10,000 Earths' Worth of Fresh Dust Found Near Star Explosion
Astronomers have at last found definitive evidence that the universe's first
dust - the celestial stuff that seeded future generations of stars and
planets - was forged in the explosions of massive stars.
The findings, made with NASA's Spitzer Space Telescope, are the most significant
clue yet in the longstanding mystery of where the dust in our very young universe
came from. Scientists had suspected that exploding stars, or supernovae, were the
primary source, but nobody had been able to demonstrate that they can create
copious amounts of dust - until now. Spitzer's sensitive infrared detectors have
found 10,000 Earth masses worth of dust in the blown-out remains of the well-known
supernova remnant Cassiopeia A.
"Now we can say unambiguously that dust - and lots of it - was formed in the ejecta
of the Cassiopeia A explosion. This finding was possible because Cassiopeia A is in
our own galaxy, where it is close enough to study in detail," said Jeonghee Rho of
NASA's Spitzer Science Center at the California Institute of Technology in Pasadena.
Rho is the lead author of a new report about the discovery appearing in the Jan.
20 issue of the Astrophysical Journal.
Space dust is everywhere in the cosmos, in our own neck of the universe and all the
way back billions of light-years away in our infant universe. Developing stars need
dust to cool down enough to collapse and ignite, while planets and living creatures
consist of the powdery substance. In our nearby universe, dust is pumped out by dying
stars like our sun. But back when the universe was young, sun-like stars hadn't been
around long enough to die and leave dust.
That's where supernovae come in. These violent explosions occur when the most massive
stars in the universe die. Because massive stars don't live very long, theorists reasoned
that the very first exploding massive stars could be the suppliers of the unaccounted-for
dust. These first stars, called Population III, are the only stars that formed without any dust.
Other objects in addition to supernovae might also contribute to the universe's first dust.
Spitzer recently found evidence that highly energetic black holes, called quasars, could,
together with supernovae, manufacture some dust in their winds
Rho and her colleagues analyzed the Cassopeia A supernova remnant, located about 11,000
light-years away. Though this remnant is not from the early universe, its proximity to us
makes it easier to address the question of whether supernovae have the ability to synthesize
significant amounts of dust. The astronomers analyzed the infrared light coming from Cassiopeia
A using Spitzer's infrared spectrograph, which spreads light apart to reveal the signatures of
different elements and molecules. "Because Spitzer is extremely sensitive to dust, we were
able to make high-resolution maps of dust in the entire structure," said Rho.
The map reveals the quantity, location and composition of the supernova remnant's dust, which
includes proto-silicates, silicon dioxide, iron oxide, pyroxene, carbon, aluminium oxide and
other compounds. One of the first things the astronomers noticed was that the dust matches up
perfectly with the gas, or ejecta, known to have been expelled in the explosion. This is the
smoking gun indicating the dust was freshly made in the ejecta from the stellar blast. "Dust
forms a few to several hundred days after these energetic explosions, when the temperature of
gas in the ejecta cools down," said Takashi Kozasa, a co-author at the Hokkaido University in Japan.
The team was surprised to find freshly-made dust deeper inside the remnant as well. This cooler
dust, mixed in with gas referred to as the unshocked ejecta, had never been seen before.
All the dust around the remnant, both warm and cold, adds up to about three percent of the
mass of the sun, or 10,000 Earths. This is just enough to explain where a large fraction,
but not all, of the universe's early dust came from. "Perhaps at least some of the unexplained
portion is much colder dust, which could be observed with upcoming telescopes, such as Herschel,"
said Haley Gomez, a co-author at University of Wales, Cardiff. The Herschel Space Observatory,
scheduled to launch in 2008, is a European Space Agency mission with significant NASA participation.
Rho also said that more studies of other supernovae from near to far are needed to put this
issue to rest. She notes that the rate at which dust is destroyed - a factor in determining
how much dust is needed to explain the dusty early universe - is still poorly understood.
The principal investigator of the research program, and a co-author of the paper, is Lawrence
Rudnick of the University of Minnesota, Twin Cities. Other co-authors include W.T. Reach of the
Spitzer Science Center; J. D. Smith of the Steward Observatory, Tucson, Ariz.; T. Delaney of
the Massachusetts Institute of Technology, Cambridge; J.A. Ennis of the University of Minnesota;
and A. Tappe of the Spitzer Science Center and the Harvard Smithsonian Center for Astrophysics,
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission
for NASA's Science Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology. Caltech manages JPL for NASA.
Spitzer's infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development
was led by Jim Houck of Cornell. For more information about Spitzer, visit http://www.nasa.gov/spitzer
Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.