Solving a supernova mystery

Type Ia supernovae are violent stellar explosions. Observations of their brightness are used to determine distances in the universe and have shown scientists that the universe is expanding at an accelerating rate. The Nobel Prize in Physics was awarded December 10 to three astronomers for their "discovery of the accelerating expansion of the Universe through observations of distant supernovae."

The PTF team, led by Professor Shri Kulkarni of the California Institute of Technology, discovered this supernova, named SN2011fe, just 11 hours after it exploded. They were able to pinpoint the explosion in the Pinwheel Galaxy to August 23 at about 4:30 p.m. Universal Time.

"For several years, I had been taking images with robotic telescopes at Palomar Observatory of the Pinwheel Galaxy every night I possibly could, hoping it would give birth to a rare cosmic feat," Kasliwal said. "When we saw SN2011fe, I fell off my chair as its brightness was too faint to be a supernova and too bright to be nova. Only follow-up observations in the next few hours revealed that this was actually an exceptionally young Type Ia supernova."

The widely accepted theory is that Type Ia supernovae are thermonuclear explosions of a white dwarf star that's part of a binary system--two stars that are physically close and orbit around a common center of mass.

There are two different models for how Type Ia supernovae are created from this type of binary system. In the so-called double-degenerate (or DD) model, the orbit between two white dwarf stars shrinks until the lighter star's path is disrupted and it moves close enough for some of its matter to be absorbed into the primary white dwarf and initiate an explosion. In the so-called single-degenerate (or SD) model, the white dwarf slowly accretes mass from a different, non-white dwarf type of star, until it reaches an ignition point. There are three potential methods for the transfer of mass and--depending on which one is used--the second star is likely to be a red giant, a helium star, or a so-called subgiant or main-sequence star.

Observations of the early stages of the supernova--presented in a paper by lead author Peter Nugent of Lawrence Berkeley Laboratory--showed direct evidence that the primary star was a type of white dwarf called a carbon-oxygen white dwarf. Very sensitive and early radio and X-ray observations, presented in a separate paper to be published in The Astrophysical Journal, show no evidence of interaction with surrounding material. Combining this data with an analysis of historical images, the team ruled out luminous red giants and the vast majority of helium stars for the second star in the binary system before the explosion.

These clues meant that the secondary star was either another white dwarf, as in the DD model, or a subgiant or main-sequence star, as created by one of the three SD model methods.

Analysis of the matter ejected by the supernova's explosion suggests that the second star is less likely to be another white dwarf. Thus, the solution to the mystery of SN2011fe's origin is probably a primary white dwarf accreting matter from a neighboring subgiant or main-sequence star.

"The fact that we discovered this supernova in its infancy, and that the Pinwheel Galaxy is in our cosmic backyard, has given us an unprecedented opportunity to make this the best studied supernova to date," Kulkarni said.