02Oct2012

Suspected Supernova From A Yellow Supergiant Star.

Artist's conception of the progenitor system of SN 2011dh. The system consists of a blue star and a yellow supergiant. Credit: Kavli IPMU/Aya Tsuboi

REMEMBER THE SUPERNOVA EVENT in M51 that set the observing world abuzz? Now a group of researchers led by Melina Bersten (Kavli IPMU) made a surprise find.

They have new evidence the event was a lot more special than any of us could believe. It would appear that a yellow supergiant (YSG) star was found at the location of SN2011dh and it was the responsible party. But how could this type of star explode? It could be because it was a member of a close binary system.

In a paper published in the September 20, 2012, issue of The Astrophysical Journal, the research team reveals their modeling techniques – important contributions to the world of astrophysics. Understanding how a progenitor star – or progenitor star system – is created and evolved is big news. As of the present, it is hypothesized that most massive stars are doomed to become supernova events when they reach the red supergiant stage. It is also possible that blue, compact stars of the Wolf-Rayet variety can also reach this critical end. However, new observations of yellow supergiant stars as possible supernovae are making astronomers questions their results and their understanding of the evolution of massive stars.

The supernova event in M51 – SN2011dh – provided astronomers with a unique chance to study nearby activity. It was classed as a Type IIb supernova, with detectable hydrogen lines in the spectra during its beginning and then evolved a helium signature. These findings point to a star which depleted its hydrogen dominated envelope before it exploded.

M51 Galaxy before(left) and after(right) the eruption of SN 2011dh. The image on the left was taken in 2009, and on the right July 8th, 2011. Credit: Conrad Jung.

Thanks to the Hubble Space Telescope, researchers were ale to study images taken before the supernova event. Two independent groups were able to locate a source that could provide a close match with the supernova. Through the use of photometry, this pre-existing source showed a close match to a yellow supergiant star. If so, even more questions arise. Just how could this type of star explode? Current stellar evolution models for low mass range point to red supergiants undergoing core collapse as a source of supernova, or compact blue stars for the larger mass alternative. The YSG sits in the middle, an area where supernova just isn’t predicted. To add even more to the confusion, early optical emission and radio observations show that SN 2011dh could have been a compact object with the YSG being a companion star or simply a chance object.

Questions abound. In trying to solve the riddle, Bersten’s team made stellar evolution calculations for two massive stars in close orbit exchanging mass. Their conclusion shows the mass-donor star could eventually explode and this closely matches the observations of the YSG object found in the pre-supernova images. Their binary models predict an accreting companion should be a massive blue star at the moment of cataclysm. Due to its high surface temperature, the companion star should glow brightly in the ultraviolet end of the spectrum with little emission in the optical range. This means the companion star was so faint that it wasn’t picked up in the pre-supernova images. To further solidify their findings, the calculations predict that once the supernova has faded, the companion star should be observable in the deep blue end.

“The present results reveal the necessity and importance of further studying the evolution and explosion of binary stars,” Bersten, the first author of the paper said. “I look forward to the observation that will confirm our prediction.”

Original Story Source: Kavli Institute News Release. Submitted by: Tammy Plotner for “Dave Reneke’s World of Space and Astronomy.

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