When stellar cataclysms known as type Ia supernovae flare up far across the universe, their brightness and consistency allow astronomers to use them as so-called standard candles to measure cosmological distances. Just over a decade ago, two teams used the supernovae to show that the universe is accelerating in its expansion due to the influence of dark energy, a shocking discovery that thrust type Ia supernovae into the astrophysical limelight. But how exactly did these cosmic mileposts come to be?
A type Ia supernova arises from the explosion of an ultradense stellar remnant known as a white dwarf, but it is less than clear how the white dwarf comes to ignite in a thermonuclear blast. The traditional view held that a white dwarf, locked in a binary pairing with another star, sucked matter from its companion, growing ever larger in size until it could no longer support its own weight. Once a white dwarf reaches the Chandrasekhar limit, roughly 1.4 times the mass of the sun, it contracts and explodes in a massive blast.
But a new study presents evidence that, for at least one kind of galaxy, the binary-accretion model should not be more than a minor contributor to the observed type Ia supernovae population. Marat Gilfanov, an astrophysicist at the Max Planck Institute for Astrophysics (M.P.A.) in Garching, Germany, and the Space Research Institute in Moscow, along with M.P.A. graduate student Ákos Bogdán looked at elliptical galaxies for x-rays expected to arise during the accretion process. In a study published in the February 18 issue of Nature, Gilfanov and Bogdán report that they found just a fraction of the x-rays expected from white dwarfs accreting matter from their neighbors. (Scientific American is part of Nature Publishing Group.) The standard path to type Ia supernovae, the study's authors wrote, should have produced 30 to 50 times the x-rays observed, indicating that accreting white dwarfs account for less than 5 percent of the explosions.
As a white dwarf draws off hydrogen-rich material from a binary companion over millions of years, Gilfanov says, it experiences a steady process of nuclear fusion on its surface that gives off tremendous amounts of radiation. That radiation should be detectable in the x-ray band, although interstellar gas and dust would absorb some of it. That is why the researchers focused on elliptical galaxies, which have less obscuring material than spiral and irregular ones. Gilfanov says they are now working on characterizing the type Ia progenitors in other galactic types, such as the spiral cousins of our own Milky Way.
Andrew Howell, a staff scientist with the Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., says that alternative origins for type Ia supernovae are becoming more compelling. "The evidence has been building for years that the classical paradigm, the single-degenerate scenario, is not enough to explain every type Ia that we see," Howell says. The favored alternative at present is the so-called double-degenerate scenario, in which two white dwarfs locked in a binary pairing spiral inward and merge, triggering an explosion. Such an explosion, which would have more fuel to burn than a single detonated white dwarf, might explain certain bright supernovae that appear to be powered by an object above the Chandrasekhar mass.
Howell says that these mergers have been less favored because it is difficult to make them work in a three-dimensional computer model, although recent work has offered promise. "Nature is telling us that these mergers happen, but we're not smart enough yet to figure out how this happens," he says.
The use of type Ia supernovae for cosmic distance measurements does not depend heavily on knowing the mechanism by which they detonate, so the new work will not unseat dark energy as a widely accepted component of the universe. But Howell notes that the mix of supernova brightnesses changes as astronomers look farther across the universe and, by extension, further back in time. Understanding the progenitors of the explosions might help unravel their evolution through cosmic time.
And Gilfanov says that resolving the underlying astrophysics of type Ia supernovae would help make standard-candle measurements more precise. "If we want to go from 10 percent to 1 percent [uncertainty] in measuring cosmological parameters," he says, astronomers need a better understanding of why white dwarfs explode in supernovae. "Dark energy will not go away, and the concept of standard candles will not go away," Gilfanov says. "It just gives us a better understanding and a better set of tools."