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Brightest supernova a new kind of exploding star?
Posted: 10 June 2011

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Listen to Robert Quimby talk to AN's Nick Howes about the Palomar Transient Factory and his latest supernova discovery.

The mystery of a supernova that exploded six years ago may have been solved, indicating a new class of supernova more luminous than any others.

Supernovae come in many flavours, which we can think of in two ways. In broad terms they are the explosions of either matter-accreting white dwarf stars or massive stars that have reached the end of their evolution. Astronomers use more technical classification: type I supernovae show no hydrogen in their spectra and can either be white dwarf or massive star explosions, and type II supernovae that do display hydrogen lines and are exclusively the domain of massive star destruction.

When supernova 2005ap was seen to explode in a faint and distant dwarf galaxy in Coma Berenices in 2005, it posed something of a puzzle. It blew up 4.7 billion light years away but, if it had exploded 33 light years away it would have had an absolute magnitude in our sky of –22.7 (the Sun’s magnitude in our sky is –27). It was intrinsically the brightest supernova ever seen, a hundred billion times more luminous than the Sun and much more luminous than a typical type II supernova, which reaches absolute magnitudes between –17 and –19, powered by the radioactive decay of nickel-56 in the supernova debris.

Images of the four supernovae – PTF09atu, PTF09cnd, PTF09cwl and PTF10cwr – seen by the PTF. On the left is before the explosion, and on the right is after the explosion. Image: Caltech/Robert Quimby/Nature.

Initial indications were that 2005ap had hydrogen lines in its spectra, but further data and analysis ruled this out. Around the same time, several other ultra-luminous supernovae – 2008es, 2006gy, 2006tf and SCP 06F6 – were seen to go off, and all except the latter were observed to contain hydrogen. At first these these were lumped in with 2005ap, but the growing realisation that 2005ap and SCP06F6 actually lacked hydrogen meant that they were not an easy match. Since then four new ultra-luminous supernovae discovered by the Palomar Transient Factory (PTF) have also been found to be hydrogen deficient and have now been identified as being very similar to 2005ap and SCP 06F6, which was discovered eight billion light years away by the Hubble Space Telescope.

The PTF is an automated, wide-field survey using 1.2-metre and 1.5-metre telescopes on Mount Palomar in California to search for transient objects in the sky, be they variable stars, novae or supernovae. For these four particular supernovae it was able to track them until they faded from view, from their hottest state to much cooler states, and follow-up observations with the ten-metre Keck telescopes in Hawaii, the 5.1 metre Hale Telescope at Mount Palomar, and the 4.2-metre William Herschel Telescope in the Canary Islands measured their spectra. Even as they cooled down, no hydrogen lines could be detected. If similar data had been available for 2005ap, it too would not have shown hydrogen lines at this stage. Once differences in redshift had been accounted for, the spectra of all six supernovae matched precisely.

The project has been led by Robert Quimby, now of the California Institute of Technology, who discovered the peculiarities of SN 2005ap when he was at the University of Texas in Austin, as well as 2006gy and 2008es. “[Supernovae] 2008es, 2006gy and 2006tf all showed clear signs of hydrogen and for the latter two we know the interaction of supernova ejecta with a hydrogen-rich medium around the star was an important contributor to the peak brightness,” he tells Astronomy Now. “But with no hydrogen the new PTF discoveries must be powered through different means.”

What do we know about this new class of supernova? Their spectra are very blue, topping out in ultraviolet wavelengths. They are hot and bright, with the temperature in their explosive layers between 10,000 and 20,000 degrees Celsius, and their debris cloud expands at the breakneck velocity of 10,000 kilometres per second. They are also longer-lived than ordinary supernovae, taking 50 days or more to fade.

From this data, and the lack of hydrogen, Quimby and his colleagues have been able to assemble plausible explanations for the extreme brightness of the supernovae in a new paper published in the 9 June edition of Nature. Their leading theory is a variation on the pair-instability model that was first developed by Alex Filippenko, Nathan Smith and David Pooley at the University of California, Berkeley to help explain the likes of 2006gy.

Artwork representing a pair-instability supernova explosion. Image: NASA/CXC/M Weiss.

Pair instability occurs in the heftiest stars with over 90 times the mass of the Sun. In their cores, temperatures grow so hot that gamma rays produced by nuclear fusion in the core collide and annihilate, producing a pair of electron–position particles. Ordinarily the gamma ray radiation coming out of the core provides the requisite pressure to hold the star up against its own gravity and, when the gamma rays disappear, the star becomes unstable and begins to collapse before utterly exploding to smithereens, not even leaving a neutron star behind.

Quimby’s Nature paper describes a possible variation of this where such a star experiences pulsations in the centuries or millennia before its destruction. In stars initially less than 130 solar masses the beginnings of the star’s contraction causes nuclear fusion to ignite in the outer layers, fusing hydrogen into helium. At some point the continued contraction is “quenched”, says Quimby, leading to the release of energy that puffs the now hydrogen-depleted layers up (similar to what occurs in pulsating variable stars or red giants) and casts them away from the star completely. “Later, when the star finally dies, the supernova ejecta collides with these previously cast off layers and this interaction is what may make our supernovae so bright,” he says.

An alternative model is that of a more typical supernova explosion, where the core fuses elements all the way up to iron, stops producing energy and collapses, but which leaves behind an extremely magnetic form of neutron star called a magnetar. The spinning magnetic field of the magnetar interacts with the expanding cloud of supernova debris, slowing down the magnetar’s rotation. Angular momentum from the magnetar is transferred into heat energy in the supernova ejecta, causing it to glow more brightly.

Automated surveys like the PTF will in future allow the detection of many more of these exotic, ultra-luminous supernovae and provide the ability to work out which of these two possibilities, if either, are correct. It may also provide an insight into the deaths of the first generations of stars in the Universe, which were thought to be very massive and have undergone processes similar to pair instability.