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Astronomers find supernova remnant being created
Posted: Feb. 28, 2000

Supernova 1987A's circumstellar ring, the brightest part of the nebula surrounding SN 1987A, is shown as the colored ellipse, while stars are shown in white. The red spots indicate the brightest features, nearly all of which have appeared recently as a result of the debris from the SN explosion (ejecta) striking the ring. This image was processed through "image deconvolution" which increases the detail evident in the data by recreating the distribution of light sources that must be responsible for creating an image formed by observing under slightly blurrier conditions. Photo: Columbia University
For the first time in history, scientists are now able to see the details of a supernova remnant in the making. Astronomers from Columbia have discovered a new brightening of the circumstellar ring around a supernova, indicating that supernova ejecta have finally begun to collide with a shell of gas blown out by the star earlier in its lifetime.

The activity was sighted on Dec. 25, 1999, by a team of Columbia astrophysicists, Stephen Lawrence, Arlin Crotts, Ben Sugerman, and Robert Uglesich, led by Patrice Bouchet of the National Optical Astronomy Observatories' Cerro Tololo Inter-American Observatory (CTIO). A "hot spot" that appeared in the circumstellar ring around the supernova in 1997 was believed to be the first impact of supernova ejecta, but no other activity sites had been observed until the CTIO and Columbia team sighted this one in December. The new hot spot is about the same brightness as the first was when it was originally found. Other, fainter impact sites are present in their observations. The scientists also determined that the original hot spot had brightened significantly since their last observation over a year ago.

"The first collision of ejecta may have been a jet of material striking the circumstellar ring, shocking the gas into emission, much like bullets hitting a target," said Crotts, a Columbia professor of astronomy. "Now the entire ring is beginning to be engulfed with shocked material from the supernova, lighting up the ejecta and circumstellar material as a supernova remnant. We have observed many examples of supernova remnants -- for example, the Crab Nebula -- but all were formed long ago. We have never before seen one in the making in any meaningful degree of detail."

Unprocessed image
The unprocessed image used from above figure showing the ring, the previously known hotspot on the north-east portion of the ring, and surrounding stars. The right panel shows the same seen after subtracting an image taken 14 months earlier, showing the increase in brightness of the hotspot, and a second, fainter source to the south and slightly east. All images were obtained 25 December 1999, using the Blanco 4-meter telescope at Cerro Tololo Interamerican Observatory (near La Serena, Chile), operated by the National Optical Astronomy Observatories for AURA, Inc., under contract with the National Science Foundation. Photo: Columbia University
The significance of the newly discovered hot spots is that they are not confined to a single location, but are distributed around the circumstellar ring. The distribution around the ring indicates that a large fraction of the ejected material is finally colliding with the whole ring, instead of a fast moving "bullet" of ejecta making a single hot spot. If so, this is the beginning the long awaited formation of a supernova remnant. Other teams making follow-up observations with the Hubble Space Telescope in late January and early February have confirmed the new hot spot and found a number of other faint, new impact sites. These other hot spots are also found in the CTIO data, at a more subtle level.

The CTIO observations used an innovative imaging system on a Blanco 4-m telescope that achieved better spatial resolution than is commonly possible from ground-based observatories. The CTIO system tips and tilts the secondary mirror of the telescope to take the "twinkle" out of starlight, producing steadier, sharper images. They also used a novel image processing technique developed by the Columbia team.

Supernova 1987A occurred when the star known as Sanduleak -69 202 ended its life in a gigantic explosion, which was observed on earth on February 23, 1987, and became known as supernova 1987A. While the radiation from that explosion traveled out at the speed of light, material from the star itself was ejected at a much lower speed, some tens of millions of miles per hour. This material is now beginning to catch up and collide with material blown out some twenty thousand years earlier by the star in a relatively gentle, slow, cool stellar wind. This collision of supernova ejecta with the wind material, now forming the circumstellar shell, was predicted to occur sometime between 1995 and 2010.

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