BY DR EMILY BALDWIN
Posted: 20 January, 2009
Striking new evidence suggests that large volumes of cosmic dust was forged by gradually dying carbon stars, contradicting theories that it forms solely in massive stellar explosions.
Cosmic dust is the building block for the formation of planets, and also life, yet scientists have long debated its origin. Until now, the most popular theory says that cosmic dust is produced in the dramatic death cries of massive stars as they explode as supernovae, but some scientists suspect that this process may destroy more dust than it produces. Furthermore, dust-producing carbon stars, like the one observed in this new study, were not thought to have existed in early galaxies.
UK Schmidt Telescope optical image of the Sculptor Dwarf spheroidal galaxy observed in this study. Image: David Malin, Anglo-Australian Observatory.
“The first carbon stars would exist about 50 million years after the first stars formed. That is roughly 500 million years after the Big Bang,” says Albert Zijlstra from the University of Manchester's Jodrell Bank Centre for Astrophysics. Zijlstra lead a team of international astronomers on a project using NASA’s Spitzer Space Telescope to observe dust forming around a dying star in a nearby galaxy. The team focused on the carbon star known as MAG 29, which is located 280,000 light years away in a small nearby galaxy called the Sculptor Dwarf.
The Sculptor Dwarf contains only four percent of the carbon and other heavy elements in the Milky Way, making it similar to primitive galaxies seen at the edge of the Universe. “All the elements heavier than helium were made after the Big Bang in successive generations of stars,” says Zijlstra. “We came up with the idea of looking at nearby galaxies poor in heavier elements to get a close-up view of how stars live and die in conditions similar to those in the first galaxies.”
Zijlstra explains that the Universe became 'dusty' quite early on, but that the dust may have been different from that observed in the Milky Way today. “It is metallicity dependent: in the metal-rich inner Milky Way, no carbon stars are found; in the outer Milky Way they become more numerous,” he says. “We have found that oxygen-rich stars at low metallicity do not produce much dust. So compared to the Milky Way, the early Universe would have seen more carbon dust produced and much less silicate.”
Optical image of the carbon star IRC+10216 in our own Milky Way Galaxy taken with FORS1 on the Very Large Telescope. Image: Izan Leao (Universidade Federal do Rio Grande do Norte, Brazil).
And on the observations of MAG 29, Cornell University astronomer Greg Sloan says, “We haven't seen carbon-rich dust in this primitive an environment before. What this tells us is that carbon stars could have been pumping out dust soon after the first galaxies were born”.
Carbon dust is trapped within pre-solar meteorites that later fell to Earth, and over 90 percent of this dust comes from carbon stars, with the remainder from supernovae. “In the early Universe, supernovae would be more important,” Zijlstra tells Astronomy Now. “At the moment, observed dust production in supernovae falls short of what is needed to explain the early Universe by a factor of a few. It is possible that we are underestimating the amount of dust produced as we can only detect heated dust and may miss dust in cold clumps in the supernova. However, it leaves open the possiblity that carbon stars are responsible for the remainder.”
Astronomers will strive to understand the quantity and composition of the Universe’s dust inventory to better understand how stars and galaxies evolve, both in the early Universe and in our galactic neighbourhood.
The research is reported in the January 16 issue of the journal Science.