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 Sweden, 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. This state-of-art, robotic telescope scans the sky rapidly over a thousand 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.

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 in the optical spectrum. 

Shortly after the supernova explosion, the team including Professor Albert Kong, from National Tsing Hua University (member of the University System of Taiwan), announced that a possible progenitor star was found in the pre-explosion high-resolution HST archival images. Following the initial alert, high-resolution optical images of the supernova were obtained with the 10m Keck I telescope together with an adaptive optics system (a special technique to improve the spatial resolution by counteracting for the distortions caused by the atmosphere). The team successfully refined the explosion position and confirmed the progenitor candidate identified in the HST images. The results combined with multi-wavelength follow-up observations indicate that the progenitor is consistent with a Wolf-Rayet star with a radius of a few times of our Sun. Wolf-Rayet stars are very hot star stars that are 10 times more massive and thousands of times brighter than the Sun, and that are losing mass rapidly by means of very strong stellar winds. 

"Although Wolf-Rayet progenitors have long been predicted for Type Ib supernovae, no concrete observational evidence has been found,“ said co-author Ray Li, a PhD student of National Tsing Hua University. “The progenitor discovery of iPTF13bvn fills the gap between theory and observation. 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. X-rays from supernova explosion are difficult to be detected because they are usually faint. By accumulating the X-ray data for nearly a month, the Taiwanese team finally found the trace of the X-rays. The results will provide important information how the shock wave of the explosion interacts with the interstellar medium.

A paper led by Yi Cao, a graduate student in astronomy at Caltech, describing these new results is published in the 2013 Sept 20 issue of The Astrophysical Journal Letters.

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