Snagging supernova spectra

Spectroscopy reveals that an extraordinarily bright, very long-lasting supernova named SN 2007bi, spotted in the night sky by a robotic telescope is the first example of the earliest type of star in the Universe.

SN 2007bi, as its name reveals, was first observed in 2007 by the international Nearby Supernova Factory (SNfactory) based at the US Department of Energy's Lawrence Berkeley National Laboratory. It was immediately obvious that spectra of the superbright supernova, which appeared in a nearby dwarf galaxy, was unusual.

Team member Alex Filippenko obtained the optical spectrum of the unusual supernova, and then Caltech researchers acquired additional spectra with the Keck telescope, as did Paolo Mazzali's team from the Max Planck Institute for Astrophysics in Garching, Germany, using the Very Large Telescope (VLT) in Chile.

Rollin Thomas of CRD, a member of C3 and the SNfactory, aided the early analysis, using the Franklin supercomputer at the National Energy Research Scientific Computing Center (NERSC) to run a code he developed to generate numerous synthetic spectra for comparison with the real spectrum.

The detailed look by astronomers at the University of California at Berkeley working with Avishay Gal-Yam of Israel's Weizmann Institute of Science suggested that the supernova originated in the explosion of a giant star with a mass at least 200 times that of our Sun. More intriguingly, however, the spectrum shows the presence of few chemical elements other than hydrogen and helium, implying that the star must have shone in the very early universe.

"Because the core alone was some 100 solar masses, the long-hypothesized phenomenon called pair instability must have occurred," explains astrophysicist Peter Nugent of the SNfactory. "In the extreme heat of the star's interior, energetic gamma rays created pairs of electrons and positrons, which bled off the pressure that sustained the core against collapse."

Heavyweight stars, as they die, are expected to collapse and form a black hole, this star underwent nuclear runaway that blew it to pieces and although the behaviour has decades-old theory to support it, it has not been observed until now.

SN 2007bi was observed by the Palomar-QUEST Survey, an automated search on the Oschin Telescope at Caltech's Palomar Observatory and categorised by the SNfactory. The SNfactory has so far discovered nearly a thousand supernovae of all types and amassed thousands of spectra, but has focused on those designated Type Ia, the "standard candles" used to study the expansion history of the Universe and which provided the early evidence for dark energy. SN 2007bi, however, turned out not to be a Type Ia. For one thing, it was at least ten times as bright.

"The Keck and VLT spectra clearly indicated that an extremely large amount of material was ejected by the explosion, including a record amount of radioactive nickel, which caused the expanding gases to glow very brightly," explains Mazzali.

The data suggest that the core of the huge star had fused to oxygen near the end of its life, and was incredibly hot. At this point, the most energetic photons formed electron-positron pairs, robbing the core of pressure and causing it to collapse, which triggered the runaway nuclear event that generated large amounts of radioactive nickel. It was this that energized the ejected gas and allowed the supernova to remain visible for such a long time.

"It's significant that the first unambiguous example of a pair-instability supernova was found in a dwarf galaxy," says the team's Peter Nugent. "These are incredibly small, very dim galaxies that contain few elements heavier than hydrogen and helium, so they are models of the early Universe."

Dwarf galaxies are ubiquitous but so faint and dim - "they take only a few pixels on a camera," adds Nugent, "and until recently, with the development of wide-field projects like the SNfactory, astronomers had wanted to fill the chip with galaxies." But there is now new interest in what Gal-Yam and his collaborators call "fossil laboratories to study the early Universe."

Says Filippenko, "In the future, we might end up detecting the very first generation of stars, early in the history of the Universe, through explosions such as that of SN 2007bi - long before we have the capability of directly seeing the pre-explosion stars."