The Last Piece Of The Supernova Puzzle: A Lurking Blue Companion

With the discovery of a companion blue star, scientists have a better understanding of how supernovas form.

AsianScientist (Sep. 15, 2014) – Researchers have found evidence of a hot binary companion star to a yellow supergiant star which had become a bright supernova. The results have been published in the Astrophysical Journal Letters and have wide implications for our knowledge of binary systems and supernova mechanisms.

The question of how massive stars spend their short lives until they become supernovae is of great interest for astrophysicists. According to the standard theory—which is only applicable to isolated stars—only cool and extended (red supergiants) or hot and blue (Wolf-Rayet stars) are able to become supernovae.

However, growing evidence suggests that most massive stars are not lonely singles but they belong to close binary systems with profuse interactions. Episodes of mass transfer between the members of binary star systems affect the way the stars evolve, meaning that there are a great many more potential scenarios for the final stages of supernova progenitors.

The nearby supernova SN 2011dh, which occurred in 2011 in the well-known whirlpool galaxy M51, presented an excellent example that could not be explained by the standard theory. What appeared to be a yellow supergiant star was detected at the location of the supernova in images obtained before the explosion, but yellow supergiant stars in isolation were not thought capable of becoming supernovae.

Controversy arose in the astronomy community with several experts proposing that the actual progenitor must have been an unseen bright blue object, such as a Wolf Rayet star. However, the team led by Dr. Melina C. Bersten at Kavli Institute for the Physics and Mathematics of the Universe (IPMU) at the University of Tokyo, showed that the exploding star must have been extended, like a yellow supergiant, and that it must have belonged to a binary system.

The supernova explosion process. The artist's impression (top row) and the corresponding Hubble space telescope images (below). Credit: Kavli IPMU, NASA.
The supernova explosion process. The artist’s impression (top row) and the corresponding Hubble space telescope images (below). Credit: Kavli IPMU, NASA.

“We produced detailed models that self-consistently explained every property of SN 2011dh through the explosion of a yellow supergiant star in a binary system,” said Dr. Bersten.

In March 2013, the Bersten’s proposal was given substantial support when the disappearance of the yellow supergiant was observed, indicating that it and not a bright blue star was the exploding object.

“At that time there was just one piece of the puzzle missing to confirm our model: we had to find the companion star that, according to our calculations, was a hot, compact object,” said Dr. Omar G. Benvenuto, co-author of the study from the University of La Plata, Argentina.

With that goal, the group set out to obtain Hubble space telescope (HST) observing time, which was granted in 2013 and recently executed on August 7, 2014. Images were obtained in the ultraviolet regime, where the companion star was expected to be most clearly visible. A point source was clearly detected in the new images at the exact location of the supernova.

“One of the most exciting moments in my career as an astronomer was when I displayed the newly arrived HST images and saw the object right there, where we had anticipated it to be all along” said first author Dr. Gastón Folatelli from the Kavli IPMU, who led the efforts to obtain the new HST observations.

The case of SN 2011dh beautifully illustrates the advantages of an active feedback between theory and observation.

“As a scientist, for me it is like a dream come true to make a prediction and have it confirmed step by step as the supernova evolves and facts are revealed. It is a rare case for astronomy, where events usually take much longer to develop. We are very happy with how the story of SN 2011dh proceeded,” concluded Dr. Bersten.

The article can be found at: Folatelli et al. (2014) A Blue Point Source at the Location of Supernova 2011dh.

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Source: Kavli Institute for the Physics and Mathematics of the Universe.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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