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By Phil Williams
pwilliam@franklin.uga.edu
When low-mass stars called red supergiants die, they fade away on a wimpy wind--or so scientists have thought. New research suggests that the opposite may be true. These stars, in fact, may die with a bang and not with a whimper.
The study, published this month in Science, may lead researchers to a new understanding of red supergiants, which are studied to resolve issues in nucleosynthesis, stellar structure and the evolution of stars.
“This discovery was really a gigantic surprise,” says Michael Duncan, Research Professor of Chemistry at UGA. “One of the beauties of doing fundamental science is that you never quite know where it may lead.”
Other authors of the Science paper are Gerard Meijer, Gert von Helden and Deniz van Heijnsbergen of the FOM Institute for Plasma Physics in Nieuwegein; S. Hony and L.B.F.M. Waters of the University of Amsterdamand A.G.G.M. Tielens of the University of Groningen, all in the Netherlands.
Red supergiants are more properly called asymptotic giant branch stars, or AGBs. The term actually denotes a stage of development rather than a specific kind of star. During their relatively short AGB phase, intermediate-mass stars become their brightest but experience heavy mass loss that leads them rapidly to the planetary-nebula phase and a final cooling to white dwarfs. White dwarfs are extremely hot, Earth-sized objects that fade and cool for billions of years until they become black, cold cinders.
Duncan’s involvement in the discovery was the kind of scientific serendipity that often leads to unexpected breakthroughs. His area, the study of gas-phase metal clusters, has lately taken a huge step forward due to a collaboration with Gerard Meijer, whom he met at a scientific meeting at Ohio State University in 1998, and Meijer’s colleagues in the Netherlands.
“He was talking about the free-electron laser called FELIX [Free-Electron Laser for Infrared Excitation] that had been built at the FOM Institute, and I happened to ask him if it had ever been used to study gas-phase metal clusters,” says Duncan. “From that, our collaboration was born.”
There are probably no more than 20 free-electron lasers in the world, and only five in the United States. Priority for use at the U.S. machines goes largely to medical or industrial applications. FELIX is the only free-electron laser optimized for measuring infrared signals or “spectra” of chemicals, and seemed a perfect match for the metal-cluster experiments.
“After meeting Meijer, we realized his team had the free-electron laser, and I had the pulsed molecular-beam machine and experience working with metal clusters, and we needed to find a way to make them work together,” says Duncan. Luckily, Meijer received at about that time a large grant from the Dutch government, and so the team in the Netherlands was able to construct the molecular-beam machine that Duncan had been using and mate it with the free-electron laser.
The result was a machine that could detect the infrared spectra of gas-phase metals and thus give important clues to how they are structured. The new apparatus worked beautifully, and when Duncan visited the lab last summer, the team achieved the first direct infrared spectra of these clusters ever done.
These spectra in themselves will likely open a new era in the study of how gas-phase metals are structured, but a chance meeting with other Dutch scientists at the FOM Institute initiated a startling discovery that led the research from the lab to the stars.
“These astronomers were visiting the FELIX lab and hearing about work on polyaromatic hydrocarbons, which are important in the composition of interstellar space,” says Duncan. “It just so happened that our work on gas-phase metals was on a machine nearby, and they asked what it was. Meijer and von Helden showed them the machine and the spectra we had. That’s when their jaws dropped.”
The astronomers realized immediately that the infrared spectra that the group had elicited from their study of titanium carbide nanocrystals corresponded almost exactly to spectra of unknown origin seen again and again in AGB stars. The discovery created a problem, however.
Meteorites containing micrometer-sized graphite grains with embedded titanium carbide (TiC) grains have been discovered on Earth. Isotopic analysis has identified AGB stars as the birthplace of these grains, though there had been no direct link. Astronomers believe that as AGB stars begin to die, newly synthesized elements such as TiC are mixed to the surface where they spread over the galaxy in a wind, most often in the form of stardust.
The problem lies in the fact that the abundance of titanium in low-mass stars is so low that “high densities are required just to get high-enough collision gains to grow to the sizes observed in graphite stardust.”
The identification of the infrared spectra around AGB stars as gas-phase titanium carbide, however, changes that picture. Because of the low amounts of titanium in the stars and the apparent large amount in the ejecta, the event creating them must be caused by something that releases tremendous energy over a relatively short period of time. As the authors write, “The TiC identification suggests that rather than with a wimpy wind, low-mass stars end their lives with (almost) a bang.”
Studies of the infrared spectra of AGB stars is just beginning to take off. A 1998 conference in France reported what it called the “first mature results” of this research. The new study should add fuel to the fires of speculation about how stars are formed, how they live--and ultimately, how they die.
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