Companion Star Hidden for 21 Years in Supernova's Glow

Artist's impression of supernova 1993J, an exploding star in the galaxy M81 whose light reached us 21 years ago. The blue helium-burning companion star can be seen at the center of the expanding nebula of debris from the supernova / NASA, ESA, and G. Baco
Janet Fang 10 Sep 2014, 16:58

Twenty-one years ago, astronomers detected light from an unusual class of supernova, and they suspected that the explosion originated in a double-star system -- where one star intensifies the mass-loss from the aging, primary star of the pair. Now using NASA’s Hubble Space Telescope, they’ve spotted that companion star, which had been hidden this whole time by the glow of the explosion. 

This explosion, designated SN 1993J, happened in the galaxy M81, about 11 million light-years away in the direction of Ursa Major. A supernova goes off once every second somewhere in the universe. When a massive star approaches the end of its life, it burns through all of its material, and its iron core collapses; the rebounding outer material is what we see as a supernova. The rare Type IIb supernova combines the features of a supernova explosion in a binary system with what’s seen when a single massive star explodes.

Like all Type IIb supernovae, SN 1993J didn’t have a large amount of hydrogen present in the explosion. But how it lost its hydrogen was a mystery. "This is like a crime scene, and we finally identified the robber," says Alex Filippenko from the University of California, Berkeley. "The companion star stole a bunch of hydrogen before the primary star exploded." And this companion continues to burn as a blue, super-hot helium star. 

Researchers spent that last two decades searching for this lost companion and its glow, called continuum emission. Observations made ten years ago at the W.M. Keck Observatory on Mauna Kea showed some evidence for spectral absorption lines that could come from this suspected companion. But with a crowded field of view, those features could also come from other stars along SN 1993J’s line of sight.

"A binary system is likely required to lose the majority of the primary star's hydrogen envelope prior to the explosion. The problem is that, to date, direct observations of the predicted binary companion star have been difficult to obtain since it is so faint relative to the supernova itself," UC Berkeley’s Ori Fox explains in a news release

Since the companion star is so hot, its continuum glow should largely be in UV light, which can only be detected above our atmosphere. "We were able to get that UV spectrum with Hubble," says Azalee Bostroem of the Space Telescope Science Institute (STScI). “This conclusively shows that we have an excess of continuum emission in the UV, even after the light from other stars have been subtracted.” 

After collecting the UV light, the team constructed a multi-wavelength spectrum that matches what they’d predicted for the companion star’s glow. They were able to estimate the surviving star's luminosity and mass, which helps describe conditions preceding the explosion.

This detection is the first time astronomers have been able to put constraints on the properties of a companion star in Type IIb supernovae. The findings were published in the Astrophysical Journal in July.

Image: NASA, ESA, and G. Bacon (STScI)

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