A couple weeks ago I wrote about the discovery of a supernova ten times more powerful than so-called type Ia supernova. This superluminous supernova known as SN2007bi was not only extremely bright, but it remained bright for about five months. Such a long brightness period is indicative of a type of supernova known as a pair instability supernova, so called because the intense gamma rays produced in the core during the explosion form pairs of electrons and positrons (electron pairs).

It’s generally thought that SN2007bi is a clear example of a pair instability supernova due to its intensity and long brightness period, but now new supernova observations suggest that SN2007bi wasn’t a pair-instability supernova after all. But if it wasn’t, then how could it remain so bright for so long?

In a recent paper in Nature (behind a paywall, but the arxiv version is here), a research team looked at data from two new superluminous supernovae, known as PTF 12dam and PS1-11ap. Both of these were extremely bright and had long brightness periods, just like SN2007bi. But while SN2007bi was only observed once it was close to maximum brightness, the new supernovae were observed as they grew brighter.

This is important because pair instability supernovae should not only stay bright for a long time, they should also take a long time to become bright. This is because the debris from a pair-instability supernova is so massive and dense that it takes time (on the order of a year) for the heat and radiation from the explosion to filter through the debris.

If you’ve ever cooked with a heavy cast-iron pan as opposed to an aluminum pan, you know that the cast iron pan takes much longer to heat up, but once hot it stays hot for a long time. A pair-instability supernova is similar. It’s so massive that it takes a long time to rise to maximum brightness, and once bright it takes a long time to fade.

These two new supernovae did not take a long time to reach maximum brightness. They reached their maximum relatively quickly (about two months). This means they can’t be pair instability supernovae. But they still took a long time to fade, just like SN2007bi. This means our best pair-instability candidate might not have been one after all. But if it wasn’t, then how could it remain so bright for so long?

The answer seems to be a magnetic heating effect. When a supernova explodes, the core typically collapses into a neutron star. The magnetic fields of a neutron star can be quite strong, and in their most extreme case the neutron star is often referred to as a magnetar. The intense magnetic field of a magnetar can interact with surrounding material, causing it to superheat. This is similar to a magneto, where a rotating magnet can produce pulses of electric current.

Credit: M. Nicholl et al.

The authors propose that a large supernova (hypernova) can produce a magnetar when it explodes. Since the hypernova isn’t as massive as a pair-instability supernova, it can brighten fairly quickly. Normally, it would fade more quickly as well, but the intense magnetic field of the magnetar heats the surrounding debris, causing it to remain hot (and therefore bright) for much longer. When the team compared the theoretical brightness curve of a magnetar hypernova with the observed brightness of PTF 12dam, they found it matched really well. You can see this in the figure below (taken from the paper). The black curve shows the theoretical brightness of a magnetar supernova, and the black circles show the observed brightness. The other lines are theoretical pair-instability brightness curves of different masses.

Not only does the data match the magnetar theory quite well, it is also clear that a magnetar supernova fades slowly in a way that looks a lot like a pair instability supernova. This means we can’t prove a supernova is a pair-instability one simply by observing it after maximum brightness. We also need to observe its brightening period. Since we didn’t observe the brightening of SN2007bi, we can’t be sure about it. It may have been a pair instability supernova, or it may have been a magnetar supernova.

So we’re now not sure whether we’ve observed a pair instability explosion or not, but we now have a better understanding of superluminous supernovae.

Sometimes in science you have to take a step back before you can take a step forward.

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In the early moments of the universe, hydrogen and helium were formed through a process known as baryogenesis. Trace amounts of other elements such as lithium were also produced, but none of the heavier elements. This means that the first generation of stars were composed of hydrogen and helium, and it is only through fusion in their cores that the heavier elements we see today were created. The carbon, oxygen and iron in our bodies was produced through that process, which is why it is often said that we are star stuff.

Because heavier elements are only produced in stellar interiors, one way we can determine the age of a star is by the amount of these elements in its atmosphere, what is known as its metallicity. Younger stars will tend to have a higher metallicity, since they tend to form from the dust of earlier stars. Because of this, there has been a great deal of effort to discover very low metallicity stars. We have found some stars with quite low metallicity, indicating that they are second generation stars.

There has also been a search for first generation stars, but this has not been successful. Many of these stars would likely have been quite large, and therefore would burn through its hydrogen rather quickly, dying in a tremendous supernova explosion. One of the unanswered questions about these first generation stars is just how large they could get. The larger a star is, the shorter its lifetime. The general consensus is that first generation stars could have been as large as 200 – 300 solar masses, but proving that is a challenge.

Now a new paper in Science has found evidence to support this idea. The team looked at a low metallicity star known as SDSS J0018-0939. It is a dim orange star that seems to be a second-generation star. They looked at the abundances of 18 different elements within the star’s spectrum, and found that elements with an even atomic number (such as titanium and nickel) are more abundant than elements with odd atomic numbers (such as scandium and cobalt). This suggests that its progenitor star had some dying mechanism to produce more even elements than odd.

But this is something you would expect to see in the death of supermassive stars. When very large first-generation stars had fused most of their hydrogen into helium in their cores, they would then collapse quickly under their own weight. This would trigger a very large supernova (hypernova) known as a pair-instability supernova. In such a supernova, the gamma rays produced by nuclear collision are so energetic that they decay into electrons and positrons. These interact less strongly with helium, allowing more helium nuclei to fuse to produce heavier elements.  Since helium has an atomic number of 2, more even elements would be produced.

Simply finding higher even/odd element ratios in a single star isn’t enough to prove that first generation stars were quite large, but it does show that the idea is feasible. We’ll have to look at more second-generation stars to see if the idea truly holds up.

Paper: W. Aoki, et al. A chemical signature of first-generation very massive stars. Science, 345 (6199): 912-915 (2014).

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