Seeing fresh dust at a young supernova: Source of the planets?
Dust: It’s the bane of astronomers — since it blocks many wavelengths of light, dimming our view of the universe. But it’s also the source of planets. And that helps explain the excitement about the discovery of masses of dust created within months after a supernova was detected in 2010.
By “dust,” astronomers mean tiny grains of various elements that have condensed to a stable condition. Dust may form vast clouds in solar nebulae, and eventually contract into stars and planets.
In a new report in Nature, Christa Gall and colleagues reported on spectral observations of dust formed by supernova 2010jl, a staggeringly bright light created by the explosion of a massive star about 150 million light years distant. That’s what astronomers term the “nearby universe.”
As expected, they saw plenty of dust, and, less expectedly, a two-step process, with tiny grains (diameter: about one millionth of a meter) followed by larger grains (about 4.5 microns) in an observation made 30 months after the explosion.
Life: In dust we trust!
To those who follow the history of life, dust is a must. Life starts only on planets, and the rocky planets in our solar system (Mercury, Venus, Earth and Mars) are conglomerations of cooled dust. “Dust plays a crucial role in planets,” says Gall, at the department of physics and astronomy at Aarhus University in Denmark. “The surface has to be cool, the molecules have to cool to condense to make a physically compact planet or star.”
Carbon is the most common element in cosmic dust, which also contains silicon, oxygen, iron, magnesium and other elements, Gall says. In the early phase, at least, hydrogen and helium are rare, even though they are the most common elements in the universe.
“We know dust needs a temperature lower than 2,000° Kelvin (about 1700°C),” says Gall. “If it’s hotter, it would evaporate instantly. And it needs high density.”
The star that exploded, called the progenitor, was apparently quite hefty, 40 or 50 times the mass of our sun, and thus contained the energy to make a titanic explosion that sent photons hurtling all the way to the observatory in Northern Chile.
It’s still not clear how cosmic dust forms. Via press release, co-author Jens Hjorth from the Niels Bohr Institute of the University of Copenhagen, Denmark said, “Our detection of large grains soon after the supernova explosion means that there must be a fast and efficient way to create them. We really don’t know exactly how this happens.”
Gall gives an educated guess: Before the explosion, the progenitor star shed its outer shell. Then the exploding supernova sent out a huge shock wave that collided with the erupted shell. After the shock wave passed, a dense, cool region was left.
And that is where dust started forming quite early in the explosion. The cool dense shell “is the perfect place for dust formation,” Gall says. “If you have the right elements from the progenitor, given the high density, we assume this would enable the rapid formation of dust.”
Dust is also distributed when low-mass stars like the sun die, Gall told us. It cannot form from cold gas in interstellar space.
Planets: we lust for dust!
Once stable dust grains exist, elements can gather on their surface, enlarging the grains into the ones seen by Gall’s crew in the later observations. And those larger grains are thought to be more resistant to destruction in the high-energy environment surrounding the supernova.
“With smaller grains, there is a larger chance that they will get evaporated, but the large grains may survive” further shocks near the supernova, Gall says. Larger grains have been seen around supernova 1987a, and in meteorites. “That means we can retain the dust, or at least a large fraction of it, as it passes into the interstellar medium, and so we can explain the large dust masses in galaxies.”
The Milky Way contains a mass of dust equal to 10 million suns, and dusty galaxies may have ten times more. So next time you dust the furniture, remember that we now have a better idea of the birthplace of cosmic dust.
– David J. Tenenbaum