LSU astronomers recently discovered the solution to a long-standing fundamental problem of astrophysics: what produces thermonuclear, or Type Ia, supernovae, which are tremendous explosions where the light is often brighter than a whole galaxy? LSU Professor of Physics and Astronomy Bradley Schaefer and graduate student Ashley Pagnotta have proven that these supernova are caused by a pair of white dwarf stars. Their results will appear in Nature.

"Thermonuclear stars are caused by white dwarf stars reaching a maximum mass where its carbon and oxygen constituents have a runaway explosion similar to an H-bomb," said Prof. Schaefer. "The issue of determining their origin has been a fundamental problem in the field of astronomy."

The LSU team's solution represents the culmination of more than 40 years of worldwide study focused on this issue, often referred to as the "progenitor problem."

The possible types of precursor system types, called progenitors, were considered to be either a pair of white dwarfs in a close binary orbit that spiral into each other due to gravitational attraction (called the double-degenerate model) or another type of binary where the ordinary companion star in orbit around the white dwarf is feeding material onto the white dwarf until it reaches the critical mass (called the single-degenerate model).

For decades the debate has raged, with no decisive evidence, and currently a roughly evenly divided opinion amongst astronomers.

"Many possible explanations have previously been suggested, and all but one of these requires that a companion star near to the exploding white dwarf be left behind after the explosion," said Schaefer. "So, a possible way to distinguish between the various progenitor models is to look deep in the center of an old supernova remnant to find (or not find) the ex-companion star."

The progenitor problem has increased greatly in importance over the last decade, to the point that the latest Decadal Review by the National Academy of Sciences placed the question among the top nine questions currently facing astronomy. The star system that produces the Type Ia thermonuclear supernova was previously determined to be a closely orbiting pair of white dwarf stars that spiraled inward for an explosive collision.

Schaefer and Pagnotta used images from the Hubble Space Telescope of a supernova remnant named SNR 0509-67.5 to illustrate the lack of any possible surviving companion star to the exploding white dwarf, allowing the rejection of all possible classes of progenitors except for the close pair of white dwarfs.

Any such result naturally requires extensive data processing and analysis as well as detailed theory calculations before it can be considered finalized. When finished, the central region of SNR 0509-67.5 was found to be starless to a very deep limit (visual magnitude 26.9).

The faintest possible ex-companion star for all models (except the double degenerate) is a factor of 50 times brighter than the observed limit, and this makes for the rejection of all explanations except for the pair of white dwarf stars.

"The logic here is the same as expressed by Sherlock Holmes in "The Sign of the Four," that 'when you have eliminated the impossible, whatever remains, however improbable, must be the truth,'" said Schaefer. "For SNR 0509-67.5, all but one model has been eliminated as impossible, so the one model remaining must be the truth."

This discovery is one of several recent announcements in LSU Department of Physics and Astronomy that mark supervnova-related breakthroughs. In addition to this discovery:

+ Schaefer was also recently invited to the Nobel Prize Ceremony in Stockholm, Sweden, in recognition for his research leading to the discovery of the accelerated expansion of the universe due to an unknown form of energy embedded in the fabric of space. This energy, known as "dark energy," has been described as one of the greatest enigmas in physics today.

+ LSU physicists Mike Cherry, Gary Case and graduate student James Rodi, together with an international team of colleagues using the Gamma-ray Burst Monitor, or GBM, on NASA's Fermi gamma-ray space telescope, discovered that the Crab Nebula, once considered to be a source of energy so stable that astronomers used it to calibrate their instruments, is dimming, a revelation that stunned astronomers.

+ Geoff Clayton, professor of physics and astronomy, and several collaborators published a study providing details on the first time that "cold" dust has been recorded around Supernova 1987A, suggesting that supernovae in general might be responsible for the large clouds of such debris in other galaxies created shortly after the Big Bang. Supernova 1987A is the most observed, well-studied and famous stellar explosion known to man, in part because it is located close enough so that when its light reached Earth in 1987, it could be seen with the naked eye.