Star discovered that survived a supernova in a binary system

The supernova SN 2001ig
The supernova SN 2001ig

An article published in “The Astrophysical Journal” describes the detection of a companion of the type IIb supernova known as SN 2001ig. A team of astronomers used the Hubble Space Telescope to discover the first case of a binary system in which one of the stars survived when its partner exploded into a supernova. The two stars were not just close but there was an interaction that had an influence on the supernova.

The supernova SN 2001ig was sighted on December 10, 2001 in the galaxy NGC 7424, about 40 million light years from Earth. In the following years various telescopes were used to observe the evolution of that supernova, classified as type IIb, anomalous because almost all the hydrogen gets lost before the explosion. The first hypothesis was that the progenitor star had a very fast wind, so much so as to push away its outer layers, but subsequent research found none of them.

The second hypothesis for the origin of type IIb supernovae was that they occur in binary systems in which one of the two stars snatches its companion’s outer layers. The process is illustrated in the bottom image (NASA, ESA, and A. Feild (STScI)), in which two stars orbit around each other (1), the most massive one evolves more rapidly and its outer layers are stripped by its partner (2), the star explodes (3) and when the brightness drops enough, the companion can be sighted (4).

The problem was to verify that hypothesis and the first step was made in 2004 thanks to observations with the Gemini South telescope in the area around the supernova SN 2001ig. In that case the astronomers found the first possible traces of the supernova’s companion and consequently its possible position.

However, only in 2016 Stuart Ryder of the Australian Astronomical Observatory (AAO) and his team managed to detect that star by pointing the Hubble space telescope at the coordinates found 12 years earlier. At that point, that supernova’s brightness had faded and Hubble’s sensitivity to ultraviolets allowed to find and photograph the target sought for so long.

The team also includes Ori Fox of the Space Telescope Science Institute (STScI), who in 2014 participated in another similar research. In that case, Hubble detected only a spectrum and not a complete image of the companion of the supernova SN 1993J, also a type IIb one. This new detection, with a photo, is a better proof that such a mechanism can trigger that kind of supernova. The stripped gas formed a shell around the companion that protected it during the explosion.

The top image (NASA, ESA, S. Ryder (Australian Astronomical Observatory), and O. Fox (STScI)) shows a part of the NGC 7424 galaxy with the area in which the supernova was identified in the smallest inset. The insets above show, in order, the images taken by ESO’s Very Large Telescope (VLT) in 2002, by the Gemini South telescope in 2004 with the most interesting area in the central inset whose contents are shown in the inset on the right, observed by the Hubble Space Telescope in 2016.

This type of research is only at the beginning, also because this discovery doesn’t rule out the possibility of massive stars with powerful winds that sweep away their outer layers at the origin of type IIb supernovae. Sometimes different mechanisms can lead to the same result so the next step will be to look for additional cases of that type of supernova study them in depth.

Stages of a type IIb supernova
Stages of a type IIb supernova

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