Astrophysicist’s Algorithm Leads to Discovery of Super-massive Supernova

A group of scientists affiliated with the SuperNova Legacy Survey (SNLS) have found startling evidence that there is more than one kind of Type Ia supernova, a class of exploding stars which until now has been regarded as essentially uniform in all important respects. Supernova SNLS-03D3bb is more than twice as bright as most Type Ia supernovae but has much less kinetic energy, and appears to be 1.5 times as massive as a typical Type Ia.

The lead authors of the report, which appeared in the Sept. 21, 2006 issue of Nature, include Andrew Howell, formerly of the Physics Division at Berkeley Lab, and now at the University of Toronto, and Peter Nugent, an astrophysicist in CRD. Nugent’s research enabled scientists to distinguish SNLS-03D3bb from previously known Type Ia supernovae. While working at NERSC, Nugent developed an algorithm that could take a handful of photometric data points early in the evolution of a candidate supernova, positively identify it as a Type Ia, and accurately predict its time of maximum brightness.

The report’s other lead authors are Mark Sullivan of the University of Toronto and Richard Ellis of the California Institute of Technology. These and many of the other authors of the Nature paper are members of the Supernova Cosmology Project based at Berkeley Lab.

Because almost all Type Ia supernovae found so far are not only remarkably bright but remarkably uniform in their brightness, they are regarded as the best astronomical "standard candles" for measurement across cosmological distances. In 1998, after observations of many distant Type Ia supernovae, the Supernova Cosmology Project and the rival High-Z Supernova Search Team announced their discovery that the expansion of the universe is accelerating —  a finding that would soon be attributed to the unknown something called dark energy, which fills the universe and opposes the mutual gravitational attraction of matter.

"Type Ia supernovae are thought to be reliable distance indicators because they have a standard amount of fuel — the carbon and oxygen in a white dwarf star — and they have a uniform trigger," Nugent said. "They are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass, which is about 1.4 times the mass of our sun. The fact that SNLS-03D3bb is well over that mass kind of opens up a Pandora's box."

Classification of supernova types is based on their spectra. Type Ia spectra have no hydrogen lines but do have silicon absorption lines, a clue to the chemistry of their explosions. The white dwarf progenitors of Type Ia supernovae, typically about two-thirds the mass of the sun, are thought to accrete additional mass from a binary companion until they approach the Chandrasekhar limit. Increasing pressure causes the carbon and oxygen in the center of the star to fuse, producing the elements up to nickel on the periodic table. The energy released in this process blows the star to pieces in a titanic thermonuclear explosion.

Why is SNLS-03D3bb brighter than the typical Type Ia supernovae? One clue was the elements needed to produce the extra brightness. "A Type Ia of normal brightness makes about 60 percent of a solar mass worth of nickel 56, the rest being other elements. But SNLS-03D3bb is more than twice as bright as normal; it must have more than twice as much nickel 56,” Nugent said. “The only way to get that is with a progenitor that's 50 percent more massive than the Chandrasekhar mass."

Moreover, in most brighter supernovae, the matter ejected from the explosion travels at a higher velocity. But the ejecta of SNLS-03D3bb were unusually slow. The velocity of supernova ejecta depends on the kinetic energy released in the explosion, which is the difference between the energy released in thermonuclear burning minus the binding energy that acts to hold the star together.

It's possible that a very rapidly spinning star could be more massive. It's also possible that two white dwarfs, with a combined mass well over the Chandrasekhar limit, could collide and explode.

In old, dead galaxies even the biggest stars are small, Nugent explains. The only kinds of Type Ia supernovae possible in these galaxies are likely to be the binary-system, mass-accreting, Chandrasekhar-mass type. But young star-forming galaxies produce massive objects and could be rich in white-dwarf plus white-dwarf binary systems, so-called "double-degenerate" systems.

It was partly in hopes of developing a quick and dependable way to identify candidate Type Ia supernovae that Nugent and coauthor Richard Ellis initially approached Sullivan and other members of the SNLS, with its large data base of supernovae. One of the first Type Ia's studied this way turned out to be SNLS-03D3bb itself.

Nugent regards the discovery of the first demonstrable super-Chandrasekhar supernova as an exciting prospect: "For the first time since 1993" – when the brightness versus light-curve shape relationship was developed – "we now have a strong direction to look for the next parameter that describes the brightness of a Type Ia supernova. This search may lead us to a much better understanding of their progenitors, and the systematics of using them as cosmological probes."