Astronomers Solve Supernova Mystery

NASA, ESA, G. Bacon (STScI). Artist’s impression of supernova 1993J. Using the Hubble Space Telescope, astronomers have identified the blue helium-burning companion star at the center of the expanding nebula of supernova debris.

The companion star responsible for triggering an unusual sort of supernova has been identified after 20 years of searching, confirming our ideas on how these events occur. "This is like a crime scene, and we finally identified the robber," says, University of California (UC), Berkeley Professor Alex Filippenko, "The companion star stole a bunch of hydrogen before the primary star exploded." 

Supernovae have been classified depending on their spectrums, reflecting the conditions that caused the star that formed them to explode. Type IIb supernova are one of the rarest sorts. While Type Ib supernova have no hydrogen and little silicon, normal Type II show hydrogen. Type IIb, on the other hand, start off looking like a typical Type II supernova before switching to appear more like a Ib supernova.

Both Type II and Ib supernovae are a result of massive stars running out of fuel leading to collapse, followed by a rebounding explosion, unlike the very different source of Ia events. The difference is that in Tybe Ib supernova the hydrogen has been expelled by strong stellar winds, or stripped away by a companion star, before the explosion happens. This is possible because old stars become layered with heavy elements at their core and hydrogen on the outside.

Type IIb supernovae initially have only a little hydrogen in their spectrum, but enough to be classified as Type II. However, this disappears after the brightness has passed its peak. It is thought that in these cases, most of the hydrogen has been removed by a large companion star. Enough is left to be initially visible, but the outermost layer becomes transparent as the gasses rush away after the explosion

Type IIb supernova are rare, and ones close enough for identification of the companion star are particularly scarce. Consequently SN 1993J, which occurred a cosmically tiny 11 million light years away in galaxy M81, has become one of the most intensely studied stellar explosions of all time. However, until now the identity of the companion star has remained a mystery.

“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,” says Dr. Ori Fox, also of Berkeley. While this is a problem with finding all supernova companions, the search was particularly difficult in this case because the area around the supernova is crowded with stars.

However, in The Astrophysical Journal Fox and Flippenko have identified a likely candidate, based on Hubble measurements of its intense ultraviolet light, an anticipated feature of a star that has acquired additional hydrogen in this way. The finding increases confidence in our models of Type IIb supernovae, and indeed Type II events in general.

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