Scenes from a Supernova


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(l-r) Drs. Avishay Gal-Yam and Eran Ofek in the Martin Kraar Observatory on the Weizmann campus


In late August, automatic telescopes searching for signs of developing supernovae spotted one just a half a day into the explosion process. Not only was this a very early observation, the supernova was a mere 6.4 megaparsecs away – the closest one in the last 25 years. Weizmann Institute scientists were part of the discovery team, and they realized they had a unique opportunity: The technological advances of the past few decades would enable them to explore this supernova with methods never applied before. Soon, a number of other telescopes were pointed at the supernova’s home in the Pinwheel Galaxy.

The new sighting was a Type 1a supernova; these kinds of exploding stars are such a regular feature of the Universe that astrophysicists use them to measure cosmic distances. The theory of dark energy, for instance, for which the 2011 Nobel Prize in Physics was awarded, is based on supernova measurements which revealed that the expansion of the Universe is accelerating. Unfortunately, says team member Dr. Eran Ofek, “we know embarrassingly little about the physical processes underlying these supernovae.”

The Weizmann scientists, Ofek and Dr. Avishay Gal-Yam of the Particle Physics and Astrophysics Department, participate in the Palomar Transient Factory (PTF), a project for identifying new supernovae in the Universe. The PTF team recently published three new papers based on their initial observations and analysis of the nearby supernova, two of them in Nature and one in The Astrophysical Journal.

The new supernova was observed with X-ray and radio wave telescopes, both Earth- and satellite-based. In addition, a number of images of the Pinwheel Galaxy had been produced by the Hubble Space Telescope over the years, and the researchers went back to these to see if they could detect pre-explosion signs of the system that gave rise to the supernova.  
The site of SN 2011fe in galaxy M101 as imaged by the Hubble Space Telescope/ Advanced Camera for Surveys
To the scientists’ surprise, the X-ray and radio observations yielded no significant data; 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 several of the proposed scenarios for the setup that might have caused the explosion.

These scenarios fall into two broad categories, both of them involving ancient, dense stars called white dwarfs. In one of them, 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 PTF 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, showed that the exploding star was, as predicted, a white dwarf. The picture that emerged was of an extremely compact star – its 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 – excluding, for instance, a red giant scenario.

“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.”

A Marriage Written in the Stars

Dr. Eran Ofek, who recently joined the Weizmann Institute’s Particle Physics and Astrophysics Department after six years in California, started working almost as soon as his plane landed in Israel: “We arrived on August 23rd, and on the 24th we got notification of the new supernova. Instead of setting up house or shopping for a car, I was on line with other astrophysicists arranging observation time and collecting data.”

Fortunately for Ofek, who is married and the father of two small daughters, his wife, Dr. Orly Gnat, was forgiving. She is also an astrophysicist, who is currently undertaking postdoctoral research at the Hebrew University of Jerusalem. Ofek and Gnat met when they were undergraduates at Tel Aviv University. After receiving their doctorates from Tel Aviv University, they went on to postdoctoral positions at the California Institute of Technology (Caltech) and have now returned to Israel.

At Caltech, Ofek was involved in setting up the Palomar Transient Factory – a project based at the Palomar Observatory in California that searches for transient bursts of light indicating stellar explosions. The Weizmann Institute is a partner in PTF, along with scientists from all over the US and Europe. Ofek was responsible for the telescopes’ robotic capabilities: These autonomous seekers “decide” for themselves which parts of the sky to search and analyze their observations independently before notifying team members of possible supernova sightings. Ofek: “The PTF has identified more supernovae than any other survey, and we have expanded the number of known types of supernovae, as well.”
Dr. Avishay Gal Yam's research is supported by the Nella and Leon Benoziyo Center for Astrophysics; the Yeda-Sela Center for Basic Research; the Legacy Heritage Fund Program of the Israel Science Foundation; the Peter and Patricia Gruber Awards; and the Lord Sieff of Brimpton Memorial Fund. 
Dr. Eran Ofek's research is supported by the Willner Family Leadership Institute.