No Star Left Behind: Fruitless Search for Supernova Survivor Hints at Unexpected Origins

A type Ia supernova is perhaps the ultimate combination of insult and injury—a star steals material from a companion star, reaches critical mass, becomes unstable, and then unleashes a nuclear blast powerful enough to decimate or destroy its already diminished victim.

The culprit in these cases is clear: type Ia supernovae arise from the cataclysmic explosions of small, dense stars known as white dwarfs. But the victim's identity is clouded, limiting the precision of cosmological distance estimates that rely on these luminous beacons as markers. Traditionally, scientists believed that with the victims were sunlike main-sequence stars or swollen giant stars. But recent studies have pointed to a major role for a lesser known mechanism—pairings of two white dwarfs in which one cannibalizes its orbital companion before exploding as a supernova.

Now a study in the September 27 issue of Nature bolsters the latter argument, concluding that only a small minority of type Ia supernovae stem from main-sequence or giant stars. (Scientific American is part of Nature Publishing Group.) Supernovae driven by double white dwarfs, then, could be the rule rather than the exception.

Across the universe, type Ia supernovae are abundant, but few have occurred in the Milky Way in recorded human history. Supernova 1006, so named for the year A.D. in which it was first seen on Earth, was one such rare occurrence. But its origins remain unclear. So Jonay González Hernández, of the Institute of Astrophysics of the Canary Islands and the University of La Laguna in Spain, and his colleagues looked for remains of the companion star from which the ill-fated white dwarf siphoned material before exploding.

Supernova 1006 exploded 7,100 light-years from Earth, so any surviving relic of the blast should lie at about that distance. But the researchers found only a small handful of stars near the supernova site, and all of them were swollen red-giant stars. "Giant stars are not predicted to be companion stars of progenitors of type Ia supernovae, since the impact of the violent explosion would remove all the envelope of the giant star," leaving only a smaller white dwarf–esque star behind, González Hernández explains.

The lack of a surviving companion seems to rule out any large star as a partner, because the core of such a star should have weathered the blast and should remain visible today. So the companion could well have been another white dwarf, which would have left no trace. In conjunction with other, mostly fruitless searches for supernova survivors, the researchers estimate that fewer than 20 percent of type Ia supernovae originate from the classically assumed scenario of a white dwarf sucking matter away from a normal (non–white dwarf) companion star.

Not all researchers agree. Astronomer Andrew Howell of the Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., notes that he and his colleagues recently found evidence for just such a "normal" companion for a type Ia supernova discovered in 2011 in a galaxy some 675 million light-years away. He calls the 20 percent claim in the new Nature paper "a vast overstatement," noting that a normal star somewhat smaller than the sun also would not leave any detectable traces of itself and would fit the bill for the companion to supernova 1006. In Howell's view, though, the 20 percent figure might apply to red-giant progenitors specifically.

So the actual percentage of type Ia supernovae produced by various stellar combinations may remain in dispute. But if anything seems clear from the recent studies on the origins of type Ia supernovae, it is that these blasts are hardly homogeneous in terms of their progenitor stars. Howell says that such variety won't compromise the idea of dark energy, the mysterious entity driving the accelerating expansion of the universe, which was first identified in studies that used the glow of distant type Ia supernovae to trace cosmic distance. The evidence for dark energy, he says, depends on the relative brightnesses of supernovae "and being able to calibrate them using the colors of the supernovae and the shapes of their lightcurves." Unraveling the progenitors of these cosmic beacons—and, in turn, their intrinsic brightness—would, however, help researchers improve the precision of their cosmological measurements.