Astronomers see our origins in 20-year-old star explosion

Now, nearly 20 years after the star’s death, the explosion is revealing signs of life --- in the form of dust particles that are the building blocks of rocky planets and all living creatures. And astronomers once again are captivated. "Supernova 1987A is changing right before our eyes," said Dr. Eli Dwek, a cosmic dust expert at NASA Goddard Space Flight Center in Greenbelt, Maryland. Dwek and his colleague Dr. Patrice Bouchet of the Observatoire de Paris have been following for several years this rapidly changing supernova, named SN 1987A for the year it was discovered in the Large Magellanic Cloud, a dwarf galaxy. "What we are seeing is a milestone in the evolution of a supernova."

SN1987A-f1.jpg Figure 1 : The image at the left shows the dust ring observed with the Gemini 8-m Telescope in Chile (red) superimposed on the image obtained in the visible by the Hubble Space Telescope (yellow). Click on the image to enlarge it

Using infrared telescopes, Bouchet, Dwek and their colleagues (1) detected silicate dust created by the star from before it exploded. This dust survived the explosion ; was swept up and pushed out by shock waves ; and is now, nearly 20 years later, finally approaching a ring of gas surrounding the embers of the dead star, making it "visible" to infrared detectors.

Dust --- chemical particles and crystals finer than beach sand --- is both a frustration and a fascination for astronomers. Dust can obscure observations of distant stars. Yet dust is the stuff from which all solid bodies are formed. This is why dust, as bland as it sounds, is one of the most important topics in astronomy and astrobiology. Dust is everywhere in the universe, yet astronomers know so little about its origin. Clearly dust is made in stars and hurled into space by supernovae. But the devil is in the details. How much dust is made in a star ? How much is vaporized by the star explosion and how much survives ? And how do wispy dust clouds form planets and ultimately life ? These are the questions that scientists such as Patrice Bouchet and Eli Dwek want to answer. With 1987A, they have a perfect laboratory to watch the process unfold. This is new territory for astronomers, said Bouchet, who led the SN 1987A infrared observations with the Gemini South telescope in Chile and NASA’s Spitzer Observatory. His team is witnessing processes never before seen. This is the first time scientists have direct evidence of dust from a large star surviving a supernova ; the first time they detect cold dust intermingled in hot, X-ray-emitting gas of millions of degrees ; and the first time they are witnessing sputtering, the process in which dust is broken down into smaller particles. They frankly don’t know what to expect, and they have already stumbled upon a few surprises. Infrared detectors are crucial for this kind of observation. Infrared is a less-energetic form of radiation than visible light. The infrared detectors are similar to night-vision goggles. The dust is over a hundred degrees below zero and too cold to emit visible light. Optical telescopes can see gas but not the dust. Through infrared imaging, the science team determined that the dust is in the region of the equatorial ring of gas around SN 1987A. Thus, it took many years for the dust to travel out from the explosion and into the interstellar medium, a distance of a little less than one light-year. "This much was expected," said Bouchet. "The collision between the ejecta of SN 1987A and the equatorial ring predicted to occur sometime in the interval of 1995 to 2007 is now underway." The silicate dust detected could not have formed in the supernova blast itself, because of the high energies involved, and could only have come from the progenitor star. Detection of silicates was a bit of a surprise and thrill, Dwek said. The key finding, however, is that the team has detected far less dust than expected. This could mean that a supernova blast wave can destroy more dust than thought possible. If confirmed, this will have broad implications for determining dust concentrations throughout the universe. Yet this is a work in progress. "Overall, we are witnessing the interaction of the supernova blast wave with its surrounding medium, creating an environment that is rapidly evolving at all wavelengths," said Bouchet. To that effect, scientists are planning a series of new observations of SN 1987A now that it has once again become very interesting. Who knows what will be revealed once the dust settles.

(1) The team includes Patrice Bouchet (GEPI, Paris Observatory), Eli Dwek (NASA, GSFC), John Danziger (Trieste Observatory, Italy), Richard Arendt (NASA, SSAI), James De Buizer (Gemini Observatory), Sangwook Park (Pennsylvania State University, USA), Nicholas Suntzeff (Cerro Tololo Interamerican Observatory, Chile), and Robert Kirshner and Peter Challis (Harvard-Smithsonian, CfA, Cambridge MA, USA).

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Reference Article accepted in Astrophysical Journal, astro-ph/0601495

Contact Patrice Bouchet (Observatoire de Paris, GEPI)

Dernière modification le 4 mars 2013