In August, as amateurs and professionals alike turned their telescopes on the nearest Type Ia supernova discovered in decades, University of California, Berkeley, research astronomer Weidong Li focused instead on what could not be seen. Li pulled up images of the northern sky taken over the past nine years by the Hubble Space telescope in hopes of seeing the supernova's "progenitor" star, but his initial disappointment - no star at all was visible - quickly turned to excitement.

"We soon realized that the non-detection limit of the Hubble Space Telescope may be stringent enough to let us rule out some proposed progenitor systems," he said.

Li's analysis of archival Hubble data, published in the journal Nature, is the first ever to narrow down otherwise viable progenitor systems for producing Type Ia supernovae, which are the foundation of discoveries for which three scientists - including UC Berkeley physics professor Saul Perlmutter - last weekend were awarded the 2011 Nobel Prize in Physics.

"Our paper is the first ever to exclude directly some of the major candidates for Type Ia supernovae," said coauthor Joshua Bloom, UC Berkeley associate professor of astronomy.

While the star that explodes to produce a Type Ia supernova is thought to be a white dwarf composed primarily of carbon and oxygen, the white dwarf presumably explodes because it pulls matter from a binary companion that to date has been a mystery.

Li and UC Berkeley colleagues, including Bloom and Alex Filippenko, UC Berkeley professor of astronomy, analyzed high-resolution Hubble data and calculated that the companion could not have been a red giant star, which typically are bloated and bright, or most types of helium stars, which also would have been visible.

This fits with theoretical modeling of the exploding white dwarf by the supernova's discoverers: Peter Nugent, head of the Computational Cosmology Center at Lawrence Berkeley National Laboratory, and his colleagues. Together, these findings make astronomers confident that the companion was most likely a normal star like the Sun, a somewhat evolved star called a subgiant, or perhaps a white dwarf.

Nugent and his colleagues, including physicists from UC Berkeley, also will publish their conclusions in Nature.

"Together the two papers suggest that a main-sequence star companion agrees with all the observed properties of the supernova progenitor luminosity limits and the evolution of the supernova," said coauthor Saurabh Jha, an assistant professor of astronomy at Rutgers University.

Type Ia Supernovae Indicate Accelerating Expansion of Universe
Type Ia supernovae are bright and visible across huge cosmic distances, which allowed Perlmutter, his Nobel laureate colleagues and their teams to employ them as "standard candles" to measure the universe. In 1998, these studies revealed that the expansion of the universe is accelerating, now widely believed to require the presence of a mysterious "dark energy."

"The discovery of the accelerating expansion of the Universe has revolutionized physics, and the repulsive dark energy may provide key clues to the long-sought quantum theory of gravity," said Filippenko, who was a member of both of the teams that made the Nobel Prize-winning discovery. "But the actual origins of Type Ia supernovae have remained mysterious, and various aspects of the explosion are not well understood."

An opportunity to study such explosions at close hand came on August 24, 2011, when Nugent, while looking at data from the Palomar Transient Factory (PTF), which searches for short-lived events in space, spotted a remarkable object in the nearby Pinwheel Galaxy (Messier 101) some 21 million light-years away.

Subsequently named SN 2011fe, it garnered worldwide attention as the nearest such supernova in the past 25 years. Although its brightness was 40 times too faint for the naked human eye to see, for a month the supernova could be easily viewed through binoculars, a rare occurrence among amateur astronomers and the general public.

"These are the sorts of important events that the Palomar Transient Factory was designed to uncover," said PTF principal investigator Shrinivas Kulkarni, a professor at the California Institute of Technology.

Li immediately thought of looking for the original star and its companion in past Hubble data, seeing it as "a golden opportunity to investigate the properties of the progenitor system through direct imaging."

Within two days, Li and about 30 collaborators from around the world were able to obtain images of the supernova from the 10-meter Keck Telescope in Hawaii and its adaptive optics system. Using these high-resolution images, they were able to pinpoint the exact location of the supernova. Lo and behold, Hubble images showed that nothing was there.

Undeterred, Li and his colleagues used the red, green and blue wavelength observations from HST to set a stringent limit on how bright the progenitor and its companion could be.

The authors were able to exclude the presence of a luminous red giant star, and most types of helium stars. The only options consistent with the derived limits were a faint white dwarf - making the progenitor system a double-degenerate binary star system - or a main-sequence or subgiant companion star to the now-exploded white dwarf.

"Our next step is to detect the surviving companion star, perhaps using the James Webb Space Telescope when it comes online. That will give us another opportunity to reveal more secrets about this supernova and ultimately help us understand the explosion physics of Type Ia supernovae, and possibly refine them as an even better cosmological distance ladder," Li said.

Among the paper's coauthors are Adam A. Miller, S. Bradley Cenko, Peter E. Nugent, Mohan Ganeshalingam, Jeffrey M. Silverman and Ken J. Shen of UC Berkeley's Department of Astronomy, and colleagues from UC Santa Barbara, Arizona State University and the Weizmann Institute of Science in Rehovot, Israel.