Explosive stars with good table manners

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An exploding star known as a Type Ia supernova plays a key role in our understanding of the universe. Studies of Type Ia supernovae led to the discovery of dark energy, which garnered the 2011 Nobel Prize in Physics. Yet the cause of this variety of exploding star remains elusive. 

These images from Swift’s Ultraviolet/Optical Telescope (UVOT) show the nearby spiral galaxy M101 before and after the appearance of SN 2011fe (circled, right), which was discovered on Aug. 24, 2011. At a distance of 21 million light-years, it was the nearest Type Ia supernova since 1986. Left: View constructed from images taken in March and April 2007. Right: The supernova was so bright that most UVOT exposures were short, so this view includes imagery from August through November 2011 to better show the galaxy [Credit: NASA/Swift/Peter Brown, Univ. of Utah]

All evidence points to a white dwarf that feeds off its companions star, gaining mass, growing unstable, and ultimately detonating. But does that white dwarf draw material from a Sun-like star, an evolved red giant star, or from a second white dwarf? Or is something more exotic going on? Clues can be collected by searching for “cosmic crumbs” left over from the white dwarf’s last meal. 

In two comprehensive studies of SN 2011fe – the closest Type Ia supernova in the past two decades – there is new evidence that indicates that the white dwarf progenitor was a particularly picky eater, leading scientists to conclude that the companion star was not likely to be a Sun-like star or an evolved giant. 

“It’s hard to understand how a white dwarf could eat itself to death while showing such good table manners,” said Alicia Soderberg of the Harvard-Smithsonian Center for Astrophysics (CfA). 

Soderberg and her colleagues examined SN 2011fe with a suite of instruments in wavelengths ranging from X-rays to radio. They saw no sign of stellar material recently devoured by the white dwarf. Instead, the explosion occurred in a remarkably clean environment. 

“This white dwarf was a tidy eater,” said Laura Chomiuk of the CfA, lead author of one of the two papers. 

Additional studies using NASA’s Swift satellite, which examined a large number of more distant Type Ia supernovae, appear to rule out giant stars as companions for the white-dwarf progenitors. 

Taken together, these studies suggest that Type Ia supernovae likely originate from a more exotic scenario, possibly the explosive merger of two white dwarfs. 

“This is an exciting time in Type Ia supernova research since it brings us closer to solving one of the longest-standing mysteries in the life cycles of stars,” said Raffaella Margutti of the CfA, lead author of the second paper.  

The two papers on SN 2011fe are available online at http://arxiv.org/abs/1201.0994 and http://arxiv.org/abs/1202.0741.  

Source: Harvard-Smithsonian Center for Astrophysics [March 20, 2012]

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