star explode!