Honing in on supernova origins

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Pasadena, CA—Type Ia supernovae are important stellar phenomena, used to measure the expansion of the universe. But astronomers know embarrassingly little about the stars they come from and how the explosions happen. New research from a team led by Harvard University and including Carnegie’s Josh Simon, Chris Burns, Nidia Morrell, and Mark Phillips examined 23 Type Ia supernovae and helped identify the formation process for at least some of them. Their work will be published in The Astrophysical Journal and is available online.

   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, 2011, to three astronomers for their "discovery of the accelerating expansion of the Universe through observations of distant supernovae.”

   Type Ia supernovae are believed to be thermonuclear explosions of a white dwarf star that’s part of a binary system--two stars that are physically close together and orbit around a common center of mass. But there are two different possibilities for how Type Ia supernovae are created from this type of binary system.

   In the so-called double-degenerate model, the orbit between two white dwarf stars gradually shrinks until the lighter star gets so close to its companion that it is ripped apart by tidal forces. Some of the lighter star’s matter is then absorbed into the primary white dwarf, causing an explosion. In the competing single-degenerate model, the white dwarf slowly accretes mass from an ordinary, non-white dwarf star, until it reaches an ignition point.

   “Previous studies have produced conflicting results. The conflict disappears if both types of explosion are happening,” explained lead author Ryan Foley of the Harvard-Smithsonian Center for Astrophysics.

   The research team studied 23 Type Ia supernovae to look for signatures of gas around the supernovae, which should be present only in single-degenerate systems. They found that the more powerful explosions tended to come from “gassy” systems, or systems with outflows of gas. However, only a fraction of supernovae show evidence for outflows--the remainder likely come from double-degenerate systems.

   This finding has important implications for how astronomers use supernovae to measure the universe’s expansion. “To maximize the accuracy of our measurements we may have to separate the two kinds of Type Ia supernovae,” Simon said. “This study gives us one potential way to tell them apart.”

Funding for this research was provided in part by a Clay Fellowship, the ISF, the Minerva foundations, an ARCHES award, the Lord Sieff of Brimpton Fund, a Minerva fellowship, CONICYT, the Millennium Center for Supernova Science, and the NSF

The HET is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universitat Munchen, and Georg-August-Universitat Gottingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.