Solving a supernova mystery

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Pasadena, CA— A team of scientists, including Carnegie’s Mansi M. Kasliwal, has observed the early stages of a Type Ia supernova that is only 21 million light years away from Earth--the closest of its kind discovered in 25 years. The Palomar Transient Factory team’s detection of a supernova less than half a day after it exploded will refine and challenge our understanding of these stellar phenomena. Their breakthrough observations are published December 15 in Nature.

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.


Caption: SN 2011fe in the Pinwheel Galaxy (M101) at maximum brightness, a composite of optical data from the Las Cumbres Observatory Global Telescope Network 0.8m Byrne Observatory Telescope at the Sedgwick Reserve and (purple) hydrogen emission data from the Palomar Transient Factory. The left side shows the galaxy with no labels and the right shows the same with the SN2011fe labled.


The Palomar Transient Factory (PTF) is based on the 48-inch Oschin Schmidt telescope and the 60-inch telescope of the Palomar Observatory of the California Institute of Technology and is a collaboration of the following institutions: California Institute of Technology, Columbia University, Las Cumbres Observatory Global Telescope, Lawrence Berkeley Laboratory, Oxford University, the University of California at Berkeley and the Weizmann Institute of Science. The Principal Investigator of PTF is Professor S. R. Kulkarni.

This work received financial, staff and computing support from the National Energy Research Scientific Computing Center, the Department of Energy, the Department of Energy Scientific Discovery through Advanced Computing Program, the Royal Society, the National Science Foundation,  NASA, the TABASGO Foundation, Gary and Cynthia Bengier, the Richard and Rhoda Goldman Fund, NASA’s Hubble Fellowship and the Carnegie-Princeton Fellowship, the Sylvia & Jim Katzman Foundation, NASA’s Einstein Fellowship, Hilary Lipsitz, and the American Museum of Natural History. The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Radio Astronomy) with support fromthe Netherlands Organization for Scientific Research. The Liverpool Telescope is operated with financial support from the UK Science and Technology Facilities Council. The W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, was made possible by the generous financial support of the W. M. Keck Foundation. Data from the NASA/IPAC Extragalactic Database, operated by the Jet Propulsion Laboratory at the California Institute of Technology was used, as was data from the UK Swift Science Data Centre at the University of Leicester.