Powerful new survey telescopes led by the California Institute of Technology (Caltech) are being combined with the W. M. Keck Observatory to provide insight into rare, exotic cosmic explosions. Caltech’s intermediate Palomar Transient Factory (iPTF) recently described the first direct detection of the progenitor of a rare type of supernova in a nearby galaxy. The findings were published n the September 20 issue of Astrophysical Journal Letters [http://dx.doi.org/10.1088/2041-8205/775/1/L7].
The paper describes the detection of a Type Ib supernova, a rare explosion in which the progenitor star lacks an outer layer of hydrogen, the most abundant element in the universe. It has proven difficult to pin down which kinds of stars give rise to Type Ib supernovae. One of the most promising ideas, according to graduate student and lead author Yi Cao, is they originate from Wolf-Rayet stars. These objects are 10 times more massive and thousands of times brighter than the Sun and have lost their hydrogen envelope by means of very strong stellar winds. Until recently, no solid evidence existed to support this theory. Cao and colleagues believe that the young supernova they discovered, iPTF13bvn, occurred at a location formerly occupied by a likely Wolf-Rayet star.
Supernova iPTF13bvn was spotted on June 16, less than a day after the onset of its explosion. With the aid of the world-leading adaptive optics system installed on the Keck II telescope, one of Keck Observatory’s two 10-meter telescopes in Hawaii, the team obtained a high-resolution image of this supernova to determine its precise position. Then they compared the Keck Observatory image to a series of pictures of the same galaxy (NGC 5806) taken by the Hubble Space Telescope in 2005, and found one starlike source spatially coincident to the supernova. Its intrinsic brightness, color, and size — as well as its mass-loss history, inferred from supernova radio emissions — were characteristic of a Wolf-Rayet star.
“All evidence is consistent with the theoretical expectation that the progenitor of this Type Ib supernova is a Wolf-Rayet star,” said Cao. “Our next step is to check for the disappearance of this progenitor star after the supernova fades away. We expect that it will have been destroyed in the supernova explosion.”
Though Wolf-Rayet progenitors have long been predicted for Type Ib supernova, the new work represents the first time researchers have been able to fill the gap between theory and observation, according to study coauthor and Mansi Kasliwal from the Carnegie Institution for Science. “This is a big step in our understanding of the evolution of massive stars and their relation to supernovae,” she said.
The iPTF builds on the legacy of the Caltech-led 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-the-art, robotic telescope scans the sky rapidly over a thousand square degrees each night to search for transients.