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Variability in type
Ia supernovae


Posted: August 13, 2009

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Newly discovered sources of variability in type Ia supernovae could cause problems in their use for calculating cosmic distances and the expansion of the Universe.

Type 1a supernovae result from the violent explosion of a white dwarf star, and since their initial conditions are thought to be roughly the same in all cases, they tend to have the same luminosity. Their predictable and uniform brightness makes them ideal "standard candles," offering astronomers a useful way to measure cosmic distances. Furthermore, the discovery of dark energy – a mysterious force that is driving the expansion of the Universe – was based on observations of type 1a supernovae.

This image based on a computer simulation of a type 1a supernova shows the turbulent and asymmetric flame of the runaway thermonuclear burning that consumes the white dwarf star. Image by F. Ropke.

In a new study published this week in Nature, sources of variability in type 1a supernovae are revealed that could play havoc with their reputation as such reliable distance markers.

"As we begin the next generation of cosmology experiments, we will want to use type 1a supernovae as very sensitive measures of distance," says lead author Daniel Kasen, a Hubble postdoctoral fellow at the University of California, Santa Cruz. "We know they are not all the same brightness, and we have ways of correcting for that, but we need to know if there are systematic differences that would bias the distance measurements."

Kasen and his coauthors used supercomputers to simulate type 1a supernovae explosions to explore what causes those differences in brightness. The simulations were based on how and where the ignition process begins inside the white dwarf and where it makes the transition from slow-burning combustion to explosive detonation. They found that due to the chaotic nature of the processes involved, the explosions where largely asymmetric.

"Since ignition does not occur in the dead centre, and since detonation occurs first at some point near the surface of the exploding white dwarf, the resulting explosions are not spherically symmetric," describes coauthor Stan Woosley. The simulations showed that the asymmetry of the explosions is a key factor in determining the brightness of type 1a supernovae.

Multiwavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova. (NASA/CXC/JPL-Caltech/Calar Alto O. Krause et al.)

Although this variability would not produce systematic errors in measurements assuming a large enough sample was taken, discrepancies could result from differences in the chemical compositions of stars born at different times in the history of the Universe. The synthesis of new elements during supernovae explosions is sensitive to differences in the geometry of the first sparks that initially ignite inside the core of the white dwarf.

Nickel-56 is especially important, because the radioactive decay of this unstable isotope creates the afterglow that astronomers are able to observe for months or years after the explosion. "The decay of nickel-56 is what powers the light curve," says Kasen. "The explosion is over in a matter of seconds, so what we see is the result of how the nickel heats the debris and how the debris radiates light."

Another source of variability results from these asymmetric explosions looking different when viewed at different angles, accounting for variations in brightness of up to 20 percent. But this effect is random and creates scatter in the measurements that can be statistically reduced by observing large numbers of supernovae.

It's not all bad news though, since the variability seen in the computer models agrees with observations of known type 1a supernovae. "Most importantly, the width and peak luminosity of the light curve are correlated in a way that agrees with what observers have found," says Woosley. "So the models are consistent with the observations on which the discovery of dark energy was based."

The results of two-dimensional models are reported in the new Nature paper, and more detailed three-dimensional studies are currently under way which will be used to refine distance estimates across the Universe and to make measurements of the expansion rate of the Universe more precise.