Friday, May 11, 2007

University of Chicago Team Models a Type 1a Supernova

A team at the University of Chicago has announced the first successful computer model that replicates a type-1a supernova. They use the numerical code FLASH, which does fully compressible MHD along with Newtonian gravity. FLASH has been used on a number of other projects in stellar modeling and cosmology. In fact, if you wanted to model one of Joe's Population III stars, FLASH would probably be the way to do it.

What is especially significant about the Chicago group's accomplishment (aside from the really cool pictures) is that they have managed to make the star go supernova and completely blow itself up. Type-1a supernova are known by observation to completely unbind the star, leaving only an expanding shell of high velocity, super-heated stuff.

Type 1a supernova occur in binary systems where both stars have small enough masses to avoid core collapse supernovae. The first star ends it's normal lifetime by burning all of it's hydrogen and then all of it's helium in it's core. The core is then made of carbon and oxygen, but the star is unable to produce the pressures and temperatures needed to fuse carbon and oxygen. With the star's power source turned off, the star begin to contract under the pressure of gravity until the area around the core is hot enough for hydrogen or helium fusion. When hydrogen and helium ignite, it blows off the outer layers of the star and converts what is left into mostly carbon and oxygen, leaving only a carbon/oxygen core with no power source. This will contract under gravity until stopped by the electron degeneracy pressure if it's mass is less than 1.44 solar masses. So our binary system consists of a neutron star and a regular main sequence star and everything is happy for a while.

Things start to get exciting again when the second star nears the end of it's normal life and expands into a red giant/subgiant. Because the star has expanded so much, it's outer layers are very loosely bound by gravity and the white dwarf companion can begin to steal matter from the outer layers. As this matter is deposited onto the white dwarf, it's mass increased until it approaches the 1.44 solar mass limit. Before it hits the limit, however, temperatures and pressures inside the core hit the point where carbon fusion begins and a runaway burning of carbon and oxygen occurs. Essentially, the star burns all of its carbon and most of its oxygen all at once creating a huge explosion that blows the star to bits.

The University of Chicago team has modeled this process. You can find some really cool movies here. This is especially important to cosmology, because type-1a supernovae are thought to be standard candles and are one of the evidences for dark energy. If the Chicago team can find ways to make the output of type-1a supernovae vary, it will cast doubt on their support for dark energy. On the other hand, if there turns out to be only one way to get the supernovae to blow up that is consistent with observations, this would go a long way to settling the debate of the type-1a supernova measurements of dark energy.

1 comment:

  1. Those are really some impressive movies Nick. Make the visualizations I have done look childish.

    It is true, Type Ia supernovas are the "standard candles" for determining galactic distances. If they are unreliable, that could alter physics like dark energy.

    However, gravitational lensing is becoming a second way to do far distance measurements. In fact, it is becoming a better way in some respects. For example, the farthest measured galaxy known is calculated using gravitational lensing.

    If gravitational lensing show dark energy we are still "safe" is supernovas become unreliable.


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