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Discovering The First Type-Ib SN progenitor
Progenitor Star of a Stellar Explosion Found
From a series of old images by the Hubble Space telescope (HST) taken in 2005, astronomers from National Tsing Hua University in Taiwan, as well as the international project iPTF, find the progenitor star of a newly discovered stellar explosion, known as Type Ib supernova, for the first time in the history of astronomy. The unprecedented discovery has been published in The Astrophysical Journal Letters.

The iPTF project is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Center in Switzerland; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan. It was built on the legacy of the Palomar Transient Factory (PTF), designed in 2008 to systematically chart the transient sky by using a robotic observing system mounted on the 48-inch Samuel Oschin Telescope on Palomar Mountain near San Diego, California. Also led by the Caltech, this state-of-art, robotic telescope scans the sky rapidly over thousands of square degrees each night to search for astronomical objects whose brightness changes over time scales from hours to days.

One important type of these transients is supernovae, massive exploding stars at the end of their life time. Since its commissioning four years ago, the PTF scorecard stands at over 2,000 spectroscopically classified supernovae. The unique feature of iPTF is a brand new software pipeline that is geared towards fully automated, rapid response and follow-up within hours of discovery of a new supernova.

Based on observations obtained with iPTF on 16 June 2013, the scientific collaboration discovered a supernova, called iPTF13bvn, in the nearby galaxy NGC 5806, located in the constellation Virgo about 80 million light years away from us. Based on a series of follow-up spectroscopic observations, the supernova is classified as Type Ib because there is no hydrogen feature but a neutral helium line in the optical spectrum.

Shortly after the supernova explosion, Professor Albert Kong, from National Tsing Hua University, announced to his global collaboration a possible progenitor star was found in the pre-explosion high-resolution HST archival images.

Following the initial alert, his PhD student, Ray Li, and other members in the collaboration immediately performed several independent checks on the old HST images and all agreed the possible progenitor detection as a blue star. However, the supernova position fixed by the iPTF is insufficient to fully support the claim, for which it could be an unrelated object close to the supernova. A lack of a high-precision position makes the discovery far from a significant scientific finding. The team thus observed iPTF13bvn with an adaptive optics system (a special technique to improve the spatial resolution by counteracting for the distortions caused by the atmosphere) mounted on the 10 m Keck I telescope in Mauna Kea, Hawaii. The new observation successfully refines the explosion position and confirms the progenitor discovery. The results combined with multi-wavelength follow-up observations allow the team to identify the progenitor as a Wolf-Rayet star with a radius of a few times of our Sun. Wolf-Rayet stars are very hot star that have shed their outer atmosphere.

"Although Wolf-Rayet progenitors have long been predicted for Type Ib supernovae, no concrete observational evidence has been found,“ said co-author Ray Li. “The progenitor discovery of iPTF13bvn fills the gap between the theory and the reality. It is an important issue in astronomy!"

“We may be lucky to discover the progenitor star because HST happened to observe the host galaxy, NGC 5806, in 2005 to help pinpoint the location of a supernova that exploded in 2004. But luck favors the prepared," said Albert Kong.

The team members from Taiwan also discovered the X-ray emission from the supernova by using NASA’s Swift X-ray Telescope and the results will provide important information how the shock wave of the explosion interacts with the interstellar medium.

A paper led by Yi Cao, from the California Institute of Technology, describing these new results is published in the 2013 Sept 20 issue of The Astrophysical Journal Letters.
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