A star accompanying a rare type of supernova in close orbit has been discovered by astronomers using NASA's Hubble Space Telescope. Confirming a long-held theory that the explosion originated in a binary star arrangement, observations verify that the companion star precipitated the destruction of the aging primary star by drawing off mass until its core collapsed and triggered a supernova event.
Verification of this phenomenon has allowed scientists to further improve their understanding of the properties and role of a companion star in Type IIb supernovas, where a giant star has lost most of its hydrogen due to the gravitational pull of a more massive companion in a binary system, leaving a core almost entirely composed of helium. In this instance, the astronomers were able to estimate the surviving star's luminosity and mass, which provided further insight into the conditions that preceded the explosion.
"A binary system is likely required to lose the majority of the primary star's hydrogen envelope prior to the explosion." explains lead researcher Ori Fox of the University of California at Berkeley. "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."
First detected in 1993, this Type IIb supernova was given the designation SN 1993J. Located in the M81 galaxy approximately 11 million light-years from Earth in the constellation of Ursa Major, SN 1993J is the closest discovered instance of this type of supernova. However, due to the crowded field of view in this area, astronomers were unable – until now – to determine if the observed spectral absorption data signified a companion object next to the supernova, or if it was interference from other stars in the field containing SN 1993J.
The spectral absorption data first detected using the Keck Observatory on Mauna Kea, Hawaii in 2004 led to the theory that there may be a close companion star in the region. However, confirming evidence of this in what is known as a "continuum emission" (radiation that forms a continuous spectrum) of UV light was difficult to obtain as it is generally only detectable in space due to the absorption effects of Earth’s atmosphere.
"We were able to get that UV spectrum with Hubble," said Azalee Bostroem, team member at 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 has been subtracted."
To achieve this goal, astronomers combined data collected from Earth-based optical light telescope images with pictures taken with two Hubble instruments collecting ultraviolet light. The astronomers were then able to create a multi-wavelength spectrum analysis that confirmed what had been predicted and provide evidence that the light detected was the glow of a helium-burning companion star.
The results of this research were published in the Astrophysical Journal.
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