NASA’s Hubble Space Telescope has captured the first image of a surviving companion to a supernova, the most compelling evidence that some supernovas originate in double-star systems.
A supernova is a transient astronomical event that occurs during the last evolutionary stages of a massive star’s life. Seventeen years ago, astronomers witnessed a supernova go off 40 million light-years away in the galaxy called NGC 7424, located in the southern constellation Grus, the Crane, NASA said. The image of the companion star was seen in the fading afterglow of that supernova, called SN 2001ig.
“We know that the majority of massive stars are in binary pairs,” said Stuart Ryder from the Australian Astronomical Observatory (AAO). “Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together,” said Ryder, lead author of the study published in the Astrophysical Journal.
The companion to the supernova’s progenitor star was no innocent bystander to the explosion, researchers said. It siphoned off almost all of the hydrogen from the doomed star’s stellar envelope, the region that transports energy from the star’s core to its atmosphere, they said. Millions of years before the primary star went supernova, the companion’s thievery created an instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the catastrophe.
The supernova called SN 2001ig is categorised as a Type IIb stripped-envelope supernova. This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion. This type of exploding star was first identified in 1987 by Alex Filippenko of the University of California, Berkeley.
How stripped-envelope supernovas lose that outer envelope is not entirely clear. Looking for a binary companion after a supernova explosion is neither an easy task. First, it has to be at a relatively close distance to Earth for Hubble to see such a faint star. SN 2001ig and its companion are about at that limit. Within that distance range, not many supernovas go off. Even more importantly, astronomers have to know the exact position through very precise measurements.
In 2002, shortly after SN 2001ig exploded, scientists pinpointed the precise location of the supernova with the European Southern Observatory’s Very Large Telescope (VLT) in Chile. In 2004, they then followed up with the Gemini South Observatory in Chile. This observation first hinted at the presence of a surviving binary companion.
Knowing the exact coordinates, Ryder's team were able to focus Hubble on that location 12 years later, as the supernova’s glow faded. With Hubble’s exquisite resolution and ultraviolet capability, they were able to find and photograph the surviving companion – something only Hubble could do.
In the future, the astronomers hope to use the James Webb Space Telescope to continue their search.