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 & 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 the Jan. 12 issue of 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 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."
- Bradley E. Schaefer, Ashley Pagnotta. An absence of ex-companion stars in the type Ia supernova remnant SNR 0509−67.5. Nature, 2012; 481 (7380): 164 DOI: 10.1038/nature10692
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