August 27, 2013
For two years it was believed that a blue, smaller star of the two in a binary star system, had exploded. Now that the dust has settled, it is apparent that the bigger star is missing.
Supernova in the Whirlpool galaxy, 2011dh.
Two years ago, at the end of May 2011, astronomers have observed a supernova in M51 galaxy, better known as the Whirlpool galaxy. The supernova, designated 2011dh, was observed at the edge of M51, approximately 25 million light-years away from Earth.
It was captured by many amateur astronomers because Whirlpool galaxy is very beautiful and popular. "It's really one of the nearest galaxies, and it's a galaxy that's beautiful and very famous," says Schuyler Van Dyk, astronomer at the California Institute of Technology, USA.
Astronomers don't know when a star is about to go supernova and when it does, they can't be sure which star exactly exploded. But because the Whirlpool is so famous, there were plenty pre-supernova images that astronomers could use to determine where the explosion originated.
"Within days of discovery of the supernova we went to the Hubble Space Telescope data archive, and it turned out that one of the former directors of the HST had orchestrated this beautiful mosaic of M51—this glorious picture in various colors," Van Dyk says.
The supernova in question, 2011dh, was a type IIb supernova, which occurs only 10% percent of the time and while astronomers have some general explanations for this type, the exact chain of events is unknown. This is why it was interesting to find a yellow supergiant star in the Hubble images from 2005 at the exact spot where the supernova appeared in 2011.
However, many astronomers thought that the profile (type IIb) of the explosion didn't match what they expected from the collapse of a supergiant. Their data pointed to the explosion of a smaller star, maybe a companion to said yellow supergiant in a binary system. The supergiant could have stripped hydrogen from its smaller partner with its gravitational pull, which is characteristic of a type IIb supernovae occurring in binary systems, when two stars are locked to each other by gravity.
"We thought initially that the progenitor was essentially this very stripped star, very blue, and so it was unseen in the Hubble images", Van Dyk said. "The yellow star was hiding the bluer star that actually exploded – that was our conjecture."
Justyn Maund, Queen's University, Belfast, UK, and colleagues thought, however, that the giant star that Hubble had spotted had indeed been the progenitor. "They said that the yellow star was the star that exploded," Van Dyk says. "They had other data that was more consistent with a more extended progenitor. So there we were."
In March 2013, almost two years after the supernova was first observed in the Whirlpool galaxy and now that the remnants of the supernova have nearly dissipated, Van Dyk and his team took another look with the Hubble Space Telescope and found that the supergiant has indeed disappeared, meaning it was the source of the supernova.
"We just wanted to see what the evolution of the supernova was," Van Dyk says. "We fully expected the yellow supergiant to still be there in these images this year. The other team was actually correct, and we were fully contrite in that way."
Now, astronomers will be eager to observe the yellow supergiant's surviving partner, if they were locked in a binary system, to shed a new light on type IIb supernovae. This will be possible in November, as the light from supernova finally fades and Whirlpool returns to it's pre-2011 state. "You should actually be able to see the companion star in the binary system," Van Dyk says, "If [astronomers following the evolution of 2011dh] see the binary companion, then that lends a lot of credence to this binary pathway to this type of supernova. And that would be really important."