A supernova with a view

December 15, 2011

Type 1a supernovae are such regular features of the Universe that astrophysicists use them to measure cosmic distances. But we still don't know exactly what makes these giant explosions occur. Now, scientists at the Weizmann Institute of Science, as part of an international effort to study supernovae, are beginning to clear up the mystery of why certain stars explode in a brilliant display at the ends of their lives.

New research began last August, when the automatic telescopes at the Palomar Transient Factory (PTF) in California that search for signs of developing supernova spotted one just a half a day into the explosion process. Not only was this a very early observation, but the supernova was in the Pinwheel Galaxy a mere 6.4 Megaparsecs away - the closest one in the last 25 years.

The scientists participating in PTF, including Drs. Eran Ofek and Avishay Gal-Yam of the Particle Physics and Astrophysics Department, have recently published three new papers based on their initial observations and analysis, two of them appearing in Nature and one in the Astrophysical Journal.

Data on the new supernova came from X-ray and radio wave telescopes, both Earth- and satellite-based. In addition, the researchers went over images of the Pinwheel Galaxy taken by the Hubble Space Telescope over the years to see if they could detect pre-explosion signs of the system that gave rise to the supernova.

To the scientist's surprise, the X-ray and radio observations yielded no significant data, and the archival study did not reveal what was there beforehand. But, like the dog in the Sherlock Holmes story that didn't bark, this lack turned out to be a significant clue: It allowed them to eliminate some of the various scenarios proposed for the type of setup causing the explosion.

These scenarios fall into two broad categories, both of them involving ancient, dense stars called white dwarfs. In one, two white dwarfs merge, and their combined mass becomes unstable, ending in a thermonuclear blast. In the other, the heavy white dwarf siphons off material from a companion star until it exceeds its stable weight limit, again causing an explosion. Proposed companion stars run the gamut from huge, gaseous red giants to smaller, sun-like stars.

The team's results, including an analysis of the material thrown off in the blast and of the 'shock breakout' that takes place as the light released in the shockwave passes through the mass of erupting material (conducted by Itay Rabinak, a student of Prof. Eli Waxman of the same department), showed that the exploding star was, as predicted, a white dwarf: an extremely compact star with a diameter much smaller than that of our sun. And while the team didn't manage to discount either category, they set an upper limit on the size of a possible companion, showing it could not have been a particularly large star, such as a red giant.

'Although we can't rule out a white dwarf merger,' says Ofek, 'our results point to another likely scenario in which a medium range star - close to our sun's size - supplied the white dwarf with the extra material needed to turn it into a supernova.'
Dr. Avishay Gal-Yam's research is supported by the Peter and Patricia Gruber Award; the Nella and Leon Benoziyo Center for Astrophysics; the Yeda-Sela Center for Basic Research; and the Lord Sieff of Brimpton Memorial Fund.

Dr. Eran Ofek's research is supported by the Willner Family Leadership Institute.

The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to 2,700 scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment. Weizmann Institute news releases are posted on the World Wide Web at http://wis-wander.weizmann.ac.il, and are also available at http://www.eurekalert.org.

Weizmann Institute of Science

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