December 14, 2011
August 24th Supernova Was A ‘White Dwarf’ Star
Scientists have discovered that a supernova that exploded in August was a "white dwarf" star, and its companion star could not have been a "red giant."
The thermonuclear Type Ia supernova exploded on August 24th in the Pinwheel galaxy, located in the "Big Dipper" star constellation.
These supernovae are used to measure dark energy, which scientists believe is related to the expansion of the universe.
The supernova, which is located about 21 million light-years away, could be seen in early September with binoculars. The explosion gave scientists their best chance yet to study a thermonuclear supernova up close.
"We caught the supernova just 11 hours after it exploded, so soon that we were later able to calculate the actual moment of the explosion to within 20 minutes," Peter Nugent of the U.S. Department of Energy's Lawrence Berkeley National Laboratory said in a press release. "Our early observations confirmed some assumptions about the physics of Type Ia supernovae, and we ruled out a number of possible models. But with this close-up look, we also found things nobody had dreamed of."
The star that explodes to produce a Type Ia supernova is thought to be a white dwarf composed primarily of carbon and oxygen. This star presumably explodes because it pulls matter from a binary companion.
Berkeley research astronomer Weidong Li and colleagues analyzed Hubble Space Telescope data taken of the August 24th supernova.
Li and colleagues calculated that the companion star could not have been a red giant star, which typically are bloated and bright.
Nugent said these findings make astronomers confident that the companion was most likely a normal star like the Sun, a somewhat evolved star called a sub giant, or perhaps a white dwarf.
Papers from both teams of Li and Nugent were published in the December 15 issue of Nature.
"Together the two papers suggest that a main-sequence star companion agrees with all the observed properties of the supernova progenitor luminosity limits and the evolution of the supernova," coauthor Saurabh Jha, an assistant professor of astronomy at Rutgers University, said in a press release.
Once the August 24th event occurred, Li and 30 collaborators from around the world began to obtain images of the supernova from the 10-meter Keck Telescope in Hawaii and its adaptive optics system.
Using images from this telescope, they were able to pinpoint the exact location of the supernova. They then used Hubble images to determine that there was no companion star in the past data.
The team then used the red, green and blue wavelength observations from the Hubble to set a stringent limit on how bright the white dwarf and its companion could be.
They were able to eliminate the presence of a red giant star, and most types of helium stars, and determine that the main star was a white dwarf.
"Our next step is to detect the surviving companion star, perhaps using the James Webb Space Telescope when it comes online. That will give us another opportunity to reveal more secrets about this supernova and ultimately help us understand the explosion physics of Type Ia supernovae, and possibly refine them as an even better cosmological distance ladder," Li said in a press release.
Image Caption: The Palomar Transient Factory caught SN 2011fe in the Pinwheel Galaxy in the vicinity of the Big Dipper on 24 August, 2011. Found just hours after it exploded and only 21 million light years away, the discovery triggered the closest-ever look at a young Type Ia supernova. (Image by B. J. Fulton, Las Cumbres Observatory Global Telescope Network
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