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Supernovae: Type Ia, two ways.

Two new studies confirm that the white dwarfs that explode as Type la supernovae can approach death on two different paths.

When white dwarfs accrete too much material, they burst through a strict weight restriction called the Chandrasekhar limit and trigger a thermonuclear blast deep inside themselves, self-annihilating in a Type Ia supernova.

For many years, astronomers have debated just how the white dwarf maxes out its mass. There are two scenarios: either it siphons gas from a "living" companion star until it just can't swallow any more (called the single-degenerate model), or it merges with another dead star like itself (the double-degenerate model). The growing sense is that white dwarfs probably die both ways.

Two papers in the May 21st Nature support this idea. In the first, Yi Cao (Caltech) and colleagues detected an ultraviolet pulse in light from the Type la supernova iPTF14atg that lasted for four days after the explosion. The authors propose the UV pulse arose when the exploding white dwarf's ejecta slammed into its companion star. The signal would then have disappeared once the ejecta engulfed the companion enough to hide the shocked gas created by the collision.

In the second study, Rob Oiling (University of Maryland) and colleagues used archival observations from the Kepler mission to study three supernovae. Two of these supernovae (KSN 2011b and KSN 2012a) are clearly Type la; the third (KSN 2011c) is probably one. The team used the same theoretical predictions as Cao's team did to analyze their data and found no sign of ejecta slamming into companion stars.

Although Kepler doesn't look in UV (where Cao's team saw iPTF14atg's ejecta shock), Oiling says that the telescope's sensitivity is so tremendous that, had there been a signal from material colliding with a companion star, his team would have seen it in visible light. Thus, the astronomers favor a dual white dwarf death as the origin for these three (likely) Type la events.

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Title Annotation:News Notes
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Article Type:Brief article
Date:Sep 1, 2015
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